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Date: Wed, 21 Jun 2017 20:16:11 +0200
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Quentin Casasnovas <quentin.casasnovas@...cle.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
Vegard Nossum <vegard.nossum@...cle.com>,
Michal Zalewski <lcamtuf@...il.com>,
Kees Cook <keescook@...omium.org>,
andreyknvl <andreyknvl@...gle.com>,
Alexander Potapenko <glider@...gle.com>,
Victor Chibotaru <tchibo@...gle.com>,
syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH 2/2] kcov: add AFL-style tracing
On Thu, Nov 17, 2016 at 8:52 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> 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.
Hi Quentin,
Was it merged somewhere? I still don't see this upstream.
KCOV was merged via mm tree originally. Perhaps remail this to mm/Andrew.
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