[PATCH] kunit/memcpy: Avoid pathological compile-time string size

[Kees Cook]

The memcpy() KUnit tests are trying to sanity-check run-time behaviors,
but tripped compile-time warnings about a pathological condition of a
too-small buffer being used for input. Avoid this by explicitly resizing
the buffer, but leaving the string short. Avoid the following warning:

lib/memcpy_kunit.c: In function 'strtomem_test':
include/linux/string.h:303:42: warning: 'strnlen' specified bound 4 exceeds source size 3 [-Wstringop-overread]
  303 |         memcpy(dest, src, min(_dest_len, strnlen(src, _dest_len)));     \
include/linux/minmax.h:32:39: note: in definition of macro '__cmp_once'
   32 |                 typeof(y) unique_y = (y);               \
      |                                       ^
include/linux/minmax.h:45:25: note: in expansion of macro '__careful_cmp'
   45 | #define min(x, y)       __careful_cmp(x, y, <)
      |                         ^~~~~~~~~~~~~
include/linux/string.h:303:27: note: in expansion of macro 'min'
  303 |         memcpy(dest, src, min(_dest_len, strnlen(src, _dest_len)));     \
      |                           ^~~
lib/memcpy_kunit.c:290:9: note: in expansion of macro 'strtomem'
  290 |         strtomem(wrap.output, input);
      |         ^~~~~~~~
lib/memcpy_kunit.c:275:27: note: source object allocated here
  275 |         static const char input[] = "hi";
      |                           ^~~~~

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Link: https://lore.kernel.org/linux-mm/202209070728.o3stvgVt-lkp@xxxxxxxxx
Fixes: 14fa26474621 ("string: Introduce strtomem() and strtomem_pad()")
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 lib/memcpy_kunit.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/memcpy_kunit.c b/lib/memcpy_kunit.c
index d22fa3838ee9..2b5cc70ac53f 100644
--- a/lib/memcpy_kunit.c
+++ b/lib/memcpy_kunit.c
@@ -272,7 +272,7 @@ static void memset_test(struct kunit *test)
 
 static void strtomem_test(struct kunit *test)
 {
-	static const char input[] = "hi";
+	static const char input[sizeof(unsigned long)] = "hi";
 	static const char truncate[] = "this is too long";
 	struct {
 		unsigned long canary1;
-- 
2.34.1