[PATCH] lib/strscpy: avoid KASAN false positive

[Andrey Ryabinin]

strscpy() performs the word-at-a-time optimistic reads. So it may
may access the memory past the end of the object, which is perfectly fine
since strscpy() doesn't use that (past-the-end) data and makes sure the
optimistic read won't cross a page boundary.

But KASAN doesn't know anything about that so it will complain.
Let's just fallback to the byte-at-a-time reads under CONFIG_KASAN=y
to avoid false-positives.

Reported-by: Dave Jones <davej@xxxxxxxxxxxxxxxxx>
Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
---
 lib/string.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/lib/string.c b/lib/string.c
index ebbb99c775bd..8b93d2519d5a 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -199,6 +199,13 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
 		max = 0;
 #endif
 
+	/*
+	 * KASAN won't be happy about word-at-a-time
+	 * optimistic reads, so let's avoid them.
+	 */
+	if (IS_ENABLED(CONFIG_KASAN))
+		max = 0;
+
 	while (max >= sizeof(unsigned long)) {
 		unsigned long c, data;
 
-- 
2.13.0