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Date: Thu, 17 Nov 2016 08:52:42 +0100
From: Dmitry Vyukov <dvyukov@...gle.com>
To: syzkaller <syzkaller@...glegroups.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
Vegard Nossum <vegard.nossum@...cle.com>,
Michal Zalewski <lcamtuf@...il.com>,
Kees Cook <keescook@...omium.org>,
Quentin Casasnovas <quentin.casasnovas@...cle.com>
Subject: Re: [PATCH 2/2] kcov: add AFL-style tracing
On Wed, Nov 16, 2016 at 10:27 PM, Quentin Casasnovas
<quentin.casasnovas@...cle.com> wrote:
> AFL uses a fixed-size buffer (typically 64 KiB) where each byte is
> a counter representing how many times an A -> B branch was taken.
> Of course, since the buffer is fixed size, it's a little imprecise
> in that e.g. two different branches could map to the same counter,
> but in practice it works well.
>
> See afl:docs/technical_details.txt for more information.
>
> Here is a small test program that demonstrates the new capability:
>
> #include <sys/ioctl.h>
> #include <sys/mman.h>
> #include <sys/types.h>
>
> #include <errno.h>
> #include <error.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
>
> #include <linux/types.h>
>
> #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
> #define KCOV_INIT_TABLE _IOR('c', 2, unsigned long)
> #define KCOV_ENABLE _IO('c', 100)
> #define KCOV_DISABLE _IO('c', 101)
>
> int main(int argc, char *argv[])
> {
> int fd = open("/sys/kernel/debug/kcov", O_RDWR);
> if (fd == -1)
> error(1, errno, "open()");
>
> unsigned long size = 1 << 10;
> if (ioctl(fd, KCOV_INIT_TABLE, size) != 0)
> error(1, errno, "ioctl(KCOV_INIT_TABLE)");
>
> void *mem = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> if (mem == MAP_FAILED)
> error(1, errno, "mmap()");
>
> /* Start kernel instrumentation */
> if (ioctl(fd, KCOV_ENABLE, 0) != 0)
> error(1, errno, "ioctl(KCOV_ENABLE)");
>
> printf("Hello world!\n");
>
> /* End kernel instrumentation*/
> if (ioctl(fd, KCOV_DISABLE, 0) != 0)
> error(1, errno, "ioctl(KCOV_DISABLE)");
>
> /* Hex dump of memory area */
> unsigned char *mem2 = mem;
> for (unsigned int i = 0; i < size / sizeof(i); ++i) {
> printf("%02x ", mem2[i]);
> if (i % 32 == 31)
> printf("\n");
> }
>
> close(fd);
> return 0;
> }
>
> This patch is a collaboration between Quentin Casasnovas and Vegard Nossum.
>
> v2: As per Dmitry's suggestions:
> - Initialize location after barrier
> - Avoid additional indirections in __sanitizer_cov_trace_pc
> - Renamed KCOV_INIT_AFL/KCOV_MODE_AFL to KCOV_INIT_TABLE/KCOV_MODE_TABLE.
>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Cc: Michal Zalewski <lcamtuf@...il.com>
> Cc: Kees Cook <keescook@...omium.org>
> Signed-off-by: Quentin Casasnovas <quentin.casasnovas@...cle.com>
> Signed-off-by: Vegard Nossum <vegard.nossum@...cle.com>
> ---
> include/linux/kcov.h | 6 ++++++
> include/linux/sched.h | 10 ++++++++--
> include/uapi/linux/kcov.h | 1 +
> kernel/kcov.c | 37 ++++++++++++++++++++++++++++++++++++-
> 4 files changed, 51 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/kcov.h b/include/linux/kcov.h
> index 2883ac98c280..5450b8296113 100644
> --- a/include/linux/kcov.h
> +++ b/include/linux/kcov.h
> @@ -18,6 +18,12 @@ enum kcov_mode {
> * Covered PCs are collected in a per-task buffer.
> */
> KCOV_MODE_TRACE = 1,
> + /*
> + * AFL-style collection.
> + * Covered branches are hashed and collected in a fixed size buffer
> + * (see AFL documentation for more information).
> + */
> + KCOV_MODE_TABLE = 2,
> };
>
> #else
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 348f51b0ec92..31f1bde64961 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1919,8 +1919,14 @@ struct task_struct {
> #ifdef CONFIG_KCOV
> /* Coverage collection mode enabled for this task (0 if disabled). */
> enum kcov_mode kcov_mode;
> - /* Size of the kcov_area. */
> - unsigned kcov_size;
> + union {
> + /* Size of the kcov_area. */
> + unsigned kcov_size;
> + /* Mask to fit within kcov_area */
> + unsigned kcov_mask;
> + };
> + /* Hash of previous branch taken, to differentiate A > B from B > A */
> + unsigned long kcov_prev_location;
> /* Buffer for coverage collection. */
> void *kcov_area;
> /* kcov desciptor wired with this task or NULL. */
> diff --git a/include/uapi/linux/kcov.h b/include/uapi/linux/kcov.h
> index 574e22ec640d..19b8ff763243 100644
> --- a/include/uapi/linux/kcov.h
> +++ b/include/uapi/linux/kcov.h
> @@ -4,6 +4,7 @@
> #include <linux/types.h>
>
> #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
> +#define KCOV_INIT_TABLE _IOR('c', 2, unsigned long)
> #define KCOV_ENABLE _IO('c', 100)
> #define KCOV_DISABLE _IO('c', 101)
>
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index c2aa93851f93..8056013df16b 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -5,6 +5,7 @@
> #include <linux/types.h>
> #include <linux/file.h>
> #include <linux/fs.h>
> +#include <linux/hash.h>
> #include <linux/mm.h>
> #include <linux/printk.h>
> #include <linux/slab.h>
> @@ -83,6 +84,18 @@ void notrace __sanitizer_cov_trace_pc(void)
> area[pos] = _RET_IP_;
> WRITE_ONCE(area[0], pos);
> }
> + } else if (mode == KCOV_MODE_TABLE) {
> + unsigned char *area;
> + unsigned long location;
> +
> + /* See above */
> + barrier();
> +
> + location = _RET_IP_;
> + area = t->kcov_area;
> +
> + ++area[(t->kcov_prev_location ^ location) & t->kcov_mask];
> + t->kcov_prev_location = hash_long(location, BITS_PER_LONG);
> }
> }
> EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
> @@ -106,6 +119,7 @@ void kcov_task_init(struct task_struct *t)
> t->kcov_size = 0;
> t->kcov_area = NULL;
> t->kcov = NULL;
> + t->kcov_prev_location = hash_long(0, BITS_PER_LONG);
> }
>
> void kcov_task_exit(struct task_struct *t)
> @@ -205,6 +219,23 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
> kcov->size = size * sizeof(unsigned long);
> kcov->mode = KCOV_MODE_TRACE;
> return 0;
> + case KCOV_INIT_TABLE:
> + size = arg;
> +
> + if (kcov->mode != KCOV_MODE_DISABLED)
> + return -EBUSY;
> +
> + /*
> + * We infer the index in the table buffer from the return
> + * address of the caller and need a fast way to mask the
> + * relevant bits.
> + */
> + if (!is_power_of_2(size))
> + return -EINVAL;
> +
> + kcov->size = size;
> + kcov->mode = KCOV_MODE_TABLE;
> + return 0;
> case KCOV_ENABLE:
> /*
> * Enable coverage for the current task.
> @@ -221,8 +252,12 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
> return -EBUSY;
> t = current;
> /* Cache in task struct for performance. */
> - t->kcov_size = kcov->size / sizeof(unsigned long);
> + if (kcov->mode == KCOV_MODE_TRACE)
> + t->kcov_size = kcov->size / sizeof(unsigned long);
> + else if (kcov->mode == KCOV_MODE_TABLE)
> + t->kcov_mask = kcov->size - 1;
> t->kcov_area = kcov->area;
> + t->kcov_prev_location = hash_long(0, BITS_PER_LONG);
> /* See comment in __sanitizer_cov_trace_pc(). */
> barrier();
> WRITE_ONCE(t->kcov_mode, kcov->mode);
> --
Thanks!
Reviewed-and-tested-by: Dmitry Vyukov <dvyukov@...gle.com>
I've tested that KCOV_MODE_TRACE works for me with these two patches applied.
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