kcov: support comparison operands collection

Enables kcov to collect comparison operands from instrumented code.
This is done by using Clang's -fsanitize=trace-cmp instrumentation
(currently not available for GCC).

The comparison operands help a lot in fuzz testing.  E.g.  they are used
in Syzkaller to cover the interiors of conditional statements with way
less attempts and thus make previously unreachable code reachable.

To allow separate collection of coverage and comparison operands two
different work modes are implemented.  Mode selection is now done via a
KCOV_ENABLE ioctl call with corresponding argument value.

Link: http://lkml.kernel.org/r/20171011095459.70721-1-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>

2 files changed, 32 insertions, 4 deletions

diff --git a/include/linux/kcov.h b/include/linux/kcov.h
index f5d8ce4..3ecf6f5 100644
--- a/include/linux/kcov.h
+++ b/include/linux/kcov.h
@@ -8,19 +8,23 @@ struct task_struct;
 
 #ifdef CONFIG_KCOV
 
-void kcov_task_init(struct task_struct *t);
-void kcov_task_exit(struct task_struct *t);
-
 enum kcov_mode {
 	/* Coverage collection is not enabled yet. */
 	KCOV_MODE_DISABLED = 0,
+	/* KCOV was initialized, but tracing mode hasn't been chosen yet. */
+	KCOV_MODE_INIT = 1,
 	/*
 	 * Tracing coverage collection mode.
 	 * Covered PCs are collected in a per-task buffer.
 	 */
-	KCOV_MODE_TRACE = 1,
+	KCOV_MODE_TRACE_PC = 2,
+	/* Collecting comparison operands mode. */
+	KCOV_MODE_TRACE_CMP = 3,
 };
 
+void kcov_task_init(struct task_struct *t);
+void kcov_task_exit(struct task_struct *t);
+
 #else
 
 static inline void kcov_task_init(struct task_struct *t) {}
diff --git a/include/uapi/linux/kcov.h b/include/uapi/linux/kcov.h
index 33eabbb..9529867 100644
--- a/include/uapi/linux/kcov.h
+++ b/include/uapi/linux/kcov.h
@@ -8,4 +8,28 @@
 #define KCOV_ENABLE			_IO('c', 100)
 #define KCOV_DISABLE			_IO('c', 101)
 
+enum {
+	/*
+	 * Tracing coverage collection mode.
+	 * Covered PCs are collected in a per-task buffer.
+	 * In new KCOV version the mode is chosen by calling
+	 * ioctl(fd, KCOV_ENABLE, mode). In older versions the mode argument
+	 * was supposed to be 0 in such a call. So, for reasons of backward
+	 * compatibility, we have chosen the value KCOV_TRACE_PC to be 0.
+	 */
+	KCOV_TRACE_PC = 0,
+	/* Collecting comparison operands mode. */
+	KCOV_TRACE_CMP = 1,
+};
+
+/*
+ * 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)
+
 #endif /* _LINUX_KCOV_IOCTLS_H */