linux-kernel - Re: kfree(0)

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Message-ID: <alpine.LFD.0.999.0708180526280.3666@enigma.security.iitk.ac.in>
Date:	Sat, 18 Aug 2007 05:32:25 +0530 (IST)
From:	Satyam Sharma <satyam@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Christoph Lameter <clameter@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Tim Bird <tim.bird@...sony.com>,
	linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: kfree(0) - ok?

On Sat, 18 Aug 2007, Thomas Gleixner wrote:

> On Sat, 2007-08-18 at 00:42 +0300, Pekka Enberg wrote:
> > On 8/18/07, Christoph Lameter <clameter@....com> wrote:
> > > That was merged over my objections. IMHO ksize(NULL) should fail since we
> > > are determining the size of an unallocated object.
> > 
> > Agreed, especially as we have real zero-sized objects returned from
> > kmalloc() et al now.
> 
> Do we really ? 
> 
> If yes, who invented this 1980s reminiscence, where you got valid
> pointers for malloc(0) ?

No, not valid. Dereferencing it will oops. See ZERO_SIZE_PTR.

> This is completely stupid. You do not go into a bar and order an empty
> glass, just because you might eventually become thirsty later.

What we're doing presently is at least better than what SLAB did
previously (did return a valid pointer!), all this time :-)

I do agree with you in principle, of course. But it's not for the kind of
cases that you describe -- "kmalloc(0) just because I'd eventually want
to krealloc() it to something meaningful later". This was done because
there's a lot of lazy callsites that "don't want to write code for corner
cases explicitly". Sad, very sad, I say :-)

[ The krealloc() discussion on this thread came about when I noticed that
  it's the only callsite of ksize() that would reasonably / meaningfully
  want to deal with NULL ptrs, for whom I noticed (from Andrew's initial
  mail) that ksize(NULL) returned 0. As you know from the canonical
  semantics of realloc(), it _is_ supposed to deal with NULL ptrs, hence
  the discussion. ]

Satyam
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