On Fri, Aug 29, 2025 at 9:32 AM Rong Tao <[email protected]> wrote:
>
> From: Rong Tao <[email protected]>
>
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 <[email protected]>
> ---
> 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
>
