linux-kernel - Re: [PATCH] media: Fix invalid link creation when source entity has 0 pads

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Message-ID: <20250402002948.GC4845@pendragon.ideasonboard.com>
Date: Wed, 2 Apr 2025 03:29:48 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Gabriel <gshahrouzi@...il.com>
Cc: Ricardo Ribalda <ribalda@...omium.org>, hdegoede@...hat.com,
	mchehab@...nel.org, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	syzbot+701fc9cc0cb44e2b0fe9@...kaller.appspotmail.com,
	skhan@...uxfoundation.org, kernelmentees@...ts.linuxfoundation.org
Subject: Re: [PATCH] media: Fix invalid link creation when source entity has
 0 pads

Hi Gabriel,

On Sat, Mar 29, 2025 at 01:50:00PM -0400, Gabriel wrote:
> Hi Laurent,
> 
> I’ve analyzed the bug report, and the root cause of the
> "WARNING-media_create_pad_link" issue is a mismatch in terminal
> references in the USB descriptor.
> 
> The format type descriptor references terminal ID 6, while the audio
> streaming interface descriptor points to terminal ID 5. This
> discrepancy triggers the warning: "No streaming interface found for
> terminal 6", followed by the media pad link warning.

Can you share the USB descriptors.

> I confirmed this by changing the terminal ID in the format descriptor
> from 6 to 5, which eliminates both warnings. This shows the warning is
> correctly identifying an invalid descriptor configuration, not a
> kernel bug.

There's still something not quite right. uvc_entity->num_pads should
always be equal to the corresponding media_entity->num_pads. That's not
the case here, and I think it indicates a bug.

> Since the USB descriptor is invalid, I believe the warning is
> necessary and should remain. The code should stay as is.

There should be a warning, but I think it needs to be caught in a
different place, earlier.

> On Tue, Mar 25, 2025 at 8:13 PM Laurent Pinchart wrote:
> >
> > On Tue, Mar 25, 2025 at 06:05:00PM -0400, Gabriel wrote:
> > > Hi Ricardo,
> > >
> > > > I cannot reach that URL
> > > I was unable to access the URL from my email client when I initially
> > > sent the email, but a couple of hours later, I was able to. Initially,
> > > copying and pasting the URL into the browser provided a workaround.
> > >
> > > > Shouldn't it be?:
> > > > Fixes: 4ffc2d89f38a ("[media] uvcvideo: Register subdevices for each entity")
> > > You're right, I incorrectly referenced the wrong commit. However, I’m
> > > not certain if it should reference a96aa5342d57 (Fixes: a96aa5342d57 -
> > > '[media] uvcvideo: Ignore entities for terminals with no supported
> > > format') as it's the latest commit affecting the line I'm changing or
> > > the one you mentioned.
> > >
> > > > Shouldn't source->num_pads be the same as remote->num_pads?
> > > The fuzzer (Syzkaller) that triggered the warning appears to have
> > > encountered a case where source->num_pads and remote->num_pads were
> > > different. When analyzing the case in GDB, remote->num_pads was 1,
> > > while source->num_pads was 0.
> >
> > This seems like the real bug that should be fixed.
> >
> > > > Are you sure that your kernel does not contain?
> > > > https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/media/usb/uvc/uvc_entity.c?id=41ddb251c68ac75c101d3a50a68c4629c9055e4c
> > > Yes, it should be included since I am running the upstream kernel.

-- 
Regards,

Laurent Pinchart