On Thu, Aug 28, 2025 at 4:07 AM Rong Tao <[email protected]> wrote:
>
> From: Rong Tao <[email protected]>
>
> 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 <[email protected]>
> ---
> 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
>
