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/[email protected]
Signed-off-by: Victor Chibotaru <[email protected]>
Signed-off-by: Alexander Potapenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Alexander Popov <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Vegard Nossum <[email protected]>
Cc: Quentin Casasnovas <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 24 insertions, 0 deletions

diff --git a/include/uapi/linux/kcov.h b/include/uapi/linux/kcov.h
index 33eabbb8ada1..9529867717a8 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 */