On Thu, Aug 28, 2025 at 4:07 AM Rong Tao rtoax@foxmail.com wrote:

From: Rong Tao rongtao@cestc.cn

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 rongtao@cestc.cn

kernel/bpf/helpers.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)

Add Fixes: tag?

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index 401b4932cc49..ced7132980fe 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -3672,10 +3672,12 @@ __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++) {
  

that <= is a bit subtle, and combined with extra i+j==len check below is a bit confusing, so I think it would be good to add a short comment explaining this corner case you caught and that we are doing this to ensure that we matched entire s2 (and thus we need to find NUL to confirm).

                    __get_kernel_nofault(&c2, s2__ign + j, char, err_out);
                    if (c2 == '\0')
                            return i;

•                   if (i + j == len)
  

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

-- 2.51.0