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Message-ID: <CANpmjNNmyXd9YkYSTpWrKRqBzJp5bBaEZEuZLHK9Tw-D6NDezQ@mail.gmail.com>
Date: Thu, 17 Apr 2025 21:43:20 +0200
From: Marco Elver <elver@...gle.com>
To: Alexander Potapenko <glider@...gle.com>
Cc: quic_jiangenj@...cinc.com, linux-kernel@...r.kernel.org,
kasan-dev@...glegroups.com, x86@...nel.org,
Aleksandr Nogikh <nogikh@...gle.com>, Andrey Konovalov <andreyknvl@...il.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, Dmitry Vyukov <dvyukov@...gle.com>,
Ingo Molnar <mingo@...hat.com>, Josh Poimboeuf <jpoimboe@...nel.org>,
Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3/7] kcov: x86: introduce CONFIG_KCOV_ENABLE_GUARDS
On Wed, 16 Apr 2025 at 10:55, Alexander Potapenko <glider@...gle.com> wrote:
>
> The new config switches coverage instrumentation to using
> __sanitizer_cov_trace_pc_guard(u32 *guard)
> instead of
> __sanitizer_cov_trace_pc(void)
>
> Each callback receives a unique 32-bit guard variable residing in the
> __sancov_guards section. Those guards can be used by kcov to deduplicate
> the coverage on the fly.
>
> As a first step, we make the new instrumentation mode 1:1 compatible with
> the old one.
>
> Cc: x86@...nel.org
> Signed-off-by: Alexander Potapenko <glider@...gle.com>
> ---
> arch/x86/kernel/vmlinux.lds.S | 1 +
> include/asm-generic/vmlinux.lds.h | 14 ++++++-
> include/linux/kcov.h | 2 +
> kernel/kcov.c | 61 +++++++++++++++++++++----------
> lib/Kconfig.debug | 16 ++++++++
> scripts/Makefile.kcov | 4 ++
> scripts/module.lds.S | 23 ++++++++++++
> tools/objtool/check.c | 1 +
> 8 files changed, 101 insertions(+), 21 deletions(-)
>
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 0deb4887d6e96..2acfbbde33820 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -390,6 +390,7 @@ SECTIONS
> . = ALIGN(PAGE_SIZE);
> __bss_stop = .;
> }
> + SANCOV_GUARDS_BSS
Right now this will be broken on other architectures, right?
> /*
> * The memory occupied from _text to here, __end_of_kernel_reserve, is
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 0d5b186abee86..3ff150f152737 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -102,7 +102,8 @@
> * sections to be brought in with rodata.
> */
> #if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG) || \
> -defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
> + defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG) || \
> + defined(CONFIG_KCOV_ENABLE_GUARDS)
> #define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
> #else
> #define TEXT_MAIN .text
> @@ -121,6 +122,17 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
> #define SBSS_MAIN .sbss
> #endif
>
> +#if defined(CONFIG_KCOV_ENABLE_GUARDS)
> +#define SANCOV_GUARDS_BSS \
> + __sancov_guards(NOLOAD) : { \
> + __start___sancov_guards = .; \
> + *(__sancov_guards); \
> + __stop___sancov_guards = .; \
> + }
> +#else
> +#define SANCOV_GUARDS_BSS
> +#endif
> +
> /*
> * GCC 4.5 and later have a 32 bytes section alignment for structures.
> * Except GCC 4.9, that feels the need to align on 64 bytes.
> diff --git a/include/linux/kcov.h b/include/linux/kcov.h
> index e1f7d793c1cb3..7ec2669362fd1 100644
> --- a/include/linux/kcov.h
> +++ b/include/linux/kcov.h
> @@ -107,6 +107,8 @@ typedef unsigned long long kcov_u64;
> #endif
>
> void __sanitizer_cov_trace_pc(void);
> +void __sanitizer_cov_trace_pc_guard(u32 *guard);
> +void __sanitizer_cov_trace_pc_guard_init(uint32_t *start, uint32_t *stop);
> void __sanitizer_cov_trace_cmp1(u8 arg1, u8 arg2);
> void __sanitizer_cov_trace_cmp2(u16 arg1, u16 arg2);
> void __sanitizer_cov_trace_cmp4(u32 arg1, u32 arg2);
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 8fcbca236bec5..b97f429d17436 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -193,27 +193,15 @@ static notrace unsigned long canonicalize_ip(unsigned long ip)
> return ip;
> }
>
> -/*
> - * Entry point from instrumented code.
> - * This is called once per basic-block/edge.
> - */
> -void notrace __sanitizer_cov_trace_pc(void)
> +static void sanitizer_cov_write_subsequent(unsigned long *area, int size,
notrace is missing.
Can we give this a more descriptive name? E.g. "kcov_append" ?
> + unsigned long ip)
> {
> - struct task_struct *t;
> - unsigned long *area;
> - unsigned long ip = canonicalize_ip(_RET_IP_);
> - unsigned long pos;
> -
> - t = current;
> - if (!check_kcov_mode(KCOV_MODE_TRACE_PC, t))
> - return;
> -
> - area = t->kcov_state.s.area;
> /* The first 64-bit word is the number of subsequent PCs. */
> - pos = READ_ONCE(area[0]) + 1;
> - if (likely(pos < t->kcov_state.s.size)) {
> - /* Previously we write pc before updating pos. However, some
> - * early interrupt code could bypass check_kcov_mode() check
> + unsigned long pos = READ_ONCE(area[0]) + 1;
> +
> + if (likely(pos < size)) {
> + /*
> + * Some early interrupt code could bypass check_kcov_mode() check
> * and invoke __sanitizer_cov_trace_pc(). If such interrupt is
> * raised between writing pc and updating pos, the pc could be
> * overitten by the recursive __sanitizer_cov_trace_pc().
> @@ -224,7 +212,40 @@ void notrace __sanitizer_cov_trace_pc(void)
> area[pos] = ip;
> }
> }
> +
> +/*
> + * Entry point from instrumented code.
> + * This is called once per basic-block/edge.
> + */
> +#ifndef CONFIG_KCOV_ENABLE_GUARDS
Negation makes it harder to read - just #ifdef, and swap the branches below.
> +void notrace __sanitizer_cov_trace_pc(void)
> +{
> + if (!check_kcov_mode(KCOV_MODE_TRACE_PC, current))
> + return;
> +
> + sanitizer_cov_write_subsequent(current->kcov_state.s.area,
> + current->kcov_state.s.size,
> + canonicalize_ip(_RET_IP_));
> +}
> EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
> +#else
> +void notrace __sanitizer_cov_trace_pc_guard(u32 *guard)
> +{
> + if (!check_kcov_mode(KCOV_MODE_TRACE_PC, current))
> + return;
> +
> + sanitizer_cov_write_subsequent(current->kcov_state.s.area,
> + current->kcov_state.s.size,
> + canonicalize_ip(_RET_IP_));
> +}
> +EXPORT_SYMBOL(__sanitizer_cov_trace_pc_guard);
> +
> +void notrace __sanitizer_cov_trace_pc_guard_init(uint32_t *start,
> + uint32_t *stop)
> +{
> +}
> +EXPORT_SYMBOL(__sanitizer_cov_trace_pc_guard_init);
> +#endif
>
> #ifdef CONFIG_KCOV_ENABLE_COMPARISONS
> static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
> @@ -252,7 +273,7 @@ static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
> start_index = 1 + count * KCOV_WORDS_PER_CMP;
> end_pos = (start_index + KCOV_WORDS_PER_CMP) * sizeof(u64);
> if (likely(end_pos <= max_pos)) {
> - /* See comment in __sanitizer_cov_trace_pc(). */
> + /* See comment in sanitizer_cov_write_subsequent(). */
> WRITE_ONCE(area[0], count + 1);
> barrier();
> area[start_index] = type;
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 35796c290ca35..a81d086b8e1ff 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2135,6 +2135,8 @@ config ARCH_HAS_KCOV
> config CC_HAS_SANCOV_TRACE_PC
> def_bool $(cc-option,-fsanitize-coverage=trace-pc)
>
> +config CC_HAS_SANCOV_TRACE_PC_GUARD
> + def_bool $(cc-option,-fsanitize-coverage=trace-pc-guard)
>
> config KCOV
> bool "Code coverage for fuzzing"
> @@ -2151,6 +2153,20 @@ config KCOV
>
> For more details, see Documentation/dev-tools/kcov.rst.
>
> +config KCOV_ENABLE_GUARDS
The "ENABLE" here seems redundant.
Just KCOV_GUARDS should be clear enough.
> + depends on KCOV
> + depends on CC_HAS_SANCOV_TRACE_PC_GUARD
> + bool "Use fsanitize-coverage=trace-pc-guard for kcov"
The compiler option is an implementation detail - it might be more
helpful to have this say "Use coverage guards for kcov".
> + help
> + Use coverage guards instrumentation for kcov, passing
> + -fsanitize-coverage=trace-pc-guard to the compiler.
> +
> + Every coverage callback is associated with a global variable that
> + allows to efficiently deduplicate coverage at collection time.
> +
> + This comes at a cost of increased binary size (4 bytes of .bss
> + per basic block, plus 1-2 instructions to pass an extra parameter).
> +
> config KCOV_ENABLE_COMPARISONS
> bool "Enable comparison operands collection by KCOV"
> depends on KCOV
> diff --git a/scripts/Makefile.kcov b/scripts/Makefile.kcov
> index 67e8cfe3474b7..ec63d471d5773 100644
> --- a/scripts/Makefile.kcov
> +++ b/scripts/Makefile.kcov
> @@ -1,5 +1,9 @@
> # SPDX-License-Identifier: GPL-2.0-only
> +ifeq ($(CONFIG_KCOV_ENABLE_GUARDS),y)
> +kcov-flags-$(CONFIG_CC_HAS_SANCOV_TRACE_PC_GUARD) += -fsanitize-coverage=trace-pc-guard
This can just be kcov-flags-y, because CONFIG_KCOV_ENABLE_GUARDS
implies CONFIG_CC_HAS_SANCOV_TRACE_PC_GUARD.
> +else
> kcov-flags-$(CONFIG_CC_HAS_SANCOV_TRACE_PC) += -fsanitize-coverage=trace-pc
> +endif
> kcov-flags-$(CONFIG_KCOV_ENABLE_COMPARISONS) += -fsanitize-coverage=trace-cmp
> kcov-flags-$(CONFIG_GCC_PLUGIN_SANCOV) += -fplugin=$(objtree)/scripts/gcc-plugins/sancov_plugin.so
>
> diff --git a/scripts/module.lds.S b/scripts/module.lds.S
> index 450f1088d5fd3..ec7e9247f8de6 100644
> --- a/scripts/module.lds.S
> +++ b/scripts/module.lds.S
> @@ -64,6 +64,29 @@ SECTIONS {
> MOD_CODETAG_SECTIONS()
> }
> #endif
> +
> +#ifdef CONFIG_KCOV_ENABLE_GUARDS
> + __sancov_guards(NOLOAD) : {
> + __start___sancov_guards = .;
> + *(__sancov_guards);
> + __stop___sancov_guards = .;
> + }
> +
> + .text : {
> + *(.text .text.[0-9a-zA-Z_]*)
> + *(.text..L*)
> + }
> +
> + .init.text : {
> + *(.init.text .init.text.[0-9a-zA-Z_]*)
> + *(.init.text..L*)
> + }
> + .exit.text : {
> + *(.exit.text .exit.text.[0-9a-zA-Z_]*)
> + *(.exit.text..L*)
> + }
> +#endif
> +
> MOD_SEPARATE_CODETAG_SECTIONS()
> }
>
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index ce973d9d8e6d8..a5db690dd2def 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -1149,6 +1149,7 @@ static const char *uaccess_safe_builtin[] = {
> "write_comp_data",
> "check_kcov_mode",
> "__sanitizer_cov_trace_pc",
> + "__sanitizer_cov_trace_pc_guard",
> "__sanitizer_cov_trace_const_cmp1",
> "__sanitizer_cov_trace_const_cmp2",
> "__sanitizer_cov_trace_const_cmp4",
> --
> 2.49.0.604.gff1f9ca942-goog
>
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