bpf/helpers: bpf_strnstr: Exact match length

strnstr should not treat the ending '\0' of s2 as a matching character,
otherwise the parameter 'len' will be meaningless, for example:

    1. bpf_strnstr("openat", "open", 4) = -ENOENT
    2. bpf_strnstr("openat", "open", 5) = 0

This patch makes (1) return 0, indicating a successful match.

Signed-off-by: Rong Tao <[email protected]>

Author: Rtoax

kernel/bpf/helpers.c

@@ -3681,6 +3681,8 @@ __bpf_kfunc int bpf_strnstr(const char *s1__ign, const char *s2__ign, size_t len
 				return -ENOENT;
 			if (c1 != c2)
 				break;
+			if (j == len - 1)
+				return i;
 		}
 		if (j == XATTR_SIZE_MAX)
 			return -E2BIG;