bpf/helpers: bpf_strnstr: Exact match length

strnstr should not treat the ending '\0' of s2 as a matching character
if the parameter 'len' equal to s2 string length, 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.

Fixes: e913705 ("bpf: Add kfuncs for read-only string operations")
Signed-off-by: Rong Tao <[email protected]>

Author: Rtoax

kernel/bpf/helpers.c

@@ -3672,10 +3672,18 @@ __bpf_kfunc int bpf_strnstr(const char *s1__ign, const char *s2__ign, size_t len
 
 	guard(pagefault)();
 	for (i = 0; i < XATTR_SIZE_MAX; i++) {
-		for (j = 0; i + j < len && j < XATTR_SIZE_MAX; j++) {
+		for (j = 0; i + j <= len && j < XATTR_SIZE_MAX; j++) {
 			__get_kernel_nofault(&c2, s2__ign + j, char, err_out);
 			if (c2 == '\0')
 				return i;
+			/**
+			 * corner case i+j==len to ensure that we matched
+			 * entire s2. for example, param len=3:
+			 *     s1: A B C D E F  -> i==1
+			 *     s2:   B C D      -> j==2
+			 */
+			if (i + j == len)
+				break;
 			__get_kernel_nofault(&c1, s1__ign + j, char, err_out);
 			if (c1 == '\0')
 				return -ENOENT;