Date: Tue, 14 Feb 2012 15:50:32 -0500
From: Nick Bowler <nbowler@elliptictech.com>
To: Christoph Lameter <cl@linux.com>
Cc: Xi Wang <xi.wang@gmail.com>, Dan Carpenter <dan.carpenter@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jesper Juhl <jj@chaosbits.net>, Jens Axboe <axboe@kernel.dk>,
        Pekka Enberg <penberg@kernel.org>, linux-kernel@vger.kernel.org,
        Matt Mackall <mpm@selenic.com>, David Rientjes <rientjes@google.com>
Subject: Re: Uninline kcalloc
Message-ID: <20120214205032.GA631@elliptictech.com>
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 <8F83835C-366C-46AC-A50A-3F680B7D2D83@gmail.com>
 <alpine.DEB.2.00.1202141032410.22250@router.home>
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On 2012-02-14 13:37 -0600, Christoph Lameter wrote:
> This patch still preserves kcalloc. But note that if kcalloc returns NULL
> then multiple conditions may have caused it. One is that the array is
> simply too large. The other may be that such an allocation is not possible
> due to fragmentation.
[...]
> +static inline long calculate_array_size(size_t n, size_t size)
> +{
> +	if (size != 0 && n > ULONG_MAX / size)
> +
> +		return 0;

This isn't right.  The above tests whether or not the result of the
multiplication will not be representable in an 'unsigned long'...

> +
> +	return n * size;

but then the result is assigned to a (signed) long, which may overflow 
if it's greater than LONG_MAX.

> +}
[...]
>  void *kcalloc(size_t n, size_t size, gfp_t flags)
>  {
> -	if (size != 0 && n > ULONG_MAX / size)
> -		return NULL;
> -	return __kmalloc(n * size, flags | __GFP_ZERO);
> +	size_t s = calculate_array_size(n, size);
> +
> +	if (s)
> +		return kzalloc(s, flags);
> +
> +	return NULL;
>  }

This hunk changes the behaviour of kcalloc if either of the two size parameters
is 0.

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

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