Merge branch 'fix-bpf_strnstr-len-error'

Rong Tao says:

====================
Fix bpf_strnstr len error

From: Rong Tao <[email protected]>

Fix bpf_strnstr() wrong 'len' parameter, bpf_strnstr("open", "open", 4)
should return 0 instead of -ENOENT. And fix a more general case when s2
is a suffix of the first len characters of s1.
====================

Link: https://patch.msgid.link/[email protected]
Signed-off-by: Andrii Nakryiko <[email protected]>

Author: anakryiko

kernel/bpf/helpers.c

@@ -3664,10 +3664,17 @@ __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;
+			/*
+			 * We allow reading an extra byte from s2 (note the
+			 * `i + j <= len` above) to cover the case when s2 is
+			 * a suffix of the first len chars of s1.
+			 */
+			if (i + j == len)
+				break;
 			__get_kernel_nofault(&c1, s1__ign + j, char, err_out);
 			if (c1 == '\0')
 				return -ENOENT;

tools/testing/selftests/bpf/progs/string_kfuncs_success.c

@@ -30,8 +30,12 @@ __test(2) int test_strcspn(void *ctx) { return bpf_strcspn(str, "lo"); }
 __test(6) int test_strstr_found(void *ctx) { return bpf_strstr(str, "world"); }
 __test(-ENOENT) int test_strstr_notfound(void *ctx) { return bpf_strstr(str, "hi"); }
 __test(0) int test_strstr_empty(void *ctx) { return bpf_strstr(str, ""); }
-__test(0) int test_strnstr_found(void *ctx) { return bpf_strnstr(str, "hello", 6); }
-__test(-ENOENT) int test_strnstr_notfound(void *ctx) { return bpf_strnstr(str, "hi", 10); }
+__test(0) int test_strnstr_found1(void *ctx) { return bpf_strnstr("", "", 0); }
+__test(0) int test_strnstr_found2(void *ctx) { return bpf_strnstr(str, "hello", 5); }
+__test(0) int test_strnstr_found3(void *ctx) { return bpf_strnstr(str, "hello", 6); }
+__test(-ENOENT) int test_strnstr_notfound1(void *ctx) { return bpf_strnstr(str, "hi", 10); }
+__test(-ENOENT) int test_strnstr_notfound2(void *ctx) { return bpf_strnstr(str, "hello", 4); }
+__test(-ENOENT) int test_strnstr_notfound3(void *ctx) { return bpf_strnstr("", "a", 0); }
 __test(0) int test_strnstr_empty(void *ctx) { return bpf_strnstr(str, "", 1); }
 
 char _license[] SEC("license") = "GPL";