linux-kernel - Re: [PATCH 03/14] lib/vsprintf.c: eliminate potential race in string()

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Message-ID: <CAHp75VePn-1kge9NEJHjyAkVJ8xhKutjGqjfB_iUMjZx-Kk0OA@mail.gmail.com>
Date:	Tue, 24 Nov 2015 00:51:44 +0200
From:	Andy Shevchenko <andy.shevchenko@...il.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/14] lib/vsprintf.c: eliminate potential race in string()

On Mon, Nov 23, 2015 at 11:29 PM, Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
> If the string corresponding to a %s specifier can change under us, we
> might end up copying a \0 byte to the output buffer. There might be
> callers who expect the output buffer to contain a genuine C string
> whose length is exactly the snprintf return value (assuming truncation
> hasn't happened or has been checked for).
>
> We can avoid this by only passing over the source string once,
> stopping the first time we meet a nul byte (or when we reach the given
> precision), and then letting widen_string() handle left/right space
> padding. As a small bonus, this code reuse also makes the generated
> code slightly smaller.
>

Could it be pair of patches: a) re-use, b) optimize for fuzzy strings?

> Cc: Ingo Molnar <mingo@...nel.org>
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
>  lib/vsprintf.c | 28 +++++++++-------------------
>  1 file changed, 9 insertions(+), 19 deletions(-)
>
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index a021e6380404..63ca52366049 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -557,32 +557,22 @@ char *widen_string(char *buf, int n, char *end, struct printf_spec spec)
>  static noinline_for_stack
>  char *string(char *buf, char *end, const char *s, struct printf_spec spec)
>  {
> -       int len, i;
> +       int len = 0;
> +       size_t lim = spec.precision;

Just a nitpick: maybe longer first?

>
>         if ((unsigned long)s < PAGE_SIZE)
>                 s = "(null)";
>
> -       len = strnlen(s, spec.precision);
> -
> -       if (!(spec.flags & LEFT)) {
> -               while (len < spec.field_width--) {
> -                       if (buf < end)
> -                               *buf = ' ';
> -                       ++buf;
> -               }
> -       }
> -       for (i = 0; i < len; ++i) {
> -               if (buf < end)
> -                       *buf = *s;
> -               ++buf; ++s;
> -       }
> -       while (len < spec.field_width--) {
> +       while (lim--) {
> +               char c = *s++;
> +               if (!c)
> +                       break;
>                 if (buf < end)
> -                       *buf = ' ';
> +                       *buf = c;
>                 ++buf;
> +               ++len;
>         }
> -
> -       return buf;
> +       return widen_string(buf, len, end, spec);
>  }
>
>  static noinline_for_stack
> --
> 2.6.1
>
> --
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-- 
With Best Regards,
Andy Shevchenko
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