linux-kernel - Re: [regression] nf_iterate(), BUG: unable to handle kernel NULL pointer dereference

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Date:	Thu, 24 Jul 2008 21:59:33 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Patrick McHardy <kaber@...sh.net>, Ingo Molnar <mingo@...e.hu>,
	David Miller <davem@...emloft.net>, w@....eu,
	davidn@...idnewall.com, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, stefanr@...6.in-berlin.de,
	rjw@...k.pl, ilpo.jarvinen@...sinki.fi,
	Dave Jones <davej@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [regression] nf_iterate(), BUG: unable to handle kernel NULL
	pointer dereference


On Fri, 2008-07-25 at 09:39 +0800, Herbert Xu wrote:
> On Thu, Jul 24, 2008 at 12:47:19PM -0500, Matt Mackall wrote:
> >
> > Let's try this again: did you know that ksize could fail depending on
> > kernel configuration? Most of us would answer no. That suggests the API
> > is bad. This ranks 12 on Rusty's spectrum of user-friendly APIs:
> 
> I think you misunderstood my argument.  I never suggested changing
> the existing ksize interface to return an error onto unsuspecting
> users.  I suggested creating a new interface that is explicitly
> designed to return an error if the underlying implementation
> is unable to support this.

I think that could probably be made to work. Perhaps something like:

size_t kmalloc_extra(void *); /* how many extra bytes in this kmalloc?
*/

Which, if it didn't work, could return a nice safe 0. We could argue
about signedness a bit, but I think this would always be safe.

This will also work with all our current kmalloc implementations. The
trouble was calling ksize() on kmem_cache_alloc objects, which happens
to work with SLAB and SLOB.

-- 
Mathematics is the supreme nostalgia of our time.

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