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Message-ID: <CACT4Y+bsTHAOtMKj_kWps1UdORWM1C8C7AN+ogPvw1gg1ugY7A@mail.gmail.com>
Date: Wed, 23 Oct 2019 13:08:49 +0200
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Marco Elver <elver@...gle.com>
Cc: LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
Alan Stern <stern@...land.harvard.edu>,
Alexander Potapenko <glider@...gle.com>,
Andrea Parri <parri.andrea@...il.com>,
Andrey Konovalov <andreyknvl@...gle.com>,
Andy Lutomirski <luto@...nel.org>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Arnd Bergmann <arnd@...db.de>,
Boqun Feng <boqun.feng@...il.com>,
Borislav Petkov <bp@...en8.de>, Daniel Axtens <dja@...ens.net>,
Daniel Lustig <dlustig@...dia.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
David Howells <dhowells@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
Jade Alglave <j.alglave@....ac.uk>,
Joel Fernandes <joel@...lfernandes.org>,
Jonathan Corbet <corbet@....net>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Luc Maranget <luc.maranget@...ia.fr>,
Mark Rutland <mark.rutland@....com>,
Nicholas Piggin <npiggin@...il.com>,
"Paul E. McKenney" <paulmck@...ux.ibm.com>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Will Deacon <will@...nel.org>,
kasan-dev <kasan-dev@...glegroups.com>,
linux-arch <linux-arch@...r.kernel.org>,
"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
linux-efi@...r.kernel.org,
"open list:KERNEL BUILD + fi..." <linux-kbuild@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>,
"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH v2 1/8] kcsan: Add Kernel Concurrency Sanitizer infrastructure
w?
On Thu, Oct 17, 2019 at 4:13 PM Marco Elver <elver@...gle.com> wrote:
>
> Kernel Concurrency Sanitizer (KCSAN) is a dynamic data-race detector for
> kernel space. KCSAN is a sampling watchpoint-based data-race detector.
> See the included Documentation/dev-tools/kcsan.rst for more details.
>
> This patch adds basic infrastructure, but does not yet enable KCSAN for
> any architecture.
>
> Signed-off-by: Marco Elver <elver@...gle.com>
> ---
> v2:
> * Elaborate comment about instrumentation calls emitted by compilers.
> * Replace kcsan_check_access(.., {true, false}) with
> kcsan_check_{read,write} for improved readability.
> * Change bug title of race of unknown origin to just say "data-race in".
> * Refine "Key Properties" in kcsan.rst, and mention observed slow-down.
> * Add comment about safety of find_watchpoint without user_access_save.
> * Remove unnecessary preempt_disable/enable and elaborate on comment why
> we want to disable interrupts and preemptions.
> * Use common struct kcsan_ctx in task_struct and for per-CPU interrupt
> contexts [Suggested by Mark Rutland].
> ---
> Documentation/dev-tools/kcsan.rst | 203 ++++++++++++++
> MAINTAINERS | 11 +
> Makefile | 3 +-
> include/linux/compiler-clang.h | 9 +
> include/linux/compiler-gcc.h | 7 +
> include/linux/compiler.h | 35 ++-
> include/linux/kcsan-checks.h | 147 ++++++++++
> include/linux/kcsan.h | 108 ++++++++
> include/linux/sched.h | 4 +
> init/init_task.c | 8 +
> init/main.c | 2 +
> kernel/Makefile | 1 +
> kernel/kcsan/Makefile | 14 +
> kernel/kcsan/atomic.c | 21 ++
> kernel/kcsan/core.c | 428 ++++++++++++++++++++++++++++++
> kernel/kcsan/debugfs.c | 225 ++++++++++++++++
> kernel/kcsan/encoding.h | 94 +++++++
> kernel/kcsan/kcsan.c | 86 ++++++
> kernel/kcsan/kcsan.h | 140 ++++++++++
> kernel/kcsan/report.c | 306 +++++++++++++++++++++
> kernel/kcsan/test.c | 117 ++++++++
> lib/Kconfig.debug | 2 +
> lib/Kconfig.kcsan | 88 ++++++
> lib/Makefile | 3 +
> scripts/Makefile.kcsan | 6 +
> scripts/Makefile.lib | 10 +
> 26 files changed, 2069 insertions(+), 9 deletions(-)
> create mode 100644 Documentation/dev-tools/kcsan.rst
> create mode 100644 include/linux/kcsan-checks.h
> create mode 100644 include/linux/kcsan.h
> create mode 100644 kernel/kcsan/Makefile
> create mode 100644 kernel/kcsan/atomic.c
> create mode 100644 kernel/kcsan/core.c
> create mode 100644 kernel/kcsan/debugfs.c
> create mode 100644 kernel/kcsan/encoding.h
> create mode 100644 kernel/kcsan/kcsan.c
> create mode 100644 kernel/kcsan/kcsan.h
> create mode 100644 kernel/kcsan/report.c
> create mode 100644 kernel/kcsan/test.c
> create mode 100644 lib/Kconfig.kcsan
> create mode 100644 scripts/Makefile.kcsan
>
> diff --git a/Documentation/dev-tools/kcsan.rst b/Documentation/dev-tools/kcsan.rst
> new file mode 100644
> index 000000000000..497b09e5cc96
> --- /dev/null
> +++ b/Documentation/dev-tools/kcsan.rst
> @@ -0,0 +1,203 @@
> +The Kernel Concurrency Sanitizer (KCSAN)
> +========================================
> +
> +Overview
> +--------
> +
> +*Kernel Concurrency Sanitizer (KCSAN)* is a dynamic data-race detector for
> +kernel space. KCSAN is a sampling watchpoint-based data-race detector -- this
> +is unlike Kernel Thread Sanitizer (KTSAN), which is a happens-before data-race
> +detector. Key priorities in KCSAN's design are lack of false positives,
> +scalability, and simplicity. More details can be found in `Implementation
> +Details`_.
> +
> +KCSAN uses compile-time instrumentation to instrument memory accesses. KCSAN is
> +supported in both GCC and Clang. With GCC it requires version 7.3.0 or later.
> +With Clang it requires version 7.0.0 or later.
> +
> +Usage
> +-----
> +
> +To enable KCSAN configure kernel with::
> +
> + CONFIG_KCSAN = y
> +
> +KCSAN provides several other configuration options to customize behaviour (see
> +their respective help text for more info).
> +
> +debugfs
> +~~~~~~~
> +
> +* The file ``/sys/kernel/debug/kcsan`` can be read to get stats.
> +
> +* KCSAN can be turned on or off by writing ``on`` or ``off`` to
> + ``/sys/kernel/debug/kcsan``.
> +
> +* Writing ``!some_func_name`` to ``/sys/kernel/debug/kcsan`` adds
> + ``some_func_name`` to the report filter list, which (by default) blacklists
> + reporting data-races where either one of the top stackframes are a function
> + in the list.
> +
> +* Writing either ``blacklist`` or ``whitelist`` to ``/sys/kernel/debug/kcsan``
> + changes the report filtering behaviour. For example, the blacklist feature
> + can be used to silence frequently occurring data-races; the whitelist feature
> + can help with reproduction and testing of fixes.
> +
> +Error reports
> +~~~~~~~~~~~~~
> +
> +A typical data-race report looks like this::
> +
> + ==================================================================
> + BUG: KCSAN: data-race in generic_permission / kernfs_refresh_inode
> +
> + write to 0xffff8fee4c40700c of 4 bytes by task 175 on cpu 4:
> + kernfs_refresh_inode+0x70/0x170
> + kernfs_iop_permission+0x4f/0x90
> + inode_permission+0x190/0x200
> + link_path_walk.part.0+0x503/0x8e0
> + path_lookupat.isra.0+0x69/0x4d0
> + filename_lookup+0x136/0x280
> + user_path_at_empty+0x47/0x60
> + vfs_statx+0x9b/0x130
> + __do_sys_newlstat+0x50/0xb0
> + __x64_sys_newlstat+0x37/0x50
> + do_syscall_64+0x85/0x260
> + entry_SYSCALL_64_after_hwframe+0x44/0xa9
> +
> + read to 0xffff8fee4c40700c of 4 bytes by task 166 on cpu 6:
> + generic_permission+0x5b/0x2a0
> + kernfs_iop_permission+0x66/0x90
> + inode_permission+0x190/0x200
> + link_path_walk.part.0+0x503/0x8e0
> + path_lookupat.isra.0+0x69/0x4d0
> + filename_lookup+0x136/0x280
> + user_path_at_empty+0x47/0x60
> + do_faccessat+0x11a/0x390
> + __x64_sys_access+0x3c/0x50
> + do_syscall_64+0x85/0x260
> + entry_SYSCALL_64_after_hwframe+0x44/0xa9
> +
> + Reported by Kernel Concurrency Sanitizer on:
> + CPU: 6 PID: 166 Comm: systemd-journal Not tainted 5.3.0-rc7+ #1
> + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> + ==================================================================
> +
> +The header of the report provides a short summary of the functions involved in
> +the race. It is followed by the access types and stack traces of the 2 threads
> +involved in the data-race.
> +
> +The other less common type of data-race report looks like this::
> +
> + ==================================================================
> + BUG: KCSAN: data-race in e1000_clean_rx_irq+0x551/0xb10
> +
> + race at unknown origin, with read to 0xffff933db8a2ae6c of 1 bytes by interrupt on cpu 0:
> + e1000_clean_rx_irq+0x551/0xb10
> + e1000_clean+0x533/0xda0
> + net_rx_action+0x329/0x900
> + __do_softirq+0xdb/0x2db
> + irq_exit+0x9b/0xa0
> + do_IRQ+0x9c/0xf0
> + ret_from_intr+0x0/0x18
> + default_idle+0x3f/0x220
> + arch_cpu_idle+0x21/0x30
> + do_idle+0x1df/0x230
> + cpu_startup_entry+0x14/0x20
> + rest_init+0xc5/0xcb
> + arch_call_rest_init+0x13/0x2b
> + start_kernel+0x6db/0x700
> +
> + Reported by Kernel Concurrency Sanitizer on:
> + CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.3.0-rc7+ #2
> + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> + ==================================================================
> +
> +This report is generated where it was not possible to determine the other
> +racing thread, but a race was inferred due to the data-value of the watched
> +memory location having changed. These can occur either due to missing
> +instrumentation or e.g. DMA accesses.
> +
> +Data-Races
> +----------
> +
> +Informally, two operations *conflict* if they access the same memory location,
> +and at least one of them is a write operation. In an execution, two memory
> +operations from different threads form a **data-race** if they *conflict*, at
> +least one of them is a *plain access* (non-atomic), and they are *unordered* in
> +the "happens-before" order according to the `LKMM
> +<../../tools/memory-model/Documentation/explanation.txt>`_.
> +
> +Relationship with the Linux Kernel Memory Model (LKMM)
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +The LKMM defines the propagation and ordering rules of various memory
> +operations, which gives developers the ability to reason about concurrent code.
> +Ultimately this allows to determine the possible executions of concurrent code,
> +and if that code is free from data-races.
> +
> +KCSAN is aware of *atomic* accesses (``READ_ONCE``, ``WRITE_ONCE``,
> +``atomic_*``, etc.), but is oblivious of any ordering guarantees. In other
> +words, KCSAN assumes that as long as a plain access is not observed to race
> +with another conflicting access, memory operations are correctly ordered.
> +
> +This means that KCSAN will not report *potential* data-races due to missing
> +memory ordering. If, however, missing memory ordering (that is observable with
> +a particular compiler and architecture) leads to an observable data-race (e.g.
> +entering a critical section erroneously), KCSAN would report the resulting
> +data-race.
> +
> +Implementation Details
> +----------------------
> +
> +The general approach is inspired by `DataCollider
> +<http://usenix.org/legacy/events/osdi10/tech/full_papers/Erickson.pdf>`_.
> +Unlike DataCollider, KCSAN does not use hardware watchpoints, but instead
> +relies on compiler instrumentation. Watchpoints are implemented using an
> +efficient encoding that stores access type, size, and address in a long; the
> +benefits of using "soft watchpoints" are portability and greater flexibility in
> +limiting which accesses trigger a watchpoint.
> +
> +More specifically, KCSAN requires instrumenting plain (unmarked, non-atomic)
> +memory operations; for each instrumented plain access:
> +
> +1. Check if a matching watchpoint exists; if yes, and at least one access is a
> + write, then we encountered a racing access.
> +
> +2. Periodically, if no matching watchpoint exists, set up a watchpoint and
> + stall some delay.
> +
> +3. Also check the data value before the delay, and re-check the data value
> + after delay; if the values mismatch, we infer a race of unknown origin.
> +
> +To detect data-races between plain and atomic memory operations, KCSAN also
> +annotates atomic accesses, but only to check if a watchpoint exists
> +(``kcsan_check_atomic_*``); i.e. KCSAN never sets up a watchpoint on atomic
> +accesses.
> +
> +Key Properties
> +~~~~~~~~~~~~~~
> +
> +1. **Memory Overhead:** No shadow memory is required. The current
> + implementation uses a small array of longs to encode watchpoint information,
> + which is negligible.
> +
> +2. **Performance Overhead:** KCSAN's runtime aims to be minimal, using an
> + efficient watchpoint encoding that does not require acquiring any shared
> + locks in the fast-path. For kernel boot with a default config on a system
> + where nproc=8 we measure a slow-down of 10-15x.
> +
> +3. **Memory Ordering:** KCSAN is *not* aware of the LKMM's ordering rules. This
> + may result in missed data-races (false negatives), compared to a
> + happens-before data-race detector.
> +
> +4. **Accuracy:** Imprecise, since it uses a sampling strategy.
> +
> +5. **Annotation Overheads:** Minimal annotation is required outside the KCSAN
> + runtime. With a happens-before data-race detector, any omission leads to
> + false positives, which is especially important in the context of the kernel
> + which includes numerous custom synchronization mechanisms. With KCSAN, as a
> + result, maintenance overheads are minimal as the kernel evolves.
> +
> +6. **Detects Racy Writes from Devices:** Due to checking data values upon
> + setting up watchpoints, racy writes from devices can also be detected.
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 0154674cbad3..71f7fb625490 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -8847,6 +8847,17 @@ F: Documentation/kbuild/kconfig*
> F: scripts/kconfig/
> F: scripts/Kconfig.include
>
> +KCSAN
> +M: Marco Elver <elver@...gle.com>
> +R: Dmitry Vyukov <dvyukov@...gle.com>
> +L: kasan-dev@...glegroups.com
> +S: Maintained
> +F: Documentation/dev-tools/kcsan.rst
> +F: include/linux/kcsan*.h
> +F: kernel/kcsan/
> +F: lib/Kconfig.kcsan
> +F: scripts/Makefile.kcsan
> +
> KDUMP
> M: Dave Young <dyoung@...hat.com>
> M: Baoquan He <bhe@...hat.com>
> diff --git a/Makefile b/Makefile
> index ffd7a912fc46..ad4729176252 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -478,7 +478,7 @@ export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS LDFLAGS_MODULE
>
> export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS KBUILD_LDFLAGS
> export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE
> -export CFLAGS_KASAN CFLAGS_KASAN_NOSANITIZE CFLAGS_UBSAN
> +export CFLAGS_KASAN CFLAGS_KASAN_NOSANITIZE CFLAGS_UBSAN CFLAGS_KCSAN
> export KBUILD_AFLAGS AFLAGS_KERNEL AFLAGS_MODULE
> export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE KBUILD_LDFLAGS_MODULE
> export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL
> @@ -900,6 +900,7 @@ endif
> include scripts/Makefile.kasan
> include scripts/Makefile.extrawarn
> include scripts/Makefile.ubsan
> +include scripts/Makefile.kcsan
>
> # Add user supplied CPPFLAGS, AFLAGS and CFLAGS as the last assignments
> KBUILD_CPPFLAGS += $(KCPPFLAGS)
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index 333a6695a918..a213eb55e725 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -24,6 +24,15 @@
> #define __no_sanitize_address
> #endif
>
> +#if __has_feature(thread_sanitizer)
> +/* emulate gcc's __SANITIZE_THREAD__ flag */
> +#define __SANITIZE_THREAD__
> +#define __no_sanitize_thread \
> + __attribute__((no_sanitize("thread")))
> +#else
> +#define __no_sanitize_thread
> +#endif
> +
> /*
> * Not all versions of clang implement the the type-generic versions
> * of the builtin overflow checkers. Fortunately, clang implements
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index d7ee4c6bad48..de105ca29282 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -145,6 +145,13 @@
> #define __no_sanitize_address
> #endif
>
> +#if __has_attribute(__no_sanitize_thread__) && defined(__SANITIZE_THREAD__)
> +#define __no_sanitize_thread \
> + __attribute__((__noinline__)) __attribute__((no_sanitize_thread))
> +#else
> +#define __no_sanitize_thread
> +#endif
> +
> #if GCC_VERSION >= 50100
> #define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1
> #endif
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 5e88e7e33abe..350d80dbee4d 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -178,6 +178,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
> #endif
>
> #include <uapi/linux/types.h>
> +#include <linux/kcsan-checks.h>
>
> #define __READ_ONCE_SIZE \
> ({ \
> @@ -193,12 +194,6 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
> } \
> })
>
> -static __always_inline
> -void __read_once_size(const volatile void *p, void *res, int size)
> -{
> - __READ_ONCE_SIZE;
> -}
> -
> #ifdef CONFIG_KASAN
> /*
> * We can't declare function 'inline' because __no_sanitize_address confilcts
> @@ -211,14 +206,38 @@ void __read_once_size(const volatile void *p, void *res, int size)
> # define __no_kasan_or_inline __always_inline
> #endif
>
> -static __no_kasan_or_inline
> +#ifdef CONFIG_KCSAN
> +# define __no_kcsan_or_inline __no_sanitize_thread notrace __maybe_unused
> +#else
> +# define __no_kcsan_or_inline __always_inline
> +#endif
> +
> +#if defined(CONFIG_KASAN) || defined(CONFIG_KCSAN)
> +/* Avoid any instrumentation or inline. */
> +#define __no_sanitize_or_inline \
> + __no_sanitize_address __no_sanitize_thread notrace __maybe_unused
> +#else
> +#define __no_sanitize_or_inline __always_inline
> +#endif
> +
> +static __no_kcsan_or_inline
> +void __read_once_size(const volatile void *p, void *res, int size)
> +{
> + kcsan_check_atomic_read((const void *)p, size);
> + __READ_ONCE_SIZE;
> +}
> +
> +static __no_sanitize_or_inline
> void __read_once_size_nocheck(const volatile void *p, void *res, int size)
> {
> __READ_ONCE_SIZE;
> }
>
> -static __always_inline void __write_once_size(volatile void *p, void *res, int size)
> +static __no_kcsan_or_inline
> +void __write_once_size(volatile void *p, void *res, int size)
> {
> + kcsan_check_atomic_write((const void *)p, size);
> +
> switch (size) {
> case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
> case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
> diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
> new file mode 100644
> index 000000000000..4203603ae852
> --- /dev/null
> +++ b/include/linux/kcsan-checks.h
> @@ -0,0 +1,147 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _LINUX_KCSAN_CHECKS_H
> +#define _LINUX_KCSAN_CHECKS_H
> +
> +#include <linux/types.h>
> +
> +/*
> + * __kcsan_*: Always available when KCSAN is enabled. This may be used
> + * even in compilation units that selectively disable KCSAN, but must use KCSAN
> + * to validate access to an address. Never use these in header files!
> + */
> +#ifdef CONFIG_KCSAN
> +/**
> + * __kcsan_check_watchpoint - check if a watchpoint exists
> + *
> + * Returns true if no race was detected, and we may then proceed to set up a
> + * watchpoint after. Returns false if either KCSAN is disabled or a race was
> + * encountered, and we may not set up a watchpoint after.
> + *
> + * @ptr address of access
> + * @size size of access
> + * @is_write is access a write
> + * @return true if no race was detected, false otherwise.
> + */
> +bool __kcsan_check_watchpoint(const volatile void *ptr, size_t size,
> + bool is_write);
> +
> +/**
> + * __kcsan_setup_watchpoint - set up watchpoint and report data-races
> + *
> + * Sets up a watchpoint (if sampled), and if a racing access was observed,
> + * reports the data-race.
> + *
> + * @ptr address of access
> + * @size size of access
> + * @is_write is access a write
> + */
> +void __kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
> + bool is_write);
> +#else
> +static inline bool __kcsan_check_watchpoint(const volatile void *ptr,
> + size_t size, bool is_write)
> +{
> + return true;
> +}
> +static inline void __kcsan_setup_watchpoint(const volatile void *ptr,
> + size_t size, bool is_write)
> +{
> +}
> +#endif
> +
> +/*
> + * kcsan_*: Only available when the particular compilation unit has KCSAN
> + * instrumentation enabled. May be used in header files.
> + */
> +#ifdef __SANITIZE_THREAD__
> +#define kcsan_check_watchpoint __kcsan_check_watchpoint
> +#define kcsan_setup_watchpoint __kcsan_setup_watchpoint
> +#else
> +static inline bool kcsan_check_watchpoint(const volatile void *ptr, size_t size,
> + bool is_write)
> +{
> + return true;
> +}
> +static inline void kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
> + bool is_write)
> +{
> +}
> +#endif
> +
> +/**
> + * __kcsan_check_read - check regular read access for data-races
> + *
> + * Full read access that checks watchpoint and sets up a watchpoint if this
> + * access is sampled. Note that, setting up watchpoints for plain reads is
> + * required to also detect data-races with atomic accesses.
> + *
> + * @ptr address of access
> + * @size size of access
> + */
> +#define __kcsan_check_read(ptr, size) \
> + do { \
> + if (__kcsan_check_watchpoint(ptr, size, false)) \
> + __kcsan_setup_watchpoint(ptr, size, false); \
> + } while (0)
> +
> +/**
> + * __kcsan_check_write - check regular write access for data-races
> + *
> + * Full write access that checks watchpoint and sets up a watchpoint if this
> + * access is sampled.
> + *
> + * @ptr address of access
> + * @size size of access
> + */
> +#define __kcsan_check_write(ptr, size) \
> + do { \
> + if (__kcsan_check_watchpoint(ptr, size, true) && \
> + !IS_ENABLED(CONFIG_KCSAN_PLAIN_WRITE_PRETEND_ONCE)) \
> + __kcsan_setup_watchpoint(ptr, size, true); \
> + } while (0)
> +
> +/**
> + * kcsan_check_read - check regular read access for data-races
> + *
> + * @ptr address of access
> + * @size size of access
> + */
> +#define kcsan_check_read(ptr, size) \
> + do { \
> + if (kcsan_check_watchpoint(ptr, size, false)) \
> + kcsan_setup_watchpoint(ptr, size, false); \
> + } while (0)
> +
> +/**
> + * kcsan_check_write - check regular write access for data-races
> + *
> + * @ptr address of access
> + * @size size of access
> + */
> +#define kcsan_check_write(ptr, size) \
> + do { \
> + if (kcsan_check_watchpoint(ptr, size, true) && \
> + !IS_ENABLED(CONFIG_KCSAN_PLAIN_WRITE_PRETEND_ONCE)) \
> + kcsan_setup_watchpoint(ptr, size, true); \
> + } while (0)
> +
> +/*
> + * Check for atomic accesses: if atomic access are not ignored, this simply
> + * aliases to kcsan_check_watchpoint, otherwise becomes a no-op.
> + */
> +#ifdef CONFIG_KCSAN_IGNORE_ATOMICS
> +#define kcsan_check_atomic_read(...) \
> + do { \
> + } while (0)
> +#define kcsan_check_atomic_write(...) \
> + do { \
> + } while (0)
> +#else
> +#define kcsan_check_atomic_read(ptr, size) \
> + kcsan_check_watchpoint(ptr, size, false)
> +#define kcsan_check_atomic_write(ptr, size) \
> + kcsan_check_watchpoint(ptr, size, true)
> +#endif
> +
> +#endif /* _LINUX_KCSAN_CHECKS_H */
> diff --git a/include/linux/kcsan.h b/include/linux/kcsan.h
> new file mode 100644
> index 000000000000..fd5de2ba3a16
> --- /dev/null
> +++ b/include/linux/kcsan.h
> @@ -0,0 +1,108 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _LINUX_KCSAN_H
> +#define _LINUX_KCSAN_H
> +
> +#include <linux/types.h>
> +#include <linux/kcsan-checks.h>
> +
> +#ifdef CONFIG_KCSAN
> +
> +/*
> + * Context for each thread of execution: for tasks, this is stored in
> + * task_struct, and interrupts access internal per-CPU storage.
> + */
> +struct kcsan_ctx {
> + int disable; /* disable counter */
> + int atomic_next; /* number of following atomic ops */
> +
> + /*
> + * We use separate variables to store if we are in a nestable or flat
> + * atomic region. This helps make sure that an atomic region with
> + * nesting support is not suddenly aborted when a flat region is
> + * contained within. Effectively this allows supporting nesting flat
> + * atomic regions within an outer nestable atomic region. Support for
> + * this is required as there are cases where a seqlock reader critical
> + * section (flat atomic region) is contained within a seqlock writer
> + * critical section (nestable atomic region), and the "mismatching
> + * kcsan_end_atomic()" warning would trigger otherwise.
> + */
> + int atomic_region;
> + bool atomic_region_flat;
> +};
> +
> +/**
> + * kcsan_init - initialize KCSAN runtime
> + */
> +void kcsan_init(void);
> +
> +/**
> + * kcsan_disable_current - disable KCSAN for the current context
> + *
> + * Supports nesting.
> + */
> +void kcsan_disable_current(void);
> +
> +/**
> + * kcsan_enable_current - re-enable KCSAN for the current context
> + *
> + * Supports nesting.
> + */
> +void kcsan_enable_current(void);
> +
> +/**
> + * kcsan_begin_atomic - use to denote an atomic region
> + *
> + * Accesses within the atomic region may appear to race with other accesses but
> + * should be considered atomic.
> + *
> + * @nest true if regions may be nested, or false for flat region
> + */
> +void kcsan_begin_atomic(bool nest);
> +
> +/**
> + * kcsan_end_atomic - end atomic region
> + *
> + * @nest must match argument to kcsan_begin_atomic().
> + */
> +void kcsan_end_atomic(bool nest);
> +
> +/**
> + * kcsan_atomic_next - consider following accesses as atomic
> + *
> + * Force treating the next n memory accesses for the current context as atomic
> + * operations.
> + *
> + * @n number of following memory accesses to treat as atomic.
> + */
> +void kcsan_atomic_next(int n);
> +
> +#else /* CONFIG_KCSAN */
> +
> +static inline void kcsan_init(void)
> +{
> +}
> +
> +static inline void kcsan_disable_current(void)
> +{
> +}
> +
> +static inline void kcsan_enable_current(void)
> +{
> +}
> +
> +static inline void kcsan_begin_atomic(bool nest)
> +{
> +}
> +
> +static inline void kcsan_end_atomic(bool nest)
> +{
> +}
> +
> +static inline void kcsan_atomic_next(int n)
> +{
> +}
> +
> +#endif /* CONFIG_KCSAN */
> +
> +#endif /* _LINUX_KCSAN_H */
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 2c2e56bd8913..9490e417bf4a 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -31,6 +31,7 @@
> #include <linux/task_io_accounting.h>
> #include <linux/posix-timers.h>
> #include <linux/rseq.h>
> +#include <linux/kcsan.h>
>
> /* task_struct member predeclarations (sorted alphabetically): */
> struct audit_context;
> @@ -1171,6 +1172,9 @@ struct task_struct {
> #ifdef CONFIG_KASAN
> unsigned int kasan_depth;
> #endif
> +#ifdef CONFIG_KCSAN
> + struct kcsan_ctx kcsan_ctx;
> +#endif
>
> #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> /* Index of current stored address in ret_stack: */
> diff --git a/init/init_task.c b/init/init_task.c
> index 9e5cbe5eab7b..e229416c3314 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -161,6 +161,14 @@ struct task_struct init_task
> #ifdef CONFIG_KASAN
> .kasan_depth = 1,
> #endif
> +#ifdef CONFIG_KCSAN
> + .kcsan_ctx = {
> + .disable = 1,
> + .atomic_next = 0,
> + .atomic_region = 0,
> + .atomic_region_flat = 0,
> + },
> +#endif
> #ifdef CONFIG_TRACE_IRQFLAGS
> .softirqs_enabled = 1,
> #endif
> diff --git a/init/main.c b/init/main.c
> index 91f6ebb30ef0..4d814de017ee 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -93,6 +93,7 @@
> #include <linux/rodata_test.h>
> #include <linux/jump_label.h>
> #include <linux/mem_encrypt.h>
> +#include <linux/kcsan.h>
>
> #include <asm/io.h>
> #include <asm/bugs.h>
> @@ -779,6 +780,7 @@ asmlinkage __visible void __init start_kernel(void)
> acpi_subsystem_init();
> arch_post_acpi_subsys_init();
> sfi_init_late();
> + kcsan_init();
>
> /* Do the rest non-__init'ed, we're now alive */
> arch_call_rest_init();
> diff --git a/kernel/Makefile b/kernel/Makefile
> index daad787fb795..74ab46e2ebd1 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -102,6 +102,7 @@ obj-$(CONFIG_TRACEPOINTS) += trace/
> obj-$(CONFIG_IRQ_WORK) += irq_work.o
> obj-$(CONFIG_CPU_PM) += cpu_pm.o
> obj-$(CONFIG_BPF) += bpf/
> +obj-$(CONFIG_KCSAN) += kcsan/
>
> obj-$(CONFIG_PERF_EVENTS) += events/
>
> diff --git a/kernel/kcsan/Makefile b/kernel/kcsan/Makefile
> new file mode 100644
> index 000000000000..c25f07062d26
> --- /dev/null
> +++ b/kernel/kcsan/Makefile
> @@ -0,0 +1,14 @@
> +# SPDX-License-Identifier: GPL-2.0
> +KCSAN_SANITIZE := n
> +KCOV_INSTRUMENT := n
> +
> +CFLAGS_REMOVE_kcsan.o = $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_core.o = $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_atomic.o = $(CC_FLAGS_FTRACE)
> +
> +CFLAGS_kcsan.o = $(call cc-option, -fno-conserve-stack -fno-stack-protector)
> +CFLAGS_core.o = $(call cc-option, -fno-conserve-stack -fno-stack-protector)
> +CFLAGS_atomic.o = $(call cc-option, -fno-conserve-stack -fno-stack-protector)
Building with clang 10, I still see:
CC kernel/kcsan/core.o
kernel/kcsan/core.o: warning: objtool:
__kcsan_check_watchpoint()+0x228: call to __stack_chk_fail() with
UACCESS enabled
kernel/kcsan/core.o: warning: objtool:
__kcsan_setup_watchpoint()+0x3be: call to __stack_chk_fail() with
UACCESS enabled
> +obj-y := kcsan.o core.o atomic.o debugfs.o report.o
> +obj-$(CONFIG_KCSAN_SELFTEST) += test.o
> diff --git a/kernel/kcsan/atomic.c b/kernel/kcsan/atomic.c
> new file mode 100644
> index 000000000000..dd44f7d9e491
> --- /dev/null
> +++ b/kernel/kcsan/atomic.c
> @@ -0,0 +1,21 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/jiffies.h>
> +
> +#include "kcsan.h"
> +
> +/*
> + * List all volatile globals that have been observed in races, to suppress
> + * data-race reports between accesses to these variables.
> + *
> + * For now, we assume that volatile accesses of globals are as strong as atomic
> + * accesses (READ_ONCE, WRITE_ONCE cast to volatile). The situation is still not
> + * entirely clear, as on some architectures (Alpha) READ_ONCE/WRITE_ONCE do more
> + * than cast to volatile. Eventually, we hope to be able to remove this
> + * function.
> + */
> +bool kcsan_is_atomic(const volatile void *ptr)
> +{
> + /* only jiffies for now */
> + return ptr == &jiffies;
> +}
> diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
> new file mode 100644
> index 000000000000..bc8d60b129eb
> --- /dev/null
> +++ b/kernel/kcsan/core.c
> @@ -0,0 +1,428 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/atomic.h>
> +#include <linux/bug.h>
> +#include <linux/delay.h>
> +#include <linux/export.h>
> +#include <linux/init.h>
> +#include <linux/percpu.h>
> +#include <linux/preempt.h>
> +#include <linux/random.h>
> +#include <linux/sched.h>
> +#include <linux/uaccess.h>
> +
> +#include "kcsan.h"
> +#include "encoding.h"
> +
> +/*
> + * Helper macros to iterate slots, starting from address slot itself, followed
> + * by the right and left slots.
> + */
> +#define CHECK_NUM_SLOTS (1 + 2 * KCSAN_CHECK_ADJACENT)
> +#define SLOT_IDX(slot, i) \
> + ((slot + (((i + KCSAN_CHECK_ADJACENT) % CHECK_NUM_SLOTS) - \
> + KCSAN_CHECK_ADJACENT)) % \
> + KCSAN_NUM_WATCHPOINTS)
> +
> +bool kcsan_enabled;
> +
> +/* Per-CPU kcsan_ctx for interrupts */
> +static DEFINE_PER_CPU(struct kcsan_ctx, kcsan_cpu_ctx) = {
> + .disable = 0,
> + .atomic_next = 0,
> + .atomic_region = 0,
> + .atomic_region_flat = 0,
> +};
> +
> +/*
> + * Watchpoints, with each entry encoded as defined in encoding.h: in order to be
> + * able to safely update and access a watchpoint without introducing locking
> + * overhead, we encode each watchpoint as a single atomic long. The initial
> + * zero-initialized state matches INVALID_WATCHPOINT.
> + */
> +static atomic_long_t watchpoints[KCSAN_NUM_WATCHPOINTS];
> +
> +/*
> + * Instructions skipped counter; see should_watch().
> + */
> +static DEFINE_PER_CPU(unsigned long, kcsan_skip);
> +
> +static inline atomic_long_t *find_watchpoint(unsigned long addr, size_t size,
> + bool expect_write,
> + long *encoded_watchpoint)
> +{
> + const int slot = watchpoint_slot(addr);
> + const unsigned long addr_masked = addr & WATCHPOINT_ADDR_MASK;
> + atomic_long_t *watchpoint;
> + unsigned long wp_addr_masked;
> + size_t wp_size;
> + bool is_write;
> + int i;
> +
> + for (i = 0; i < CHECK_NUM_SLOTS; ++i) {
> + watchpoint = &watchpoints[SLOT_IDX(slot, i)];
> + *encoded_watchpoint = atomic_long_read(watchpoint);
> + if (!decode_watchpoint(*encoded_watchpoint, &wp_addr_masked,
> + &wp_size, &is_write))
> + continue;
> +
> + if (expect_write && !is_write)
> + continue;
> +
> + /* Check if the watchpoint matches the access. */
> + if (matching_access(wp_addr_masked, wp_size, addr_masked, size))
> + return watchpoint;
> + }
> +
> + return NULL;
> +}
> +
> +static inline atomic_long_t *insert_watchpoint(unsigned long addr, size_t size,
> + bool is_write)
> +{
> + const int slot = watchpoint_slot(addr);
> + const long encoded_watchpoint = encode_watchpoint(addr, size, is_write);
> + atomic_long_t *watchpoint;
> + int i;
> +
> + for (i = 0; i < CHECK_NUM_SLOTS; ++i) {
> + long expect_val = INVALID_WATCHPOINT;
> +
> + /* Try to acquire this slot. */
> + watchpoint = &watchpoints[SLOT_IDX(slot, i)];
> + if (atomic_long_try_cmpxchg_relaxed(watchpoint, &expect_val,
> + encoded_watchpoint))
> + return watchpoint;
> + }
> +
> + return NULL;
> +}
> +
> +/*
> + * Return true if watchpoint was successfully consumed, false otherwise.
> + *
> + * This may return false if:
> + *
> + * 1. another thread already consumed the watchpoint;
> + * 2. the thread that set up the watchpoint already removed it;
> + * 3. the watchpoint was removed and then re-used.
> + */
> +static inline bool try_consume_watchpoint(atomic_long_t *watchpoint,
> + long encoded_watchpoint)
> +{
> + return atomic_long_try_cmpxchg_relaxed(watchpoint, &encoded_watchpoint,
> + CONSUMED_WATCHPOINT);
> +}
> +
> +/*
> + * Return true if watchpoint was not touched, false if consumed.
> + */
> +static inline bool remove_watchpoint(atomic_long_t *watchpoint)
> +{
> + return atomic_long_xchg_relaxed(watchpoint, INVALID_WATCHPOINT) !=
> + CONSUMED_WATCHPOINT;
> +}
> +
> +static inline struct kcsan_ctx *get_ctx(void)
> +{
> + /*
> + * In interrupt, use raw_cpu_ptr to avoid unnecessary checks, that would
> + * also result in calls that generate warnings in uaccess regions.
> + */
> + return in_task() ? ¤t->kcsan_ctx : raw_cpu_ptr(&kcsan_cpu_ctx);
> +}
> +
> +static inline bool is_atomic(const volatile void *ptr)
> +{
> + struct kcsan_ctx *ctx = get_ctx();
> +
> + if (unlikely(ctx->atomic_next > 0)) {
> + --ctx->atomic_next;
> + return true;
> + }
> + if (unlikely(ctx->atomic_region > 0 || ctx->atomic_region_flat))
> + return true;
> +
> + return kcsan_is_atomic(ptr);
> +}
> +
> +static inline bool should_watch(const volatile void *ptr)
> +{
> + /*
> + * Never set up watchpoints when memory operations are atomic.
> + *
> + * We need to check this first, because: 1) atomics should not count
> + * towards skipped instructions below, and 2) to actually decrement
> + * kcsan_atomic_next for each atomic.
> + */
> + if (is_atomic(ptr))
> + return false;
> +
> + /*
> + * We use a per-CPU counter, to avoid excessive contention; there is
> + * still enough non-determinism for the precise instructions that end up
> + * being watched to be mostly unpredictable. Using a PRNG like
> + * prandom_u32() turned out to be too slow.
> + */
> + return (this_cpu_inc_return(kcsan_skip) %
> + CONFIG_KCSAN_WATCH_SKIP_INST) == 0;
> +}
> +
> +static inline bool is_enabled(void)
> +{
> + return READ_ONCE(kcsan_enabled) && get_ctx()->disable == 0;
> +}
> +
> +static inline unsigned int get_delay(void)
> +{
> + unsigned int max_delay = in_task() ? CONFIG_KCSAN_UDELAY_MAX_TASK :
> + CONFIG_KCSAN_UDELAY_MAX_INTERRUPT;
> + return IS_ENABLED(CONFIG_KCSAN_DELAY_RANDOMIZE) ?
> + ((prandom_u32() % max_delay) + 1) :
> + max_delay;
> +}
> +
> +/* === Public interface ===================================================== */
> +
> +void __init kcsan_init(void)
> +{
> + BUG_ON(!in_task());
> +
> + kcsan_debugfs_init();
> + kcsan_enable_current();
> +#ifdef CONFIG_KCSAN_EARLY_ENABLE
> + /*
> + * We are in the init task, and no other tasks should be running.
> + */
> + WRITE_ONCE(kcsan_enabled, true);
> +#endif
> +}
> +
> +/* === Exported interface =================================================== */
> +
> +void kcsan_disable_current(void)
> +{
> + ++get_ctx()->disable;
> +}
> +EXPORT_SYMBOL(kcsan_disable_current);
> +
> +void kcsan_enable_current(void)
> +{
> + if (get_ctx()->disable-- == 0) {
> + kcsan_disable_current(); /* restore to 0 */
> + kcsan_disable_current();
> + WARN(1, "mismatching %s", __func__);
> + kcsan_enable_current();
> + }
> +}
> +EXPORT_SYMBOL(kcsan_enable_current);
> +
> +void kcsan_begin_atomic(bool nest)
> +{
> + if (nest)
> + ++get_ctx()->atomic_region;
> + else
If it's flat, shoudn't we do WARN_ON(get_ctx()->atomic_region_flat)?
> + get_ctx()->atomic_region_flat = true;
> +}
> +EXPORT_SYMBOL(kcsan_begin_atomic);
> +
> +void kcsan_end_atomic(bool nest)
> +{
> + if (nest) {
> + if (get_ctx()->atomic_region-- == 0) {
> + kcsan_begin_atomic(true); /* restore to 0 */
> + kcsan_disable_current();
> + WARN(1, "mismatching %s", __func__);
> + kcsan_enable_current();
> + }
> + } else {
WARN_ON(!get_ctx()->atomic_region_flat)?
> + get_ctx()->atomic_region_flat = false;
> + }
> +}
> +EXPORT_SYMBOL(kcsan_end_atomic);
> +
> +void kcsan_atomic_next(int n)
> +{
> + get_ctx()->atomic_next = n;
> +}
> +EXPORT_SYMBOL(kcsan_atomic_next);
> +
> +bool __kcsan_check_watchpoint(const volatile void *ptr, size_t size,
> + bool is_write)
> +{
> + atomic_long_t *watchpoint;
> + long encoded_watchpoint;
> + unsigned long flags;
> + enum kcsan_report_type report_type;
> +
> + if (unlikely(!is_enabled()))
> + return false;
> +
> + /*
> + * Avoid user_access_save in fast-path here: find_watchpoint is safe
> + * without user_access_save, as the address that ptr points to is only
> + * used to check if a watchpoint exists; ptr is never dereferenced.
> + */
> + watchpoint = find_watchpoint((unsigned long)ptr, size, !is_write,
> + &encoded_watchpoint);
> + if (watchpoint == NULL)
> + return true;
> +
> + flags = user_access_save();
> + if (!try_consume_watchpoint(watchpoint, encoded_watchpoint)) {
> + /*
> + * The other thread may not print any diagnostics, as it has
> + * already removed the watchpoint, or another thread consumed
> + * the watchpoint before this thread.
> + */
> + kcsan_counter_inc(kcsan_counter_report_races);
> + report_type = kcsan_report_race_check_race;
> + } else {
> + report_type = kcsan_report_race_check;
> + }
> +
> + /* Encountered a data-race. */
> + kcsan_counter_inc(kcsan_counter_data_races);
> + kcsan_report(ptr, size, is_write, raw_smp_processor_id(), report_type);
> +
> + user_access_restore(flags);
> + return false;
> +}
> +EXPORT_SYMBOL(__kcsan_check_watchpoint);
> +
> +void __kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
> + bool is_write)
> +{
> + atomic_long_t *watchpoint;
> + union {
> + u8 _1;
> + u16 _2;
> + u32 _4;
> + u64 _8;
> + } expect_value;
> + bool is_expected = true;
> + unsigned long ua_flags = user_access_save();
> + unsigned long irq_flags;
> +
> + if (!should_watch(ptr))
> + goto out;
> +
> + if (!check_encodable((unsigned long)ptr, size)) {
> + kcsan_counter_inc(kcsan_counter_unencodable_accesses);
> + goto out;
> + }
> +
> + /*
> + * Disable interrupts & preemptions to avoid another thread on the same
> + * CPU accessing memory locations for the set up watchpoint; this is to
> + * avoid reporting races to e.g. CPU-local data.
> + *
> + * An alternative would be adding the source CPU to the watchpoint
> + * encoding, and checking that watchpoint-CPU != this-CPU. There are
> + * several problems with this:
> + * 1. we should avoid stealing more bits from the watchpoint encoding
> + * as it would affect accuracy, as well as increase performance
> + * overhead in the fast-path;
> + * 2. if we are preempted, but there *is* a genuine data-race, we
> + * would *not* report it -- since this is the common case (vs.
> + * CPU-local data accesses), it makes more sense (from a data-race
> + * detection PoV) to simply disable preemptions to ensure as many
> + * tasks as possible run on other CPUs.
> + */
> + local_irq_save(irq_flags);
> +
> + watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
> + if (watchpoint == NULL) {
> + /*
> + * Out of capacity: the size of `watchpoints`, and the frequency
> + * with which `should_watch()` returns true should be tweaked so
> + * that this case happens very rarely.
> + */
> + kcsan_counter_inc(kcsan_counter_no_capacity);
> + goto out_unlock;
> + }
> +
> + kcsan_counter_inc(kcsan_counter_setup_watchpoints);
> + kcsan_counter_inc(kcsan_counter_used_watchpoints);
> +
> + /*
> + * Read the current value, to later check and infer a race if the data
> + * was modified via a non-instrumented access, e.g. from a device.
> + */
> + switch (size) {
> + case 1:
> + expect_value._1 = READ_ONCE(*(const u8 *)ptr);
> + break;
> + case 2:
> + expect_value._2 = READ_ONCE(*(const u16 *)ptr);
> + break;
> + case 4:
> + expect_value._4 = READ_ONCE(*(const u32 *)ptr);
> + break;
> + case 8:
> + expect_value._8 = READ_ONCE(*(const u64 *)ptr);
> + break;
> + default:
> + break; /* ignore; we do not diff the values */
> + }
> +
> +#ifdef CONFIG_KCSAN_DEBUG
> + kcsan_disable_current();
> + pr_err("KCSAN: watching %s, size: %zu, addr: %px [slot: %d, encoded: %lx]\n",
> + is_write ? "write" : "read", size, ptr,
> + watchpoint_slot((unsigned long)ptr),
> + encode_watchpoint((unsigned long)ptr, size, is_write));
> + kcsan_enable_current();
> +#endif
> +
> + /*
> + * Delay this thread, to increase probability of observing a racy
> + * conflicting access.
> + */
> + udelay(get_delay());
> +
> + /*
> + * Re-read value, and check if it is as expected; if not, we infer a
> + * racy access.
> + */
> + switch (size) {
> + case 1:
> + is_expected = expect_value._1 == READ_ONCE(*(const u8 *)ptr);
> + break;
> + case 2:
> + is_expected = expect_value._2 == READ_ONCE(*(const u16 *)ptr);
> + break;
> + case 4:
> + is_expected = expect_value._4 == READ_ONCE(*(const u32 *)ptr);
> + break;
> + case 8:
> + is_expected = expect_value._8 == READ_ONCE(*(const u64 *)ptr);
> + break;
> + default:
> + break; /* ignore; we do not diff the values */
> + }
> +
> + /* Check if this access raced with another. */
> + if (!remove_watchpoint(watchpoint)) {
> + /*
> + * No need to increment 'race' counter, as the racing thread
> + * already did.
> + */
> + kcsan_report(ptr, size, is_write, smp_processor_id(),
> + kcsan_report_race_setup);
> + } else if (!is_expected) {
> + /* Inferring a race, since the value should not have changed. */
> + kcsan_counter_inc(kcsan_counter_races_unknown_origin);
> +#ifdef CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN
> + kcsan_report(ptr, size, is_write, smp_processor_id(),
> + kcsan_report_race_unknown_origin);
> +#endif
> + }
> +
> + kcsan_counter_dec(kcsan_counter_used_watchpoints);
> +out_unlock:
> + local_irq_restore(irq_flags);
> +out:
> + user_access_restore(ua_flags);
> +}
> +EXPORT_SYMBOL(__kcsan_setup_watchpoint);
> diff --git a/kernel/kcsan/debugfs.c b/kernel/kcsan/debugfs.c
> new file mode 100644
> index 000000000000..6ddcbd185f3a
> --- /dev/null
> +++ b/kernel/kcsan/debugfs.c
> @@ -0,0 +1,225 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/atomic.h>
> +#include <linux/bsearch.h>
> +#include <linux/bug.h>
> +#include <linux/debugfs.h>
> +#include <linux/init.h>
> +#include <linux/kallsyms.h>
> +#include <linux/mm.h>
> +#include <linux/seq_file.h>
> +#include <linux/sort.h>
> +#include <linux/string.h>
> +#include <linux/uaccess.h>
> +
> +#include "kcsan.h"
> +
> +/*
> + * Statistics counters.
> + */
> +static atomic_long_t counters[kcsan_counter_count];
> +
> +/*
> + * Addresses for filtering functions from reporting. This list can be used as a
> + * whitelist or blacklist.
> + */
> +static struct {
> + unsigned long *addrs; /* array of addresses */
> + size_t size; /* current size */
> + int used; /* number of elements used */
> + bool sorted; /* if elements are sorted */
> + bool whitelist; /* if list is a blacklist or whitelist */
> +} report_filterlist = {
> + .addrs = NULL,
> + .size = 8, /* small initial size */
> + .used = 0,
> + .sorted = false,
> + .whitelist = false, /* default is blacklist */
> +};
> +static DEFINE_SPINLOCK(report_filterlist_lock);
> +
> +static const char *counter_to_name(enum kcsan_counter_id id)
> +{
> + switch (id) {
> + case kcsan_counter_used_watchpoints:
> + return "used_watchpoints";
> + case kcsan_counter_setup_watchpoints:
> + return "setup_watchpoints";
> + case kcsan_counter_data_races:
> + return "data_races";
> + case kcsan_counter_no_capacity:
> + return "no_capacity";
> + case kcsan_counter_report_races:
> + return "report_races";
> + case kcsan_counter_races_unknown_origin:
> + return "races_unknown_origin";
> + case kcsan_counter_unencodable_accesses:
> + return "unencodable_accesses";
> + case kcsan_counter_encoding_false_positives:
> + return "encoding_false_positives";
> + case kcsan_counter_count:
> + BUG();
> + }
> + return NULL;
> +}
> +
> +void kcsan_counter_inc(enum kcsan_counter_id id)
> +{
> + atomic_long_inc(&counters[id]);
> +}
> +
> +void kcsan_counter_dec(enum kcsan_counter_id id)
> +{
> + atomic_long_dec(&counters[id]);
> +}
> +
> +static int cmp_filterlist_addrs(const void *rhs, const void *lhs)
> +{
> + const unsigned long a = *(const unsigned long *)rhs;
> + const unsigned long b = *(const unsigned long *)lhs;
> +
> + return a < b ? -1 : a == b ? 0 : 1;
> +}
> +
> +bool kcsan_skip_report(unsigned long func_addr)
> +{
> + unsigned long symbolsize, offset;
> + unsigned long flags;
> + bool ret = false;
> +
> + if (!kallsyms_lookup_size_offset(func_addr, &symbolsize, &offset))
> + return false;
> + func_addr -= offset; /* get function start */
> +
> + spin_lock_irqsave(&report_filterlist_lock, flags);
> + if (report_filterlist.used == 0)
> + goto out;
> +
> + /* Sort array if it is unsorted, and then do a binary search. */
> + if (!report_filterlist.sorted) {
> + sort(report_filterlist.addrs, report_filterlist.used,
> + sizeof(unsigned long), cmp_filterlist_addrs, NULL);
> + report_filterlist.sorted = true;
> + }
> + ret = !!bsearch(&func_addr, report_filterlist.addrs,
> + report_filterlist.used, sizeof(unsigned long),
> + cmp_filterlist_addrs);
> + if (report_filterlist.whitelist)
> + ret = !ret;
> +
> +out:
> + spin_unlock_irqrestore(&report_filterlist_lock, flags);
> + return ret;
> +}
> +
> +static void set_report_filterlist_whitelist(bool whitelist)
> +{
> + unsigned long flags;
> +
> + spin_lock_irqsave(&report_filterlist_lock, flags);
> + report_filterlist.whitelist = whitelist;
> + spin_unlock_irqrestore(&report_filterlist_lock, flags);
> +}
> +
> +static void insert_report_filterlist(const char *func)
> +{
> + unsigned long flags;
> + unsigned long addr = kallsyms_lookup_name(func);
> +
> + if (!addr) {
> + pr_err("KCSAN: could not find function: '%s'\n", func);
> + return;
Would be reasonable to return ENOENT to user.
> + }
> +
> + spin_lock_irqsave(&report_filterlist_lock, flags);
> +
> + if (report_filterlist.addrs == NULL)
> + report_filterlist.addrs = /* initial allocation */
> + kvmalloc_array(report_filterlist.size,
> + sizeof(unsigned long), GFP_KERNEL);
This can fail.
> + else if (report_filterlist.used == report_filterlist.size) {
> + /* resize filterlist */
> + unsigned long *new_addrs;
> +
> + report_filterlist.size *= 2;
> + new_addrs = kvmalloc_array(report_filterlist.size,
> + sizeof(unsigned long), GFP_KERNEL);
This can fail.
Would it be easier to use krealloc? It's usefule to have a cap on list
size anyway.
> + memcpy(new_addrs, report_filterlist.addrs,
> + report_filterlist.used * sizeof(unsigned long));
> + kvfree(report_filterlist.addrs);
> + report_filterlist.addrs = new_addrs;
> + }
> +
> + /* Note: deduplicating should be done in userspace. */
> + report_filterlist.addrs[report_filterlist.used++] =
> + kallsyms_lookup_name(func);
> + report_filterlist.sorted = false;
> +
> + spin_unlock_irqrestore(&report_filterlist_lock, flags);
> +}
> +
> +static int show_info(struct seq_file *file, void *v)
> +{
> + int i;
> + unsigned long flags;
> +
> + /* show stats */
> + seq_printf(file, "enabled: %i\n", READ_ONCE(kcsan_enabled));
> + for (i = 0; i < kcsan_counter_count; ++i)
> + seq_printf(file, "%s: %ld\n", counter_to_name(i),
> + atomic_long_read(&counters[i]));
> +
> + /* show filter functions, and filter type */
> + spin_lock_irqsave(&report_filterlist_lock, flags);
> + seq_printf(file, "\n%s functions: %s\n",
> + report_filterlist.whitelist ? "whitelisted" : "blacklisted",
> + report_filterlist.used == 0 ? "none" : "");
> + for (i = 0; i < report_filterlist.used; ++i)
> + seq_printf(file, " %ps\n", (void *)report_filterlist.addrs[i]);
> + spin_unlock_irqrestore(&report_filterlist_lock, flags);
> +
> + return 0;
> +}
> +
> +static int debugfs_open(struct inode *inode, struct file *file)
> +{
> + return single_open(file, show_info, NULL);
> +}
> +
> +static ssize_t debugfs_write(struct file *file, const char __user *buf,
> + size_t count, loff_t *off)
> +{
> + char kbuf[KSYM_NAME_LEN];
> + char *arg;
> + int read_len = count < (sizeof(kbuf) - 1) ? count : (sizeof(kbuf) - 1);
> +
> + if (copy_from_user(kbuf, buf, read_len))
> + return -EINVAL;
EFAULT
> + kbuf[read_len] = '\0';
> + arg = strstrip(kbuf);
> +
> + if (!strncmp(arg, "on", sizeof("on") - 1))
I would be cleaner to use strcmp (trim trailing newline first).
Otherwise we accept anything starting with "on".
> + WRITE_ONCE(kcsan_enabled, true);
> + else if (!strncmp(arg, "off", sizeof("off") - 1))
> + WRITE_ONCE(kcsan_enabled, false);
> + else if (!strncmp(arg, "whitelist", sizeof("whitelist") - 1))
> + set_report_filterlist_whitelist(true);
> + else if (!strncmp(arg, "blacklist", sizeof("blacklist") - 1))
> + set_report_filterlist_whitelist(false);
> + else if (arg[0] == '!')
> + insert_report_filterlist(&arg[1]);
> + else
> + return -EINVAL;
> +
> + return count;
> +}
> +
> +static const struct file_operations debugfs_ops = { .read = seq_read,
> + .open = debugfs_open,
> + .write = debugfs_write,
> + .release = single_release };
> +
> +void __init kcsan_debugfs_init(void)
> +{
> + debugfs_create_file("kcsan", 0644, NULL, NULL, &debugfs_ops);
> +}
> diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
> new file mode 100644
> index 000000000000..8f9b1ce0e59f
> --- /dev/null
> +++ b/kernel/kcsan/encoding.h
> @@ -0,0 +1,94 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _MM_KCSAN_ENCODING_H
> +#define _MM_KCSAN_ENCODING_H
> +
> +#include <linux/bits.h>
> +#include <linux/log2.h>
> +#include <linux/mm.h>
> +
> +#include "kcsan.h"
> +
> +#define SLOT_RANGE PAGE_SIZE
> +#define INVALID_WATCHPOINT 0
> +#define CONSUMED_WATCHPOINT 1
> +
> +/*
> + * The maximum useful size of accesses for which we set up watchpoints is the
> + * max range of slots we check on an access.
> + */
> +#define MAX_ENCODABLE_SIZE (SLOT_RANGE * (1 + KCSAN_CHECK_ADJACENT))
> +
> +/*
> + * Number of bits we use to store size info.
> + */
> +#define WATCHPOINT_SIZE_BITS bits_per(MAX_ENCODABLE_SIZE)
> +/*
> + * This encoding for addresses discards the upper (1 for is-write + SIZE_BITS);
> + * however, most 64-bit architectures do not use the full 64-bit address space.
> + * Also, in order for a false positive to be observable 2 things need to happen:
> + *
> + * 1. different addresses but with the same encoded address race;
> + * 2. and both map onto the same watchpoint slots;
> + *
> + * Both these are assumed to be very unlikely. However, in case it still happens
> + * happens, the report logic will filter out the false positive (see report.c).
> + */
> +#define WATCHPOINT_ADDR_BITS (BITS_PER_LONG - 1 - WATCHPOINT_SIZE_BITS)
> +
> +/*
> + * Masks to set/retrieve the encoded data.
> + */
> +#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG - 1)
> +#define WATCHPOINT_SIZE_MASK \
> + GENMASK(BITS_PER_LONG - 2, BITS_PER_LONG - 2 - WATCHPOINT_SIZE_BITS)
> +#define WATCHPOINT_ADDR_MASK \
> + GENMASK(BITS_PER_LONG - 3 - WATCHPOINT_SIZE_BITS, 0)
> +
> +static inline bool check_encodable(unsigned long addr, size_t size)
> +{
> + return size <= MAX_ENCODABLE_SIZE;
> +}
> +
> +static inline long encode_watchpoint(unsigned long addr, size_t size,
> + bool is_write)
> +{
> + return (long)((is_write ? WATCHPOINT_WRITE_MASK : 0) |
> + (size << WATCHPOINT_ADDR_BITS) |
> + (addr & WATCHPOINT_ADDR_MASK));
> +}
> +
> +static inline bool decode_watchpoint(long watchpoint,
> + unsigned long *addr_masked, size_t *size,
> + bool *is_write)
> +{
> + if (watchpoint == INVALID_WATCHPOINT ||
> + watchpoint == CONSUMED_WATCHPOINT)
> + return false;
> +
> + *addr_masked = (unsigned long)watchpoint & WATCHPOINT_ADDR_MASK;
> + *size = ((unsigned long)watchpoint & WATCHPOINT_SIZE_MASK) >>
> + WATCHPOINT_ADDR_BITS;
> + *is_write = !!((unsigned long)watchpoint & WATCHPOINT_WRITE_MASK);
> +
> + return true;
> +}
> +
> +/*
> + * Return watchpoint slot for an address.
> + */
> +static inline int watchpoint_slot(unsigned long addr)
> +{
> + return (addr / PAGE_SIZE) % KCSAN_NUM_WATCHPOINTS;
> +}
> +
> +static inline bool matching_access(unsigned long addr1, size_t size1,
> + unsigned long addr2, size_t size2)
> +{
> + unsigned long end_range1 = addr1 + size1 - 1;
> + unsigned long end_range2 = addr2 + size2 - 1;
> +
> + return addr1 <= end_range2 && addr2 <= end_range1;
> +}
> +
> +#endif /* _MM_KCSAN_ENCODING_H */
> diff --git a/kernel/kcsan/kcsan.c b/kernel/kcsan/kcsan.c
> new file mode 100644
> index 000000000000..45cf2fffd8a0
> --- /dev/null
> +++ b/kernel/kcsan/kcsan.c
> @@ -0,0 +1,86 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/*
> + * The Kernel Concurrency Sanitizer (KCSAN) infrastructure. For more info please
> + * see Documentation/dev-tools/kcsan.rst.
> + */
> +
> +#include <linux/export.h>
> +
> +#include "kcsan.h"
> +
> +/*
> + * KCSAN uses the same instrumentation that is emitted by supported compilers
> + * for Thread Sanitizer (TSAN).
> + *
> + * When enabled, the compiler emits instrumentation calls (the functions
> + * prefixed with "__tsan" below) for all loads and stores that it generated;
> + * inline asm is not instrumented.
> + */
> +
> +#define DEFINE_TSAN_READ_WRITE(size) \
> + void __tsan_read##size(void *ptr) \
> + { \
> + __kcsan_check_read(ptr, size); \
> + } \
> + EXPORT_SYMBOL(__tsan_read##size); \
> + void __tsan_write##size(void *ptr) \
> + { \
> + __kcsan_check_write(ptr, size); \
> + } \
> + EXPORT_SYMBOL(__tsan_write##size)
> +
> +DEFINE_TSAN_READ_WRITE(1);
> +DEFINE_TSAN_READ_WRITE(2);
> +DEFINE_TSAN_READ_WRITE(4);
> +DEFINE_TSAN_READ_WRITE(8);
> +DEFINE_TSAN_READ_WRITE(16);
> +
> +/*
> + * Not all supported compiler versions distinguish aligned/unaligned accesses,
> + * but e.g. recent versions of Clang do.
> + */
> +#define DEFINE_TSAN_UNALIGNED_READ_WRITE(size) \
> + void __tsan_unaligned_read##size(void *ptr) \
> + { \
> + __kcsan_check_read(ptr, size); \
> + } \
> + EXPORT_SYMBOL(__tsan_unaligned_read##size); \
> + void __tsan_unaligned_write##size(void *ptr) \
> + { \
> + __kcsan_check_write(ptr, size); \
> + } \
> + EXPORT_SYMBOL(__tsan_unaligned_write##size)
> +
> +DEFINE_TSAN_UNALIGNED_READ_WRITE(2);
> +DEFINE_TSAN_UNALIGNED_READ_WRITE(4);
> +DEFINE_TSAN_UNALIGNED_READ_WRITE(8);
> +DEFINE_TSAN_UNALIGNED_READ_WRITE(16);
> +
> +void __tsan_read_range(void *ptr, size_t size)
> +{
> + __kcsan_check_read(ptr, size);
> +}
> +EXPORT_SYMBOL(__tsan_read_range);
> +
> +void __tsan_write_range(void *ptr, size_t size)
> +{
> + __kcsan_check_write(ptr, size);
> +}
> +EXPORT_SYMBOL(__tsan_write_range);
> +
> +/*
> + * The below are not required KCSAN, but can still be emitted by the compiler.
Is "for" missed before KCSAN?
> + */
> +void __tsan_func_entry(void *call_pc)
> +{
> +}
> +EXPORT_SYMBOL(__tsan_func_entry);
> +void __tsan_func_exit(void)
> +{
> +}
> +EXPORT_SYMBOL(__tsan_func_exit);
> +void __tsan_init(void)
> +{
> +}
> +EXPORT_SYMBOL(__tsan_init);
> diff --git a/kernel/kcsan/kcsan.h b/kernel/kcsan/kcsan.h
> new file mode 100644
> index 000000000000..429479b3041d
> --- /dev/null
> +++ b/kernel/kcsan/kcsan.h
> @@ -0,0 +1,140 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _MM_KCSAN_KCSAN_H
> +#define _MM_KCSAN_KCSAN_H
> +
> +#include <linux/kcsan.h>
> +
> +/*
> + * Total number of watchpoints. An address range maps into a specific slot as
> + * specified in `encoding.h`. Although larger number of watchpoints may not even
> + * be usable due to limited thread count, a larger value will improve
> + * performance due to reducing cache-line contention.
> + */
> +#define KCSAN_NUM_WATCHPOINTS 64
> +
> +/*
> + * The number of adjacent watchpoints to check; the purpose is 2-fold:
> + *
> + * 1. the address slot is already occupied, check if any adjacent slots are
> + * free;
> + * 2. accesses that straddle a slot boundary due to size that exceeds a
> + * slot's range may check adjacent slots if any watchpoint matches.
> + *
> + * Note that accesses with very large size may still miss a watchpoint; however,
> + * given this should be rare, this is a reasonable trade-off to make, since this
> + * will avoid:
> + *
> + * 1. excessive contention between watchpoint checks and setup;
> + * 2. larger number of simultaneous watchpoints without sacrificing
> + * performance.
> + */
> +#define KCSAN_CHECK_ADJACENT 1
> +
> +/*
> + * Globally enable and disable KCSAN.
> + */
> +extern bool kcsan_enabled;
> +
> +/*
> + * Helper that returns true if access to ptr should be considered as an atomic
> + * access, even though it is not explicitly atomic.
> + */
> +bool kcsan_is_atomic(const volatile void *ptr);
> +
> +/*
> + * Initialize debugfs file.
> + */
> +void kcsan_debugfs_init(void);
> +
> +enum kcsan_counter_id {
> + /*
> + * Number of watchpoints currently in use.
> + */
> + kcsan_counter_used_watchpoints,
> +
> + /*
> + * Total number of watchpoints set up.
> + */
> + kcsan_counter_setup_watchpoints,
> +
> + /*
> + * Total number of data-races.
> + */
> + kcsan_counter_data_races,
> +
> + /*
> + * Number of times no watchpoints were available.
> + */
> + kcsan_counter_no_capacity,
> +
> + /*
> + * A thread checking a watchpoint raced with another checking thread;
> + * only one will be reported.
> + */
> + kcsan_counter_report_races,
> +
> + /*
> + * Observed data value change, but writer thread unknown.
> + */
> + kcsan_counter_races_unknown_origin,
> +
> + /*
> + * The access cannot be encoded to a valid watchpoint.
> + */
> + kcsan_counter_unencodable_accesses,
> +
> + /*
> + * Watchpoint encoding caused a watchpoint to fire on mismatching
> + * accesses.
> + */
> + kcsan_counter_encoding_false_positives,
> +
> + kcsan_counter_count, /* number of counters */
> +};
> +
> +/*
> + * Increment/decrement counter with given id; avoid calling these in fast-path.
> + */
> +void kcsan_counter_inc(enum kcsan_counter_id id);
> +void kcsan_counter_dec(enum kcsan_counter_id id);
> +
> +/*
> + * Returns true if data-races in the function symbol that maps to addr (offsets
> + * are ignored) should *not* be reported.
> + */
> +bool kcsan_skip_report(unsigned long func_addr);
> +
> +enum kcsan_report_type {
> + /*
> + * The thread that set up the watchpoint and briefly stalled was
> + * signalled that another thread triggered the watchpoint, and thus a
> + * race was encountered.
> + */
> + kcsan_report_race_setup,
> +
> + /*
> + * A thread encountered a watchpoint for the access, therefore a race
> + * was encountered.
> + */
> + kcsan_report_race_check,
> +
> + /*
> + * A thread encountered a watchpoint for the access, but the other
> + * racing thread can no longer be signaled that a race occurred.
> + */
> + kcsan_report_race_check_race,
> +
> + /*
> + * No other thread was observed to race with the access, but the data
> + * value before and after the stall differs.
> + */
> + kcsan_report_race_unknown_origin,
> +};
> +/*
> + * Print a race report from thread that encountered the race.
> + */
> +void kcsan_report(const volatile void *ptr, size_t size, bool is_write,
> + int cpu_id, enum kcsan_report_type type);
> +
> +#endif /* _MM_KCSAN_KCSAN_H */
> diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
> new file mode 100644
> index 000000000000..517db539e4e7
> --- /dev/null
> +++ b/kernel/kcsan/report.c
> @@ -0,0 +1,306 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/kernel.h>
> +#include <linux/preempt.h>
> +#include <linux/printk.h>
> +#include <linux/sched.h>
> +#include <linux/spinlock.h>
> +#include <linux/stacktrace.h>
> +
> +#include "kcsan.h"
> +#include "encoding.h"
> +
> +/*
> + * Max. number of stack entries to show in the report.
> + */
> +#define NUM_STACK_ENTRIES 16
Increase it to 64 at least. No reason to truncate potentailly useful info.
> +
> +/*
> + * Other thread info: communicated from other racing thread to thread that set
> + * up the watchpoint, which then prints the complete report atomically. Only
> + * need one struct, as all threads should to be serialized regardless to print
> + * the reports, with reporting being in the slow-path.
> + */
> +static struct {
> + const volatile void *ptr;
> + size_t size;
> + bool is_write;
> + int task_pid;
> + int cpu_id;
> + unsigned long stack_entries[NUM_STACK_ENTRIES];
> + int num_stack_entries;
> +} other_info = { .ptr = NULL };
> +
> +static DEFINE_SPINLOCK(other_info_lock);
> +static DEFINE_SPINLOCK(report_lock);
> +
> +static bool set_or_lock_other_info(unsigned long *flags,
> + const volatile void *ptr, size_t size,
> + bool is_write, int cpu_id,
> + enum kcsan_report_type type)
> +{
> + if (type != kcsan_report_race_check && type != kcsan_report_race_setup)
> + return true;
> +
> + for (;;) {
> + spin_lock_irqsave(&other_info_lock, *flags);
> +
> + switch (type) {
> + case kcsan_report_race_check:
> + if (other_info.ptr != NULL) {
> + /* still in use, retry */
> + break;
> + }
> + other_info.ptr = ptr;
> + other_info.size = size;
> + other_info.is_write = is_write;
> + other_info.task_pid =
> + in_task() ? task_pid_nr(current) : -1;
> + other_info.cpu_id = cpu_id;
> + other_info.num_stack_entries = stack_trace_save(
> + other_info.stack_entries, NUM_STACK_ENTRIES, 1);
> + /*
> + * other_info may now be consumed by thread we raced
> + * with.
> + */
> + spin_unlock_irqrestore(&other_info_lock, *flags);
> + return false;
> +
> + case kcsan_report_race_setup:
> + if (other_info.ptr == NULL)
> + break; /* no data available yet, retry */
> +
> + /*
> + * First check if matching based on how watchpoint was
> + * encoded.
> + */
> + if (!matching_access((unsigned long)other_info.ptr &
> + WATCHPOINT_ADDR_MASK,
> + other_info.size,
> + (unsigned long)ptr &
> + WATCHPOINT_ADDR_MASK,
> + size))
> + break; /* mismatching access, retry */
> +
> + if (!matching_access((unsigned long)other_info.ptr,
> + other_info.size,
> + (unsigned long)ptr, size)) {
> + /*
> + * If the actual accesses to not match, this was
> + * a false positive due to watchpoint encoding.
> + */
> + other_info.ptr = NULL; /* mark for reuse */
> + kcsan_counter_inc(
> + kcsan_counter_encoding_false_positives);
> + spin_unlock_irqrestore(&other_info_lock,
> + *flags);
> + return false;
> + }
> +
> + /*
> + * Matching access: keep other_info locked, as this
> + * thread uses it to print the full report; unlocked in
> + * end_report.
> + */
> + return true;
> +
> + default:
> + BUG();
> + }
> +
> + spin_unlock_irqrestore(&other_info_lock, *flags);
> + }
> +}
> +
> +static void start_report(unsigned long *flags, enum kcsan_report_type type)
> +{
> + switch (type) {
> + case kcsan_report_race_setup:
> + /* irqsaved already via other_info_lock */
> + spin_lock(&report_lock);
> + break;
> +
> + case kcsan_report_race_unknown_origin:
> + spin_lock_irqsave(&report_lock, *flags);
> + break;
> +
> + default:
> + BUG();
> + }
> +}
> +
> +static void end_report(unsigned long *flags, enum kcsan_report_type type)
> +{
> + switch (type) {
> + case kcsan_report_race_setup:
> + other_info.ptr = NULL; /* mark for reuse */
> + spin_unlock(&report_lock);
> + spin_unlock_irqrestore(&other_info_lock, *flags);
> + break;
> +
> + case kcsan_report_race_unknown_origin:
> + spin_unlock_irqrestore(&report_lock, *flags);
> + break;
> +
> + default:
> + BUG();
> + }
> +}
> +
> +static const char *get_access_type(bool is_write)
> +{
> + return is_write ? "write" : "read";
> +}
> +
> +/* Return thread description: in task or interrupt. */
> +static const char *get_thread_desc(int task_id)
> +{
> + if (task_id != -1) {
> + static char buf[32]; /* safe: protected by report_lock */
> +
> + snprintf(buf, sizeof(buf), "task %i", task_id);
> + return buf;
> + }
> + return in_nmi() ? "NMI" : "interrupt";
in_nmi() will return a wrong thing for the other thread. We either
need to memorize it with the pid, or I would simply always print
"interrupt" b/c nmi/non-nmi is inferrable from the stack if necessary.
> +}
> +
> +/* Helper to skip KCSAN-related functions in stack-trace. */
> +static int get_stack_skipnr(unsigned long stack_entries[], int num_entries)
> +{
> + char buf[64];
> + int skip = 0;
> +
> + for (; skip < num_entries; ++skip) {
> + snprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skip]);
> + if (!strnstr(buf, "csan_", sizeof(buf)) &&
> + !strnstr(buf, "tsan_", sizeof(buf)) &&
> + !strnstr(buf, "_once_size", sizeof(buf))) {
> + break;
> + }
> + }
> + return skip;
> +}
> +
> +/* Compares symbolized strings of addr1 and addr2. */
> +static int sym_strcmp(void *addr1, void *addr2)
> +{
> + char buf1[64];
> + char buf2[64];
> +
> + snprintf(buf1, sizeof(buf1), "%pS", addr1);
> + snprintf(buf2, sizeof(buf2), "%pS", addr2);
> + return strncmp(buf1, buf2, sizeof(buf1));
> +}
> +
> +/*
> + * Returns true if a report was generated, false otherwise.
> + */
> +static bool print_summary(const volatile void *ptr, size_t size, bool is_write,
> + int cpu_id, enum kcsan_report_type type)
> +{
> + unsigned long stack_entries[NUM_STACK_ENTRIES] = { 0 };
> + int num_stack_entries =
> + stack_trace_save(stack_entries, NUM_STACK_ENTRIES, 1);
> + int skipnr = get_stack_skipnr(stack_entries, num_stack_entries);
> + int other_skipnr;
> +
> + /* Check if the top stackframe is in a blacklisted function. */
> + if (kcsan_skip_report(stack_entries[skipnr]))
> + return false;
> + if (type == kcsan_report_race_setup) {
> + other_skipnr = get_stack_skipnr(other_info.stack_entries,
> + other_info.num_stack_entries);
> + if (kcsan_skip_report(other_info.stack_entries[other_skipnr]))
> + return false;
> + }
> +
> + /* Print report header. */
> + pr_err("==================================================================\n");
> + switch (type) {
> + case kcsan_report_race_setup: {
> + void *this_fn = (void *)stack_entries[skipnr];
> + void *other_fn = (void *)other_info.stack_entries[other_skipnr];
> + int cmp;
> +
> + /*
> + * Order functions lexographically for consistent bug titles.
> + * Do not print offset of functions to keep title short.
> + */
> + cmp = sym_strcmp(other_fn, this_fn);
> + pr_err("BUG: KCSAN: data-race in %ps / %ps\n",
> + cmp < 0 ? other_fn : this_fn,
> + cmp < 0 ? this_fn : other_fn);
> + } break;
> +
> + case kcsan_report_race_unknown_origin:
> + pr_err("BUG: KCSAN: data-race in %pS\n",
> + (void *)stack_entries[skipnr]);
> + break;
> +
> + default:
> + BUG();
> + }
> +
> + pr_err("\n");
> +
> + /* Print information about the racing accesses. */
> + switch (type) {
> + case kcsan_report_race_setup:
> + pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n",
> + get_access_type(other_info.is_write), other_info.ptr,
> + other_info.size, get_thread_desc(other_info.task_pid),
> + other_info.cpu_id);
> +
> + /* Print the other thread's stack trace. */
> + stack_trace_print(other_info.stack_entries + other_skipnr,
> + other_info.num_stack_entries - other_skipnr,
> + 0);
> +
> + pr_err("\n");
> + pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n",
> + get_access_type(is_write), ptr, size,
> + get_thread_desc(in_task() ? task_pid_nr(current) : -1),
> + cpu_id);
> + break;
> +
> + case kcsan_report_race_unknown_origin:
> + pr_err("race at unknown origin, with %s to 0x%px of %zu bytes by %s on cpu %i:\n",
> + get_access_type(is_write), ptr, size,
> + get_thread_desc(in_task() ? task_pid_nr(current) : -1),
> + cpu_id);
> + break;
> +
> + default:
> + BUG();
> + }
> + /* Print stack trace of this thread. */
> + stack_trace_print(stack_entries + skipnr, num_stack_entries - skipnr,
> + 0);
> +
> + /* Print report footer. */
> + pr_err("\n");
> + pr_err("Reported by Kernel Concurrency Sanitizer on:\n");
> + dump_stack_print_info(KERN_DEFAULT);
> + pr_err("==================================================================\n");
> +
> + return true;
> +}
> +
> +void kcsan_report(const volatile void *ptr, size_t size, bool is_write,
> + int cpu_id, enum kcsan_report_type type)
> +{
> + unsigned long flags = 0;
> +
> + if (type == kcsan_report_race_check_race)
> + return;
> +
> + kcsan_disable_current();
> + if (set_or_lock_other_info(&flags, ptr, size, is_write, cpu_id, type)) {
> + start_report(&flags, type);
> + if (print_summary(ptr, size, is_write, cpu_id, type) &&
> + panic_on_warn)
> + panic("panic_on_warn set ...\n");
> + end_report(&flags, type);
> + }
> + kcsan_enable_current();
> +}
> diff --git a/kernel/kcsan/test.c b/kernel/kcsan/test.c
> new file mode 100644
> index 000000000000..68c896a24529
> --- /dev/null
> +++ b/kernel/kcsan/test.c
> @@ -0,0 +1,117 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/init.h>
> +#include <linux/kernel.h>
> +#include <linux/printk.h>
> +#include <linux/random.h>
> +#include <linux/types.h>
> +
> +#include "encoding.h"
> +
> +#define ITERS_PER_TEST 2000
> +
> +/* Test requirements. */
> +static bool test_requires(void)
> +{
> + /* random should be initialized */
> + return prandom_u32() + prandom_u32() != 0;
> +}
> +
> +/* Test watchpoint encode and decode. */
> +static bool test_encode_decode(void)
> +{
> + int i;
> +
> + for (i = 0; i < ITERS_PER_TEST; ++i) {
> + size_t size = prandom_u32() % MAX_ENCODABLE_SIZE + 1;
> + bool is_write = prandom_u32() % 2;
> + unsigned long addr;
> +
> + prandom_bytes(&addr, sizeof(addr));
> + if (WARN_ON(!check_encodable(addr, size)))
> + return false;
> +
> + /* encode and decode */
> + {
> + const long encoded_watchpoint =
> + encode_watchpoint(addr, size, is_write);
> + unsigned long verif_masked_addr;
> + size_t verif_size;
> + bool verif_is_write;
> +
> + /* check special watchpoints */
> + if (WARN_ON(decode_watchpoint(
> + INVALID_WATCHPOINT, &verif_masked_addr,
> + &verif_size, &verif_is_write)))
> + return false;
> + if (WARN_ON(decode_watchpoint(
> + CONSUMED_WATCHPOINT, &verif_masked_addr,
> + &verif_size, &verif_is_write)))
> + return false;
> +
> + /* check decoding watchpoint returns same data */
> + if (WARN_ON(!decode_watchpoint(
> + encoded_watchpoint, &verif_masked_addr,
> + &verif_size, &verif_is_write)))
> + return false;
> + if (WARN_ON(verif_masked_addr !=
> + (addr & WATCHPOINT_ADDR_MASK)))
> + goto fail;
> + if (WARN_ON(verif_size != size))
> + goto fail;
> + if (WARN_ON(is_write != verif_is_write))
> + goto fail;
> +
> + continue;
> +fail:
> + pr_err("%s fail: %s %zu bytes @ %lx -> encoded: %lx -> %s %zu bytes @ %lx\n",
> + __func__, is_write ? "write" : "read", size,
> + addr, encoded_watchpoint,
> + verif_is_write ? "write" : "read", verif_size,
> + verif_masked_addr);
> + return false;
> + }
> + }
> +
> + return true;
> +}
> +
> +static bool test_matching_access(void)
> +{
> + if (WARN_ON(!matching_access(10, 1, 10, 1)))
> + return false;
> + if (WARN_ON(!matching_access(10, 2, 11, 1)))
> + return false;
> + if (WARN_ON(!matching_access(10, 1, 9, 2)))
> + return false;
> + if (WARN_ON(matching_access(10, 1, 11, 1)))
> + return false;
> + if (WARN_ON(matching_access(9, 1, 10, 1)))
> + return false;
> + return true;
> +}
> +
> +static int __init kcsan_selftest(void)
> +{
> + int passed = 0;
> + int total = 0;
> +
> +#define RUN_TEST(do_test) \
> + do { \
> + ++total; \
> + if (do_test()) \
> + ++passed; \
> + else \
> + pr_err("KCSAN selftest: " #do_test " failed"); \
> + } while (0)
> +
> + RUN_TEST(test_requires);
> + RUN_TEST(test_encode_decode);
> + RUN_TEST(test_matching_access);
> +
> + pr_info("KCSAN selftest: %d/%d tests passed\n", passed, total);
> + if (passed != total)
> + panic("KCSAN selftests failed");
> + return 0;
> +}
> +postcore_initcall(kcsan_selftest);
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 93d97f9b0157..35accd1d93de 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2086,6 +2086,8 @@ source "lib/Kconfig.kgdb"
>
> source "lib/Kconfig.ubsan"
>
> +source "lib/Kconfig.kcsan"
> +
> config ARCH_HAS_DEVMEM_IS_ALLOWED
> bool
>
> diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
> new file mode 100644
> index 000000000000..3e1f1acfb24b
> --- /dev/null
> +++ b/lib/Kconfig.kcsan
> @@ -0,0 +1,88 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +
> +config HAVE_ARCH_KCSAN
> + bool
> +
> +menuconfig KCSAN
> + bool "KCSAN: watchpoint-based dynamic data-race detector"
> + depends on HAVE_ARCH_KCSAN && !KASAN && STACKTRACE
> + default n
> + help
> + Kernel Concurrency Sanitizer is a dynamic data-race detector, which
> + uses a watchpoint-based sampling approach to detect races.
> +
> +if KCSAN
> +
> +config KCSAN_SELFTEST
> + bool "KCSAN: perform short selftests on boot"
> + default y
> + help
> + Run KCSAN selftests on boot. On test failure, causes kernel to panic.
> +
> +config KCSAN_EARLY_ENABLE
> + bool "KCSAN: early enable"
> + default y
> + help
> + If KCSAN should be enabled globally as soon as possible. KCSAN can
> + later be enabled/disabled via debugfs.
> +
> +config KCSAN_UDELAY_MAX_TASK
> + int "KCSAN: maximum delay in microseconds (for tasks)"
> + default 80
> + help
> + For tasks, the max. microsecond delay after setting up a watchpoint.
> +
> +config KCSAN_UDELAY_MAX_INTERRUPT
> + int "KCSAN: maximum delay in microseconds (for interrupts)"
> + default 20
> + help
> + For interrupts, the max. microsecond delay after setting up a watchpoint.
> +
> +config KCSAN_DELAY_RANDOMIZE
> + bool "KCSAN: randomize delays"
> + default y
> + help
> + If delays should be randomized; if false, the chosen delay is simply
> + the maximum values defined above.
> +
> +config KCSAN_WATCH_SKIP_INST
> + int "KCSAN: watchpoint instruction skip"
> + default 2000
> + help
> + The number of per-CPU memory operations to skip watching, before
> + another watchpoint is set up; in other words, 1 in
> + KCSAN_WATCH_SKIP_INST per-CPU memory operations are used to set up a
> + watchpoint. A smaller value results in more aggressive race
> + detection, whereas a larger value improves system performance at the
> + cost of missing some races.
> +
> +config KCSAN_REPORT_RACE_UNKNOWN_ORIGIN
> + bool "KCSAN: report races of unknown origin"
> + default y
> + help
> + If KCSAN should report races where only one access is known, and the
> + conflicting access is of unknown origin. This type of race is
> + reported if it was only possible to infer a race due to a data-value
> + change while an access is being delayed on a watchpoint.
> +
> +config KCSAN_IGNORE_ATOMICS
> + bool "KCSAN: do not instrument marked atomic accesses"
> + default n
> + help
> + If enabled, never instruments marked atomic accesses. This results in
> + not reporting data-races where one access is atomic and the other is
> + a plain access.
> +
> +config KCSAN_PLAIN_WRITE_PRETEND_ONCE
> + bool "KCSAN: pretend plain writes are WRITE_ONCE"
> + default n
> + help
> + This option makes KCSAN pretend that all plain writes are WRITE_ONCE.
> + This option should only be used to prune initial data-races found in
> + existing code.
> +
> +config KCSAN_DEBUG
> + bool "Debugging of KCSAN internals"
> + default n
> +
> +endif # KCSAN
> diff --git a/lib/Makefile b/lib/Makefile
> index c5892807e06f..778ab704e3ad 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -24,6 +24,9 @@ KASAN_SANITIZE_string.o := n
> CFLAGS_string.o := $(call cc-option, -fno-stack-protector)
> endif
>
> +# Used by KCSAN while enabled, avoid recursion.
> +KCSAN_SANITIZE_random32.o := n
> +
> lib-y := ctype.o string.o vsprintf.o cmdline.o \
> rbtree.o radix-tree.o timerqueue.o xarray.o \
> idr.o extable.o \
> diff --git a/scripts/Makefile.kcsan b/scripts/Makefile.kcsan
> new file mode 100644
> index 000000000000..caf1111a28ae
> --- /dev/null
> +++ b/scripts/Makefile.kcsan
> @@ -0,0 +1,6 @@
> +# SPDX-License-Identifier: GPL-2.0
> +ifdef CONFIG_KCSAN
> +
> +CFLAGS_KCSAN := -fsanitize=thread
> +
> +endif # CONFIG_KCSAN
> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index 179d55af5852..0e78abab7d83 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -152,6 +152,16 @@ _c_flags += $(if $(patsubst n%,, \
> $(CFLAGS_KCOV))
> endif
>
> +#
> +# Enable ConcurrencySanitizer flags for kernel except some files or directories
> +# we don't want to check (depends on variables KCSAN_SANITIZE_obj.o, KCSAN_SANITIZE)
> +#
> +ifeq ($(CONFIG_KCSAN),y)
> +_c_flags += $(if $(patsubst n%,, \
> + $(KCSAN_SANITIZE_$(basetarget).o)$(KCSAN_SANITIZE)y), \
> + $(CFLAGS_KCSAN))
> +endif
> +
> # $(srctree)/$(src) for including checkin headers from generated source files
> # $(objtree)/$(obj) for including generated headers from checkin source files
> ifeq ($(KBUILD_EXTMOD),)
> --
> 2.23.0.866.gb869b98d4c-goog
>
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