linux-kernel - Re: Uninline kcalloc

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Message-ID: <20120215202427.GA7035@elliptictech.com>
Date:	Wed, 15 Feb 2012 15:24:27 -0500
From:	Nick Bowler <nbowler@...iptictech.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Xi Wang <xi.wang@...il.com>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jesper Juhl <jj@...osbits.net>, Jens Axboe <axboe@...nel.dk>,
	Pekka Enberg <penberg@...nel.org>,
	linux-kernel@...r.kernel.org, Matt Mackall <mpm@...enic.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: Uninline kcalloc

On 2012-02-15 15:17 -0500, Nick Bowler wrote:
> On 2012-02-14 15:24 -0600, Christoph Lameter wrote:
> > On Tue, 14 Feb 2012, Nick Bowler wrote:
> > 
> > > 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'...
> > 
> > Yes and so does the current kcalloc.
> 
> Well, the current kcalloc doesn't assign the result to a signed long.
> However, it does assign the result to a size_t, which makes one wonder
> why it's not testing against SIZE_MAX.

Of course, right after I send this I realize that we do not appear to
define SIZE_MAX in the kernel.  So s/SIZE_MAX/((size_t)-1)/g.

> If size_t has the same range as unsigned long on all architectures,
> then this confusion doesn't matter, but is that actually the case?
> 
> > > > +	return n * size;
> > >
> > > but then the result is assigned to a (signed) long, which may overflow
> > > if it's greater than LONG_MAX.
> > 
> > That can happen?
> 
> Yes, because LONG_MAX (the maximum value of your return type) is
> strictly less than ULONG_MAX (what you test against).  It's not hard to
> pick input numbers that multiply to something between LONG_MAX and
> ULONG_MAX, which will cause your function to return a negative value
> (standard C leaves the result of such a conversion implementation-
> defined, but I'll assume for now that it works this way for everything
> that compiles Linux).
> 
> Admittedly, your kcalloc change then assigns this negative value to a
> size_t, which will result in the correct positive value assuming
> SIZE_MAX == ULONG_MAX, but that's gratuitously roundabout.
> 
> [...]
> > > [...]
> > > >  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.
> > 
> > You want ZERO_PTR returns?
> >
> > NULL is one permissible return value of calloc() if size == 0. So we are
> > now deviating from user space conventions.
> 
> Sort of.  While standard C leaves it implementation-defined whether
> successful zero-sized allocations are possible, all sane implementations
> let them succeed.  Hence, portable C apps need to handle 0 as a special
> case, because there are insane implementations out there.  There's no
> reason for the kernel to be one of them.
> 
> Regardless, this was still a (presumably unintentional) change from
> the previous behaviour.
> 
> Cheers,
-- 
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)

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