string: Add __realloc_size hint to kmemdup()

Add __realloc_size() hint to kmemdup() so the compiler can reason about
the length of the returned buffer. (These must not use __alloc_size,
since those include __malloc which says the contents aren't defined[1]).

[1] https://lore.kernel.org/linux-hardening/[email protected]/

Cc: Rasmus Villemoes <[email protected]>
Cc: Guenter Roeck <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Signed-off-by: Kees Cook <[email protected]>

Author: kees

include/linux/fortify-string.h

@@ -659,7 +659,8 @@ __FORTIFY_INLINE void *memchr_inv(const void * const POS0 p, int c, size_t size)
 	return __real_memchr_inv(p, c, size);
 }
 
-extern void *__real_kmemdup(const void *src, size_t len, gfp_t gfp) __RENAME(kmemdup);
+extern void *__real_kmemdup(const void *src, size_t len, gfp_t gfp) __RENAME(kmemdup)
+								    __realloc_size(2);
 __FORTIFY_INLINE void *kmemdup(const void * const POS0 p, size_t size, gfp_t gfp)
 {
 	size_t p_size = __struct_size(p);

include/linux/string.h

@@ -176,7 +176,7 @@ extern void kfree_const(const void *x);
 extern char *kstrdup(const char *s, gfp_t gfp) __malloc;
 extern const char *kstrdup_const(const char *s, gfp_t gfp);
 extern char *kstrndup(const char *s, size_t len, gfp_t gfp);
-extern void *kmemdup(const void *src, size_t len, gfp_t gfp);
+extern void *kmemdup(const void *src, size_t len, gfp_t gfp) __realloc_size(2);
 extern char *kmemdup_nul(const char *s, size_t len, gfp_t gfp);
 
 extern char **argv_split(gfp_t gfp, const char *str, int *argcp);