linux-kernel - Re: [PATCH] bpf: strnchr not suitable for getting NUL-terminator

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Date: Wed, 10 Apr 2024 08:09:45 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Edward Adam Davis <eadavis@...com>, bpf@...r.kernel.org
Cc: andrii@...nel.org, ast@...nel.org, daniel@...earbox.net,
 haoluo@...gle.com, john.fastabend@...il.com, jolsa@...nel.org,
 kpsingh@...nel.org, linux-kernel@...r.kernel.org, martin.lau@...ux.dev,
 sdf@...gle.com, song@...nel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] bpf: strnchr not suitable for getting NUL-terminator


On 4/9/24 5:33 PM, Edward Adam Davis wrote:
> The strnchr() is not suitable for obtaining the end of a string with a length
> exceeding 1 and ending with a NUL character.

Could you give more detailed explanation with specific examples? I think
strnchr() does the right thing here. Note that if fmt is not NULL,
strnchrnul() never returns NULL pointer so in the change below,
'if (!fmt_end)' will be always false.

>
> Signed-off-by: Edward Adam Davis <eadavis@...com>
> ---
>   kernel/bpf/helpers.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 449b9a5d3fe3..07490eba24fe 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -826,7 +826,7 @@ int bpf_bprintf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
>   	u64 cur_arg;
>   	char fmt_ptype, cur_ip[16], ip_spec[] = "%pXX";
>   
> -	fmt_end = strnchr(fmt, fmt_size, 0);
> +	fmt_end = strnchrnul(fmt, fmt_size, 0);
>   	if (!fmt_end)
>   		return -EINVAL;
>   	fmt_size = fmt_end - fmt;