kcsan: Fix 0-sized checks

Instrumentation of arbitrary memory-copy functions, such as user-copies,
may be called with size of 0, which could lead to false positives.

To avoid this, add a comparison in check_access() for size==0, which
will be optimized out for constant sized instrumentation
(__tsan_{read,write}N), and therefore not affect the common-case
fast-path.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index e3c7d8f34f2f..82c2bef827d4 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -456,6 +456,13 @@ static __always_inline void check_access(const volatile void *ptr, size_t size,
 	long encoded_watchpoint;
 
 	/*
+	 * Do nothing for 0 sized check; this comparison will be optimized out
+	 * for constant sized instrumentation (__tsan_{read,write}N).
+	 */
+	if (unlikely(size == 0))
+		return;
+
+	/*
 	 * Avoid user_access_save in fast-path: find_watchpoint is safe without
 	 * user_access_save, as the address that ptr points to is only used to
 	 * check if a watchpoint exists; ptr is never dereferenced.