On Fri, Aug 29, 2025 at 9:32 AM Rong Tao rtoax@foxmail.com wrote:

From: Rong Tao rongtao@cestc.cn

I reworded patch subject a bit, please check the final version

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, fix just the `len == strlen(s2)` case.

And fix a more general case when s2 is a suffix of the first len characters of s1.

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

---

kernel/bpf/helpers.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index 401b4932cc49..91ad124844ae 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -3672,10 +3672,17 @@ __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;
  

•                   /**
  

fixed up comment style (extra *) when applying to bpf tree, thanks.

•                    * 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;
  

-- 2.51.0