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Subject[PATCH] kcov: add AFL-style tracing
Date
From: Quentin Casasnovas <quentin.casasnovas@oracle.com>

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_AFL _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_AFL, size) != 0)
error(1, errno, "ioctl(KCOV_INIT_AFL)");

void *mem = mmap(NULL, size * sizeof(long), 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; ++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.

Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Michal Zalewski <lcamtuf@gmail.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
---
include/linux/kcov.h | 6 ++++++
include/uapi/linux/kcov.h | 1 +
kernel/kcov.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 51 insertions(+)

diff --git a/include/linux/kcov.h b/include/linux/kcov.h
index 2883ac9..cb8eb3e 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_AFL = 2,
};

#else
diff --git a/include/uapi/linux/kcov.h b/include/uapi/linux/kcov.h
index 574e22e..0dc1cf9 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_AFL _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 a02f2dd..e6eb785 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>
@@ -34,6 +35,19 @@ struct kcov {
enum kcov_mode mode;
/* Size of arena (in long's for KCOV_MODE_TRACE). */
unsigned size;
+
+ union {
+ /* For KCOV_MODE_AFL */
+ struct {
+ /* (number of bytes) - 1, where number of
+ * bytes must be a power of 2. */
+ unsigned int mask;
+
+ /* Previous PC (for KCOV_MODE_AFL). */
+ unsigned int prev_location;
+ };
+ };
+
/* Coverage buffer shared with user space. */
void *area;
/* Task for which we collect coverage, or NULL. */
@@ -76,6 +90,19 @@ void notrace __sanitizer_cov_trace_pc(void)
area[pos] = _RET_IP_;
WRITE_ONCE(area[0], pos);
}
+ } else if (mode == KCOV_MODE_AFL) {
+ struct kcov *kcov;
+ unsigned char *area;
+ unsigned long location = _RET_IP_;
+
+ /* See above */
+ barrier();
+
+ kcov = t->kcov;
+ area = kcov->area;
+
+ ++area[(kcov->prev_location ^ location) & kcov->mask];
+ kcov->prev_location = hash_long(location, BITS_PER_LONG);
}
}
EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
@@ -200,6 +227,23 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
kcov->size = size;
kcov->mode = KCOV_MODE_TRACE;
return 0;
+ case KCOV_INIT_AFL:
+ if (kcov->mode != KCOV_MODE_DISABLED)
+ return -EBUSY;
+
+ /*
+ * AFL mode needs a fixed-size area that is a power of 2.
+ */
+ size = arg;
+ if (size < 2 || size > INT_MAX / sizeof(unsigned long))
+ return -EINVAL;
+ if (!is_power_of_2(size))
+ return -EINVAL;
+ kcov->size = size;
+ kcov->mask = (size * sizeof(unsigned long)) - 1;
+ kcov->prev_location = hash_long(0, BITS_PER_LONG);
+ kcov->mode = KCOV_MODE_AFL;
+ return 0;
case KCOV_ENABLE:
/*
* Enable coverage for the current task.
--
1.9.1
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