linux-kernel - Re: WARNING in snd_pcm_hw

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Date:   Mon, 09 Apr 2018 17:18:05 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     syzbot <syzbot+803e0047ac3a3096bb4f@...kaller.appspotmail.com>
Cc:     <alsa-devel@...a-project.org>, <syzkaller-bugs@...glegroups.com>,
        <perex@...ex.cz>, <o-takashi@...amocchi.jp>,
        <linux-kernel@...r.kernel.org>
Subject: Re: WARNING in snd_pcm_hw_params

On Fri, 06 Apr 2018 02:02:02 +0200,
syzbot wrote:
> 
> WARNING: CPU: 0 PID: 4468 at sound/core/pcm_native.c:620
> snd_pcm_hw_params_choose sound/core/pcm_native.c:620 [inline]

This is a WARN_ON() to catch the inconsistent hw_params call.
This shouldn't happen in theory, but the reality looks weirder.

The necessary "fix" is just to get rid of snd_BUG_ON(), but still
wonder which condition it hits.  If it's an empty parameter, the
previous snd_pcm_hw_refine() should have caught and returned the error
beforehand...

In anyway this can't be a real crash, let's remove the warning to stop
the chorus of syzbot birds.

thanks,

Takashi

-- 8< --
From: Takashi Iwai <tiwai@...e.de>
Subject: [PATCH] ALSA: pcm: Remove WARN_ON() at snd_pcm_hw_params() error

snd_pcm_hw_params() may hit WARN_ON() when an inconsistent parameter
set is passed.  Although this should have been caught by the former
snd_pcm_hw_refine() and returned the error, syzkaller succeeded to
pass through and hit the WARN_ON(), which in turn made it panic
unnecessarily.

For reducing this kind of noise, let's drop snd_BUG_ON() check.

Reported-by: syzbot+803e0047ac3a3096bb4f@...kaller.appspotmail.com
Signed-off-by: Takashi Iwai <tiwai@...e.de>
---
 sound/core/pcm_native.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index b84554893fab..35ffccea94c3 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -617,7 +617,7 @@ static int snd_pcm_hw_params_choose(struct snd_pcm_substream *pcm,
 			changed = snd_pcm_hw_param_first(pcm, params, *v, NULL);
 		else
 			changed = snd_pcm_hw_param_last(pcm, params, *v, NULL);
-		if (snd_BUG_ON(changed < 0))
+		if (changed < 0)
 			return changed;
 		if (changed == 0)
 			continue;
-- 
2.16.3