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Message-Id: <20171011095459.70721-3-glider@google.com>
Date: Wed, 11 Oct 2017 11:54:59 +0200
From: Alexander Potapenko <glider@...gle.com>
To: akpm@...ux-foundation.org, mark.rutland@....com,
alex.popov@...ux.com, aryabinin@...tuozzo.com,
quentin.casasnovas@...cle.com, dvyukov@...gle.com,
andreyknvl@...gle.com, keescook@...omium.org,
vegard.nossum@...cle.com
Cc: syzkaller@...glegroups.com, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: [PATCH v4 3/3] kcov: update documentation
From: Victor Chibotaru <tchibo@...gle.com>
The updated documentation describes new KCOV mode for collecting
comparison operands.
Signed-off-by: Victor Chibotaru <tchibo@...gle.com>
Signed-off-by: Alexander Potapenko <glider@...gle.com>
Cc: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Andrey Konovalov <andreyknvl@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Mark Rutland <mark.rutland@....com>
Cc: Alexander Popov <alex.popov@...ux.com>
Cc: Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Vegard Nossum <vegard.nossum@...cle.com>
Cc: Quentin Casasnovas <quentin.casasnovas@...cle.com>
Cc: syzkaller@...glegroups.com
Cc: linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org
---
v3: - update the test program to match new uapi
v2: - reflect the changes to kcov.c in the test program.
---
Documentation/dev-tools/kcov.rst | 99 ++++++++++++++++++++++++++++++++++++++--
1 file changed, 95 insertions(+), 4 deletions(-)
diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
index 44886c91e112..c2f6452e38ed 100644
--- a/Documentation/dev-tools/kcov.rst
+++ b/Documentation/dev-tools/kcov.rst
@@ -12,19 +12,30 @@ To achieve this goal it does not collect coverage in soft/hard interrupts
and instrumentation of some inherently non-deterministic parts of kernel is
disabled (e.g. scheduler, locking).
-Usage
------
+kcov is also able to collect comparison operands from the instrumented code
+(this feature currently requires that the kernel is compiled with clang).
+
+Prerequisites
+-------------
Configure the kernel with::
CONFIG_KCOV=y
CONFIG_KCOV requires gcc built on revision 231296 or later.
+
+If the comparison operands need to be collected, set::
+
+ CONFIG_KCOV_ENABLE_COMPARISONS=y
+
Profiling data will only become accessible once debugfs has been mounted::
mount -t debugfs none /sys/kernel/debug
-The following program demonstrates kcov usage from within a test program:
+Coverage collection
+-------------------
+The following program demonstrates coverage collection from within a test
+program using kcov:
.. code-block:: c
@@ -44,6 +55,9 @@ The following program demonstrates kcov usage from within a test program:
#define KCOV_DISABLE _IO('c', 101)
#define COVER_SIZE (64<<10)
+ #define KCOV_TRACE_PC 0
+ #define KCOV_TRACE_CMP 1
+
int main(int argc, char **argv)
{
int fd;
@@ -64,7 +78,7 @@ The following program demonstrates kcov usage from within a test program:
if ((void*)cover == MAP_FAILED)
perror("mmap"), exit(1);
/* Enable coverage collection on the current thread. */
- if (ioctl(fd, KCOV_ENABLE, 0))
+ if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_PC))
perror("ioctl"), exit(1);
/* Reset coverage from the tail of the ioctl() call. */
__atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
@@ -111,3 +125,80 @@ The interface is fine-grained to allow efficient forking of test processes.
That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode,
mmaps coverage buffer and then forks child processes in a loop. Child processes
only need to enable coverage (disable happens automatically on thread end).
+
+Comparison operands collection
+------------------------------
+Comparison operands collection is similar to coverage collection:
+
+.. code-block:: c
+
+ /* Same includes and defines as above. */
+
+ /* Number of 64-bit words per record. */
+ #define KCOV_WORDS_PER_CMP 4
+
+ /*
+ * The format for the types of collected comparisons.
+ *
+ * Bit 0 shows whether one of the arguments is a compile-time constant.
+ * Bits 1 & 2 contain log2 of the argument size, up to 8 bytes.
+ */
+
+ #define KCOV_CMP_CONST (1 << 0)
+ #define KCOV_CMP_SIZE(n) ((n) << 1)
+ #define KCOV_CMP_MASK KCOV_CMP_SIZE(3)
+
+ int main(int argc, char **argv)
+ {
+ int fd;
+ uint64_t *cover, type, arg1, arg2, is_const, size;
+ unsigned long n, i;
+
+ fd = open("/sys/kernel/debug/kcov", O_RDWR);
+ if (fd == -1)
+ perror("open"), exit(1);
+ if (ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE))
+ perror("ioctl"), exit(1);
+ /*
+ * Note that the buffer pointer is of type uint64_t*, because all
+ * the comparison operands are promoted to uint64_t.
+ */
+ cover = (uint64_t *)mmap(NULL, COVER_SIZE * sizeof(unsigned long),
+ PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+ if ((void*)cover == MAP_FAILED)
+ perror("mmap"), exit(1);
+ /* Note KCOV_TRACE_CMP instead of KCOV_TRACE_PC. */
+ if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_CMP))
+ perror("ioctl"), exit(1);
+ __atomic_store_n(&cover[0], 0, __ATOMIC_RELAXED);
+ read(-1, NULL, 0);
+ /* Read number of comparisons collected. */
+ n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
+ for (i = 0; i < n; i++) {
+ type = cover[i * KCOV_WORDS_PER_CMP + 1];
+ /* arg1 and arg2 - operands of the comparison. */
+ arg1 = cover[i * KCOV_WORDS_PER_CMP + 2];
+ arg2 = cover[i * KCOV_WORDS_PER_CMP + 3];
+ /* ip - caller address. */
+ ip = cover[i * KCOV_WORDS_PER_CMP + 4];
+ /* size of the operands. */
+ size = 1 << ((type & KCOV_CMP_MASK) >> 1);
+ /* is_const - true if either operand is a compile-time constant.*/
+ is_const = type & KCOV_CMP_CONST;
+ printf("ip: 0x%lx type: 0x%lx, arg1: 0x%lx, arg2: 0x%lx, "
+ "size: %lu, %s\n",
+ ip, type, arg1, arg2, size,
+ is_const ? "const" : "non-const");
+ }
+ if (ioctl(fd, KCOV_DISABLE, 0))
+ perror("ioctl"), exit(1);
+ /* Free resources. */
+ if (munmap(cover, COVER_SIZE * sizeof(unsigned long)))
+ perror("munmap"), exit(1);
+ if (close(fd))
+ perror("close"), exit(1);
+ return 0;
+ }
+
+Note that the kcov modes (coverage collection or comparison operands) are
+mutually exclusive.
--
2.15.0.rc0.271.g36b669edcc-goog
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