linux-kernel - Re: [PATCH] xfs: fix possible NULL dereference

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Message-ID: <20131022223314.GQ10553@sgi.com>
Date:	Tue, 22 Oct 2013 17:33:14 -0500
From:	Ben Myers <bpm@....com>
To:	Dave Chinner <david@...morbit.com>,
	Eric Sandeen <sandeen@...deen.net>,
	Geyslan Gregório Bem <geyslan@...il.com>
Cc:	Alex Elder <elder@...nel.org>,
	open list <linux-kernel@...r.kernel.org>,
	XFS FILESYSTEM <xfs@....sgi.com>
Subject: Re: [PATCH] xfs: fix possible NULL dereference

Hey Gents,

On Wed, Oct 23, 2013 at 09:02:54AM +1100, Dave Chinner wrote:
> On Tue, Oct 22, 2013 at 04:19:44PM -0500, Eric Sandeen wrote:
> > On 10/22/13 4:03 PM, Dave Chinner wrote:
> > > On Tue, Oct 22, 2013 at 03:49:01PM -0500, Eric Sandeen wrote:
> > >> On 10/22/13 3:39 PM, Dave Chinner wrote:
> > >>> On Tue, Oct 22, 2013 at 08:12:51AM -0200, Geyslan Gregório Bem wrote:
> > >>>> 2013/10/21 Dave Chinner <david@...morbit.com>:
> > >>>>> On Mon, Oct 21, 2013 at 07:00:59PM -0500, Eric Sandeen wrote:
> > >>>>>> On 10/21/13 6:56 PM, Dave Chinner wrote:
> > >>>>>>> On Mon, Oct 21, 2013 at 06:18:49PM -0500, Ben Myers wrote:
> > >>>>>
> > >>>>> Yes, but to continue the Devil's Advocate argument, the purpose of
> > >>>>> debug code isn't to enlightent the casual reader or drive-by
> > >>>>> patchers - it's to make life easier for people who actually spend
> > >>>>> time debugging the code. And the people who need the debug code
> > >>>>> are expected to understand why an ASSERT is not necessary. :)
> > >>>>>
> > >>>> Dave, Eric and Ben,
> > >>>>
> > >>>> This was catched by coverity (CID 102348).
> > >>>
> > >>> You should have put that in the patch description.
> > >>>
> > >>> Now I understand why there's been a sudden surge of irrelevant one
> > >>> line changes from random people that have never touched XFS before.
> > >>>
> > >>> <sigh>
> > >>>
> > >>> Ok, lets churn the code just to shut the stupid checker up. This
> > >>> doesn't fix a bug, it doesn't change behaviour, it just makes
> > >>> coverity happy. Convert it to the for loop plus ASSERT I mentioned
> > >>> in a previous message.
> > >>
> > >> You know, I respectfully disagree, but we might just have to agree
> > >> to disagree.  The code, as it stands, tests for a null ptr
> > >> and then dereferences it.  That's always going to raise some
> > >> eyebrows, coverity or not, debug code or not, drive by or not.
> > > 
> > >> So even for future developers, making the code more self-
> > >> documenting about this behavior would be a plus, whether it's by
> > >> comment, by explicit ASSERT(), or whatever.  (I don't think
> > >> that xfs_emerg() has quite enough context to make it obvious.)
> > > 
> > > Sure, but if weren't for the fact that Coverity warned about it,
> > > nobody other that us people who work on the XFS code day in, day out
> > > would have even cared about it.
> > > 
> > > That's kind of my point - again, as the Devil's Advocate - that
> > > coverity is encouraging drive-by "fixes" by people who don't
> > > actually understand any of the context, history and/or culture
> > > surrounding the code being modified.
> > 
> > They shouldn't have to, the code (or comments therein) should
> > make it obvious.  ;)  (in a perfect world...)
> 
> Obvious to whom, exactly?
> 
> That's the point I'm trying to make - "#ifdef DEBUG", two
> comments indicating that it's validating the list and printing a
> message just before it goes boom. That's pretty obvious code to
> anyone who is used to tracking down corrupted list problems...
> 
> > > I have no problems with real bugs being fixed, but if we are
> > > modifying code for no gain other than closing "coverity doesn't like
> > > it" bugs, then we *should* be questioning whether the change is
> > > really necessary.
> > 
> > But let's give Geyslan the benefit of the doubt, and realize that
> > Coverity does find real things, and even if it originated w/ a
> > Coverity CID, when one sees:
> > 
> > 	if (!a)
> > 		printk("a thing\n")
> > 
> > 	a = a->b = . . . 
> > 
> > it looks suspicious to pretty much anyone.  I don't think Geyslan
> > sent it to shut Coverity up, he sent it because it looked like
> > a bug worth fixing (after Coverity spotted it).
> > 
> > Let's not be too hard on him for trying; I appreciate it more
> > than spelling fixes and whitespace cleanups.  ;)
> 
> True, point taken. 

So, uh, lets go with the ASSERT approach then?  It seems to be a reasonable
middle ground.  ;)

Regards,
	Ben
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