linux-ext4 - Re: [PATCH] fs/ext3: set pointer = NULL, after kfree it

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Message-ID: <20121231151953.GE7564@quack.suse.cz>
Date:	Mon, 31 Dec 2012 16:19:53 +0100
From:	Jan Kara <jack@...e.cz>
To:	Chen Gang <gang.chen@...anux.com>
Cc:	Theodore Ts'o <tytso@....edu>, jack@...e.cz,
	akpm@...ux-foundation.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] fs/ext3: set pointer = NULL, after kfree it

On Wed 26-12-12 10:33:12, Chen Gang wrote:
> 于 2012年12月26日 02:51, Theodore Ts'o 写道:
> > On Mon, Dec 24, 2012 at 01:58:14PM +0800, Chen Gang wrote:
> >>
> >>   set s->base = NULL, after kfree it.
> >>
> >> Signed-off-by: Chen Gang <gang.chen@...anux.com>
> > 
> > Was this to fix something flagged by some static code checker?  The
> > only caller of ext3_xattr_block_set() is ext3_xattr_set_handle(), and
> > s->base is stored on the stack, and as soon as ext3_xattr_block_set()
> > returns, the object will disappear.  So it seems pretty hard to see
> > how this could lead to a use-after-free bug.
> > 
> >     	       	       	 		- Ted
> > 
> > 
> 
>   this patch is not for a bug
> 
>   it is only for coding habit:
>     it is better to set is to NULL, after kfree it.
>     in the future, if another functions need call ext3_xattr_block_set, too.
>       it will avoid another members to make mistake.
  Well, we set pointers to NULL after kfree() only if there's a good chance
we ever look at the pointer again. This is not the case so I don't think
this change makes the code any more safer / easier to read.

									Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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