linux-kernel - Re: [PATCH] lib: vsprintf: fix invalid arg check

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Date:	Fri, 12 Nov 2010 00:02:51 +0300
From:	Vasiliy Kulikov <segooon@...il.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	kernel-janitors@...r.kernel.org,
	André Goddard Rosa <andre.goddard@...il.com>,
	Joe Perches <joe@...ches.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Bjorn Helgaas <bjorn.helgaas@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib: vsprintf: fix invalid arg check

On Thu, Nov 11, 2010 at 12:38 -0800, David Rientjes wrote:
> On Thu, 11 Nov 2010, Vasiliy Kulikov wrote:
> 
> > > > > "size" is size_t.  If we want to check whether it was underflowed
> > > > > then we should cast it to ssize_t instead of int.  When
> > > > > sizeof(size_t) > sizeof(int) the code sees UINT_MAX as underflow,
> > > > > but it is not.
> > > > > 
> > > > 
> > > > Does this patch fix any actual observed problem?
> > 
> > I don't think so, this fix is more theoretical than practical.
> > However, maybe there is some crazy driver that fills array of 2GB with
> > s*printf().
> > 
> 
> All sizes passed to vsprintf() greater than INT_MAX are invalid; that's 
> what the original code is testing, warning, and handling correctly.

Not always correctly:

(int)(0xFFFFFFFFL + 2) = 1 is positive.

> No, it shouldn't, these functions return int.  INT_MAX is the largest 
> value we can handle successfully and that's why it is the special case for 
> sprintf() and vsprintf().
> 
> The code as it stands is correct not because of the type of the size but 
> rather the type of the return value.

OK, if the main reason here is return value type, then the correct
handling should be:

	/* Reject out-of-range values early.  Large positive sizes are
	   used for unknown buffer sizes. */
-	if (WARN_ON_ONCE((int) size < 0))
+	if (WARN_ON_ONCE(size > INT_MAX)
		return 0;

This should catch all underflows and too big integers.

-- 
Vasiliy
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