linux-kernel - Re: [PATCH bpf-next v3 1/2] bpf/helpers: bpf

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Message-ID: <CAEf4Bza9=xYzT=QcoZjiqn7rnfBP9bjtEuWLu9qZKKodwOjgZg@mail.gmail.com>
Date: Thu, 28 Aug 2025 10:04:29 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Rong Tao <rtoax@...mail.com>
Cc: ast@...nel.org, daniel@...earbox.net, vmalik@...hat.com, 
	Rong Tao <rongtao@...tc.cn>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	"open list:BPF [GENERAL] (Safe Dynamic Programs and Tools)" <bpf@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3 1/2] bpf/helpers: bpf_strnstr: Exact match length

On Thu, Aug 28, 2025 at 4:07 AM Rong Tao <rtoax@...mail.com> wrote:
>
> From: Rong Tao <rongtao@...tc.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@...tc.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
>