kcov: update documentation

The updated documentation describes new KCOV mode for collecting
comparison operands.

Link: http://lkml.kernel.org/r/20171011095459.70721-3-glider@google.com
Signed-off-by: Victor Chibotaru <tchibo@google.com>
Signed-off-by: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Popov <alex.popov@linux.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Vegard Nossum <vegard.nossum@oracle.com>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: <syzkaller@googlegroups.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files 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.