linux-kernel - Re: [PATCH] Make snd_BUG_ON() always evaluate and return the conditional expression.

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Date:	Wed, 06 Mar 2013 10:35:26 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	Christine Spang <christine.spang@...cle.com>
Cc:	Jaroslav Kysela <perex@...ex.cz>, alsa-devel@...a-project.org,
	Jamie Iles <jamie.iles@...cle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Make snd_BUG_ON() always evaluate and return the conditional expression.

At Tue, 05 Mar 2013 15:41:06 -0500,
Christine Spang wrote:
> 
> On 03/05/2013 04:05 AM, Takashi Iwai wrote:
> > At Mon,  4 Mar 2013 17:02:59 -0500,
> > Christine Spang wrote:
> >> Having snd_BUG_ON() only evaluate its conditional when CONFIG_SND_DEBUG
> >> is set leads to frequent bugs, since other similar macros in the kernel
> >> have different behavior. Let's make snd_BUG_ON() act like those macros
> >> so it will stop being accidentally misused.
> >>
> >> Signed-off-by: Christine Spang <christine.spang@...cle.com>
> > Sounds reasonable.  The dependency on CONFIG_SND_DEBUG was for
> > allowing more optimization, but since we use this for more places than
> > expected, this change would be safer indeed.
> >
> > If no one has objection, I'll apply it for 3.10 kernel.
> >
> >
> > thanks,
> >
> > Takashi
> 
> This ought to be considered for 3.9 and stable@ as
> well. It fixes NULL derefs all over the place, e.g.

No, it doesn't "fix".

> sound/core/device.c:126
> 
>     if (snd_BUG_ON(!card || !device_data))
>         return -ENXIO;
>     list_for_each_entry(dev, &card->devices, list) {
>        [...]
> 
> If card == NULL and CONFIG_SND_DEBUG is off, this code will NULL deref.

Yes, but such a condition must not happen.  If card = NULL is passed,
it's an error of the caller side, and the code there doesn't have to
take care in general.

In other words, it's a good stuff to catch a bug.  But it's never been
there for "fixing" anything per se.  That's the reason why it is
turned off when no debug option is set.

But I agree that enabling such a check would be safer, indeed, since
people tend to rely on the checks.  So I'm for your patch, but this
isn't the thing for the current tree or stable trees.

> There are some 600 other instances of snd_BUG_ON() being used dubiously
> in the current tree. Some of these instances even perform extra cleanup
> before returning in error conditions.

Give more concrete examples.  Such places must be fixed instead.

> It's really broken with
> CONFIG_SND_DEBUG off, and no major distro ships production kernels with
> this setting enabled.

If it's broken, it's the caller side, not the fact that snd_BUG_ON()
isn't compiled in.


Takashi
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