linux-kernel - Re: [PATCH] mm/sl[aou]b: make kfree() aware of error pointers

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Message-ID: <20140910152104.GS6549@mwanda>
Date:	Wed, 10 Sep 2014 18:21:04 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/sl[aou]b: make kfree() aware of error pointers

On Wed, Sep 10, 2014 at 04:26:46PM +0200, Jiri Kosina wrote:
> On Wed, 10 Sep 2014, Theodore Ts'o wrote:
> 
> > I'd much rather depending on better testing and static checkers to fix 
> > them, since kfree *is* a hot path.
> 
> BTW if we stretch this argument a little bit more, we should also kill the 
> ZERO_OR_NULL_PTR() check from kfree() and make it callers responsibility 
> to perform the checking only if applicable ... we are currently doing a 
> lot of pointless checking in cases where caller would be able to guarantee 
> that the pointer is going to be non-NULL.

What you're saying is that we should remove the ZERO_SIZE_PTR
completely.  ZERO_SIZE_PTR is a very useful idiom and also it's too late
to remove it because everything depends on it.

Returning ZERO_SIZE_PTR is not an error.  Callers shouldn't test for it.
It works like this:
1) User space says "copy zero items to somewhere."
2) The kernel says "here is a zero size pointer"
3) We do some stuff like:
	copy_from_user(zero_pointer, src, 0)
   or:
	for (i = 0; i < 0; i++)
4) The caller frees the ZERO_SIZE_PTR.
5) We return success.

If we get rid of it then we're start returning -ENOMEM all over the
place and that breaks userspace.  Or we introduce zero as a special case
for every kmalloc.

You would think there would be a lot of bugs with ZERO_SIZE_POINTERs
but they seem fairly rare to me.  There are some where we allocate a
zero length string and then put a NUL terminator at the end.

regards,
dan carpenter
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