linux-kernel - Re: [PATCH 08/14] lib/kasprintf.c: add sanity check to kvasprintf

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Message-ID: <CAHp75Vff6mPg+EPWrmZS+Y4NhpTJ_k+uOG946E3Kp3VcUHLp-w@mail.gmail.com>
Date:	Tue, 24 Nov 2015 01:10:20 +0200
From:	Andy Shevchenko <andy.shevchenko@...il.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 08/14] lib/kasprintf.c: add sanity check to kvasprintf

On Mon, Nov 23, 2015 at 11:29 PM, Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
> kasprintf relies on being able to replay the formatting and getting
> the same result (in particular, the same length). This will almost
> always work, but it is possible that the object pointed to by a %s or
> %p argument changed under us (so we might get truncated output). Add a
> somewhat paranoid sanity check and let's see if it ever triggers.
>
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
>  lib/kasprintf.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/lib/kasprintf.c b/lib/kasprintf.c
> index f194e6e593e1..7f6c506a4942 100644
> --- a/lib/kasprintf.c
> +++ b/lib/kasprintf.c
> @@ -13,19 +13,21 @@
>  /* Simplified asprintf. */
>  char *kvasprintf(gfp_t gfp, const char *fmt, va_list ap)
>  {
> -       unsigned int len;
> +       unsigned int first, second;

vsnprintf() returns plain int.
Perhaps change the type to follow the function.

>         char *p;
>         va_list aq;
>
>         va_copy(aq, ap);
> -       len = vsnprintf(NULL, 0, fmt, aq);
> +       first = vsnprintf(NULL, 0, fmt, aq);
>         va_end(aq);
>
> -       p = kmalloc_track_caller(len+1, gfp);
> +       p = kmalloc_track_caller(first+1, gfp);
>         if (!p)
>                 return NULL;
>
> -       vsnprintf(p, len+1, fmt, ap);
> +       second = vsnprintf(p, first+1, fmt, ap);
> +       WARN(first != second, "different return values (%u and %u) from vsnprintf(\"%s\", ...)",
> +            first, second, fmt);
>
>         return p;
>  }
> --
> 2.6.1
>
> --
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-- 
With Best Regards,
Andy Shevchenko
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