linux-kernel - Re: [PATCH] perf: Incorrect use of snprintf results in SEGV

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Message-ID: <20120307184455.GA13565@elliptictech.com>
Date:	Wed, 7 Mar 2012 13:44:55 -0500
From:	Nick Bowler <nbowler@...iptictech.com>
To:	Peter Seebach <peter.seebach@...driver.com>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Anton Blanchard <anton@...ba.org>, paulus@...ba.org,
	peterz@...radead.org, mingo@...e.hu, dsahern@...il.com,
	fweisbec@...il.com, yanmin_zhang@...ux.intel.com,
	emunson@...bm.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: Incorrect use of snprintf results in SEGV

On 2012-03-06 19:29 -0600, Peter Seebach wrote:
> To frame it another way:  Imagine an alternative function, called
> slenprintf(), which is just like snprintf except that it returns the
> number of bytes written instead of the number it would have liked to
> write in the event that the buffer isn't big enough.  And also
> vslenprintf(), analogous to vsnprintf().
> 
> Now consider what happens if you have one and want the semantics of the
> other:
[...]
> size_t
> mysnprintf(char *buffer, size_t len, char *fmt, ...) {
>     size_t ret;
>     va_list ap;
>     va_start(ap, fmt)
>     ret = vslenprintf(buffer, len, fmt, ap);
>     /* now what? */

To answer the question, one "solution" here is to run in a loop
allocating larger and larger buffers until ret is strictly less
than len, then (for this function) free the allocated buffer.

There are a couple functions in POSIX that work this way (*cough*
readlink *cough*), and it's *ugly*.

Cheers,
-- 
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)

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