linux-kernel - Re: [PATCH] bpf: fix uninit-value in strnchr

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Date: Wed, 10 Apr 2024 08:28:01 +0800
From: Edward Adam Davis <eadavis@...com>
To: martin.lau@...ux.dev
Cc: andrii@...nel.org,
	ast@...nel.org,
	bpf@...r.kernel.org,
	daniel@...earbox.net,
	eadavis@...com,
	haoluo@...gle.com,
	john.fastabend@...il.com,
	jolsa@...nel.org,
	kpsingh@...nel.org,
	linux-kernel@...r.kernel.org,
	sdf@...gle.com,
	song@...nel.org,
	syzbot+9b8be5e35747291236c8@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com,
	yonghong.song@...ux.dev
Subject: Re: [PATCH] bpf: fix uninit-value in strnchr

On Tue, 9 Apr 2024 10:59:17 -0700, Martin KaFai Lau wrote:
> > 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);
> 
> I don't think it is correct either.
> 
> >   	if (!fmt_end)
> 
> e.g. what will strnchrnul return if fmt is not NULL terminated?
> 
> The current code is correct as is. Comment snippet from strnchr:
> 
> /*
>   * ...
>   *
>   * Note that the %NUL-terminator is considered part of the string, and can
>   * be searched for.
>   */
> char *strnchr(const char *s, size_t count, int c)
lib/string.c
  9 /**
  8  * strnchr - Find a character in a length limited string
  7  * @s: The string to be searched
  6  * @count: The number of characters to be searched
  5  * @c: The character to search for
  4  *
  3  * Note that the %NUL-terminator is considered part of the string, and can
  2  * be searched for.
  1  */
384 char *strnchr(const char *s, size_t count, int c) 
  1 {
  2         while (count--) {
  3                 if (*s == (char)c)           // Only when the length of s is 1, can NUL char be obtained
  4                         return (char *)s;
  5                 if (*s++ == '\0')            // When the length of s is greater than 1, the loop will terminate and return NULL, without obtaining a pointer to a NUL char
  6                         break;
  7         }
  8         return NULL;
  9 }
> 
> 
> >   		return -EINVAL;
> >   	fmt_size = fmt_end - fmt;