Re: [PATCH bpf-next v4 1/2] bpf/helpers: bpf_strnstr: Exact match length

[Viktor Malik <vmalik@redhat.com>]

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;