On 8/29/25 04:12, Rong Tao wrote:

From: Rong Tao rongtao@cestc.cn

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

A good catch, thanks!

But this doesn't fix just the `len == strlen(s2)` case but a more general case when s2 is a suffix of the first len characters of s1, right? The commit message should reflect that.

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: e91370550f1f ("bpf: Add kfuncs for read-only string operations") Signed-off-by: Rong Tao rongtao@cestc.cn

---

kernel/bpf/helpers.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index 401b4932cc49..bf04881f96ec 100644 --- a/kernel/bpf/helpers.c +++ b/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
  

• 	 */
  

This is not really a good example as it's not clear whether D is a null byte or not. How about something like:

/** * 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;
  

Viktor

	__get_kernel_nofault(&c1, s1__ign + j, char, err_out);
	if (c1 == '\0')
		return -ENOENT;