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Message-ID: <CANpmjNM=AAtiXeDHgG+ec48=xwBTzphG3rpJZ3krpG2Hd1FixQ@mail.gmail.com>
Date: Tue, 22 Apr 2025 11:29:01 +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, 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 5/7] kcov: add ioctl(KCOV_UNIQUE_ENABLE)
On Wed, 16 Apr 2025 at 10:55, Alexander Potapenko <glider@...gle.com> wrote:
>
> ioctl(KCOV_UNIQUE_ENABLE) enables collection of deduplicated coverage
> in the presence of CONFIG_KCOV_ENABLE_GUARDS.
>
> The buffer shared with the userspace is divided in two parts, one holding
> a bitmap, and the other one being the trace. The single parameter of
> ioctl(KCOV_UNIQUE_ENABLE) determines the number of words used for the
> bitmap.
>
> Each __sanitizer_cov_trace_pc_guard() instrumentation hook receives a
> pointer to a unique guard variable. Upon the first call of each hook,
> the guard variable is initialized with a unique integer, which is used to
> map those hooks to bits in the bitmap. In the new coverage collection mode,
> the kernel first checks whether the bit corresponding to a particular hook
> is set, and then, if it is not, the PC is written into the trace buffer,
> and the bit is set.
>
> Note: when CONFIG_KCOV_ENABLE_GUARDS is disabled, ioctl(KCOV_UNIQUE_ENABLE)
> returns -ENOTSUPP, which is consistent with the existing kcov code.
>
> Also update the documentation.
Do you have performance measurements (old vs. new mode) that can be
included in this commit description?
> Signed-off-by: Alexander Potapenko <glider@...gle.com>
> ---
> Documentation/dev-tools/kcov.rst | 43 +++++++++++
> include/linux/kcov-state.h | 8 ++
> include/linux/kcov.h | 2 +
> include/uapi/linux/kcov.h | 1 +
> kernel/kcov.c | 129 +++++++++++++++++++++++++++----
> 5 files changed, 170 insertions(+), 13 deletions(-)
>
> diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
> index 6611434e2dd24..271260642d1a6 100644
> --- a/Documentation/dev-tools/kcov.rst
> +++ b/Documentation/dev-tools/kcov.rst
> @@ -137,6 +137,49 @@ mmaps coverage buffer, and then forks child processes in a loop. The child
> processes only need to enable coverage (it gets disabled automatically when
> a thread exits).
>
> +Unique coverage collection
> +---------------------------
> +
> +Instead of collecting raw PCs, KCOV can deduplicate them on the fly.
> +This mode is enabled by the ``KCOV_UNIQUE_ENABLE`` ioctl (only available if
> +``CONFIG_KCOV_ENABLE_GUARDS`` is on).
> +
> +.. code-block:: c
> +
> + /* Same includes and defines as above. */
> + #define KCOV_UNIQUE_ENABLE _IOW('c', 103, unsigned long)
Here it's _IOW.
> + #define BITMAP_SIZE (4<<10)
> +
> + /* Instead of KCOV_ENABLE, enable unique coverage collection. */
> + if (ioctl(fd, KCOV_UNIQUE_ENABLE, BITMAP_SIZE))
> + perror("ioctl"), exit(1);
> + /* Reset the coverage from the tail of the ioctl() call. */
> + __atomic_store_n(&cover[BITMAP_SIZE], 0, __ATOMIC_RELAXED);
> + memset(cover, 0, BITMAP_SIZE * sizeof(unsigned long));
> +
> + /* Call the target syscall call. */
> + /* ... */
> +
> + /* Read the number of collected PCs. */
> + n = __atomic_load_n(&cover[BITMAP_SIZE], __ATOMIC_RELAXED);
> + /* Disable the coverage collection. */
> + if (ioctl(fd, KCOV_DISABLE, 0))
> + perror("ioctl"), exit(1);
> +
> +Calling ``ioctl(fd, KCOV_UNIQUE_ENABLE, bitmap_size)`` carves out ``bitmap_size``
> +words from those allocated by ``KCOV_INIT_TRACE`` to keep an opaque bitmap that
> +prevents the kernel from storing the same PC twice. The remaining part of the
> +trace is used to collect PCs, like in other modes (this part must contain at
> +least two words, like when collecting non-unique PCs).
> +
> +The mapping between a PC and its position in the bitmap is persistent during the
> +kernel lifetime, so it is possible for the callers to directly use the bitmap
> +contents as a coverage signal (like when fuzzing userspace with AFL).
> +
> +In order to reset the coverage between the runs, the user needs to rewind the
> +trace (by writing 0 into the first word past ``bitmap_size``) and wipe the whole
> +bitmap.
> +
> Comparison operands collection
> ------------------------------
>
> diff --git a/include/linux/kcov-state.h b/include/linux/kcov-state.h
> index 6e576173fd442..26e275fe90684 100644
> --- a/include/linux/kcov-state.h
> +++ b/include/linux/kcov-state.h
> @@ -26,6 +26,14 @@ struct kcov_state {
> /* Buffer for coverage collection, shared with the userspace. */
> unsigned long *trace;
>
> + /* Size of the bitmap (in bits). */
> + unsigned int bitmap_size;
> + /*
> + * Bitmap for coverage deduplication, shared with the
> + * userspace.
> + */
> + unsigned long *bitmap;
> +
> /*
> * KCOV sequence number: incremented each time kcov is
> * reenabled, used by kcov_remote_stop(), see the comment there.
> diff --git a/include/linux/kcov.h b/include/linux/kcov.h
> index 7ec2669362fd1..41eebcd3ab335 100644
> --- a/include/linux/kcov.h
> +++ b/include/linux/kcov.h
> @@ -10,6 +10,7 @@ struct task_struct;
> #ifdef CONFIG_KCOV
>
> enum kcov_mode {
> + KCOV_MODE_INVALID = -1,
> /* Coverage collection is not enabled yet. */
> KCOV_MODE_DISABLED = 0,
> /* KCOV was initialized, but tracing mode hasn't been chosen yet. */
> @@ -23,6 +24,7 @@ enum kcov_mode {
> KCOV_MODE_TRACE_CMP = 3,
> /* The process owns a KCOV remote reference. */
> KCOV_MODE_REMOTE = 4,
> + KCOV_MODE_TRACE_UNIQUE_PC = 5,
> };
>
> #define KCOV_IN_CTXSW (1 << 30)
> diff --git a/include/uapi/linux/kcov.h b/include/uapi/linux/kcov.h
> index ed95dba9fa37e..fe1695ddf8a06 100644
> --- a/include/uapi/linux/kcov.h
> +++ b/include/uapi/linux/kcov.h
> @@ -22,6 +22,7 @@ struct kcov_remote_arg {
> #define KCOV_ENABLE _IO('c', 100)
> #define KCOV_DISABLE _IO('c', 101)
> #define KCOV_REMOTE_ENABLE _IOW('c', 102, struct kcov_remote_arg)
> +#define KCOV_UNIQUE_ENABLE _IOR('c', 103, unsigned long)
_IOR? The unsigned long arg is copied to the kernel, so this should be
_IOW, right?
> enum {
> /*
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 7b726fd761c1b..dea25c8a53b52 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -29,6 +29,10 @@
>
> #include <asm/setup.h>
>
> +#ifdef CONFIG_KCOV_ENABLE_GUARDS
> +atomic_t kcov_guard_max_index = ATOMIC_INIT(1);
> +#endif
> +
> #define kcov_debug(fmt, ...) pr_debug("%s: " fmt, __func__, ##__VA_ARGS__)
>
> /* Number of 64-bit words written per one comparison: */
> @@ -161,8 +165,7 @@ static __always_inline bool in_softirq_really(void)
> return in_serving_softirq() && !in_hardirq() && !in_nmi();
> }
>
> -static notrace bool check_kcov_mode(enum kcov_mode needed_mode,
> - struct task_struct *t)
> +static notrace enum kcov_mode get_kcov_mode(struct task_struct *t)
> {
> unsigned int mode;
>
> @@ -172,7 +175,7 @@ static notrace bool check_kcov_mode(enum kcov_mode needed_mode,
> * coverage collection section in a softirq.
> */
> if (!in_task() && !(in_softirq_really() && t->kcov_softirq))
> - return false;
> + return KCOV_MODE_INVALID;
> mode = READ_ONCE(t->kcov_state.mode);
> /*
> * There is some code that runs in interrupts but for which
> @@ -182,7 +185,7 @@ static notrace bool check_kcov_mode(enum kcov_mode needed_mode,
> * kcov_start().
> */
> barrier();
> - return mode == needed_mode;
> + return mode;
> }
>
> static notrace unsigned long canonicalize_ip(unsigned long ip)
> @@ -201,7 +204,7 @@ static void sanitizer_cov_write_subsequent(unsigned long *trace, int size,
>
> if (likely(pos < size)) {
> /*
> - * Some early interrupt code could bypass check_kcov_mode() check
> + * Some early interrupt code could bypass get_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().
> @@ -220,7 +223,7 @@ static void sanitizer_cov_write_subsequent(unsigned long *trace, int size,
> #ifndef CONFIG_KCOV_ENABLE_GUARDS
> void notrace __sanitizer_cov_trace_pc(void)
> {
> - if (!check_kcov_mode(KCOV_MODE_TRACE_PC, current))
> + if (get_kcov_mode(current) != KCOV_MODE_TRACE_PC)
> return;
>
> sanitizer_cov_write_subsequent(current->kcov_state.s.trace,
> @@ -229,14 +232,73 @@ void notrace __sanitizer_cov_trace_pc(void)
> }
> EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
> #else
> +
> +DEFINE_PER_CPU(u32, saved_index);
> +/*
> + * Assign an index to a guard variable that does not have one yet.
> + * For an unlikely case of a race with another task executing the same basic
> + * block, we store the unused index in a per-cpu variable.
> + * In an even less likely case the current task may lose a race and get
> + * rescheduled onto a CPU that already has a saved index, discarding that index.
> + * This will result in an unused hole in the bitmap, but such events should have
> + * minor impact on the overall memory consumption.
> + */
> +static __always_inline u32 init_pc_guard(u32 *guard)
> +{
> + /* If the current CPU has a saved free index, use it. */
> + u32 index = this_cpu_xchg(saved_index, 0);
> + u32 old_guard;
> +
> + if (likely(!index))
> + /*
> + * Allocate a new index. No overflow is possible, because 2**32
> + * unique basic blocks will take more space than the max size
> + * of the kernel text segment.
> + */
> + index = atomic_inc_return(&kcov_guard_max_index) - 1;
> +
> + /*
> + * Make sure another task is not initializing the same guard
> + * concurrently.
> + */
> + old_guard = cmpxchg(guard, 0, index);
> + if (unlikely(old_guard)) {
> + /* We lost the race, save the index for future use. */
> + this_cpu_write(saved_index, index);
> + return old_guard;
> + }
> + return index;
> +}
> +
> void notrace __sanitizer_cov_trace_pc_guard(u32 *guard)
> {
> - if (!check_kcov_mode(KCOV_MODE_TRACE_PC, current))
> - return;
> + u32 pc_index;
> + enum kcov_mode mode = get_kcov_mode(current);
>
> - sanitizer_cov_write_subsequent(current->kcov_state.s.trace,
> - current->kcov_state.s.trace_size,
> - canonicalize_ip(_RET_IP_));
> + switch (mode) {
> + case KCOV_MODE_TRACE_UNIQUE_PC:
> + pc_index = READ_ONCE(*guard);
> + if (unlikely(!pc_index))
> + pc_index = init_pc_guard(guard);
This is an unlikely branch, yet init_pc_guard is __always_inline. Can
we somehow make it noinline? I know objtool will complain, but besides
the cosmetic issues, doing noinline and just giving it a better name
("kcov_init_pc_guard") and adding that to objtool whilelist will be
better for codegen.
> +
> + /*
> + * Use the bitmap for coverage deduplication. We assume both
> + * s.bitmap and s.trace are non-NULL.
> + */
> + if (likely(pc_index < current->kcov_state.s.bitmap_size))
> + if (test_and_set_bit(pc_index,
> + current->kcov_state.s.bitmap))
> + return;
> + /* If the PC is new, write it to the trace. */
> + fallthrough;
> + case KCOV_MODE_TRACE_PC:
> + sanitizer_cov_write_subsequent(current->kcov_state.s.trace,
> + current->kcov_state.s.trace_size,
> + canonicalize_ip(_RET_IP_));
> + break;
> + default:
> + return;
> + }
> }
> EXPORT_SYMBOL(__sanitizer_cov_trace_pc_guard);
>
> @@ -255,7 +317,7 @@ static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
> u64 *trace;
>
> t = current;
> - if (!check_kcov_mode(KCOV_MODE_TRACE_CMP, t))
> + if (get_kcov_mode(t) != KCOV_MODE_TRACE_CMP)
> return;
>
> ip = canonicalize_ip(ip);
> @@ -374,7 +436,7 @@ static void kcov_start(struct task_struct *t, struct kcov *kcov,
> /* Cache in task struct for performance. */
> t->kcov_state.s = state->s;
> barrier();
> - /* See comment in check_kcov_mode(). */
> + /* See comment in get_kcov_mode(). */
> WRITE_ONCE(t->kcov_state.mode, state->mode);
> }
>
> @@ -408,6 +470,10 @@ static void kcov_reset(struct kcov *kcov)
> kcov->state.mode = KCOV_MODE_INIT;
> kcov->remote = false;
> kcov->remote_size = 0;
> + kcov->state.s.trace = kcov->state.s.area;
> + kcov->state.s.trace_size = kcov->state.s.size;
> + kcov->state.s.bitmap = NULL;
> + kcov->state.s.bitmap_size = 0;
> kcov->state.s.sequence++;
> }
>
> @@ -594,6 +660,41 @@ static inline bool kcov_check_handle(u64 handle, bool common_valid,
> return false;
> }
>
> +static long kcov_handle_unique_enable(struct kcov *kcov,
> + unsigned long bitmap_words)
> +{
> + struct task_struct *t = current;
> +
> + if (!IS_ENABLED(CONFIG_KCOV_ENABLE_GUARDS))
> + return -ENOTSUPP;
> + if (kcov->state.mode != KCOV_MODE_INIT || !kcov->state.s.area)
> + return -EINVAL;
> + if (kcov->t != NULL || t->kcov != NULL)
> + return -EBUSY;
> +
> + /*
> + * Cannot use zero-sized bitmap, also the bitmap must leave at least two
> + * words for the trace.
> + */
> + if ((!bitmap_words) || (bitmap_words >= (kcov->state.s.size - 1)))
> + return -EINVAL;
> +
> + kcov->state.s.bitmap_size = bitmap_words * sizeof(unsigned long) * 8;
> + kcov->state.s.bitmap = kcov->state.s.area;
> + kcov->state.s.trace_size = kcov->state.s.size - bitmap_words;
> + kcov->state.s.trace =
> + ((unsigned long *)kcov->state.s.area + bitmap_words);
> +
> + kcov_fault_in_area(kcov);
> + kcov->state.mode = KCOV_MODE_TRACE_UNIQUE_PC;
> + kcov_start(t, kcov, &kcov->state);
> + kcov->t = t;
> + /* Put either in kcov_task_exit() or in KCOV_DISABLE. */
> + kcov_get(kcov);
> +
> + return 0;
> +}
> +
> static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
> unsigned long arg)
> {
> @@ -627,6 +728,8 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
> /* Put either in kcov_task_exit() or in KCOV_DISABLE. */
> kcov_get(kcov);
> return 0;
> + case KCOV_UNIQUE_ENABLE:
> + return kcov_handle_unique_enable(kcov, arg);
> case KCOV_DISABLE:
> /* Disable coverage for the current task. */
> unused = arg;
> --
> 2.49.0.604.gff1f9ca942-goog
>
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