Re: [Linux-uvc-devel] [BUG] NULL pointer dereference caused by uvcvideostress test

From: Alan Jenkins
Date: Thu Oct 16 2008 - 06:01:43 EST

Laurent Pinchart wrote:
> Hi Alan,
> On Wednesday 15 October 2008, Alan Jenkins wrote:
>> Laurent Pinchart wrote:
>>> On Wednesday 15 October 2008, Alan Jenkins wrote:
>>>> If you look at the trace, it happens as "hald-probe-video" opens the
>>>> video device. This is from Ubuntu 8.04. Possibly it's significant that
>>>> I use the camera first, to make sure it works (I use Kopete, the
>>>> settings dialogue includes a video test).
>>> The NULL pointer (or rather 0x00000030 pointer) dereference happens in
>>> video_open:
>>> file->f_op = fops_get(vfl->fops);
>>> if (file->f_op->open)
>>> err = file->f_op->open(inode, file);
>>> file->f_op ends up being NULL. Either vfl->fops is NULL to begin with, or
>>> fops_get failed to get a reference to the file_operations structure.
>>> I'd be surprised if vfl->fops was NULL. To rule out that case, can you
>>> add a BUG_ON(vfl->fops == NULL) before the call to fops_get ?
>>> I'm not too familiar with the module loader, but a quick look at the code
>>> shows that the module could be marked as being unloaded
>>> (MODULE_STATE_GOING) before its exit function is called. If this is the
>>> case video_open would still be called, as the video device would still be
>>> registered, but fops_get would fail in try_module_get and return a NULL
>>> pointer. It seems the pointer returned by fops_get should be tested in
>>> video_open.
>>> I've CC'ed the v4l maintainer to get his opinion on this.
>> I put one before and one after
>> 134 BUG_ON(vfl->fops == NULL);
>> 135 file->f_op = fops_get(vfl->fops);
>> 136 BUG_ON(file->f_op == NULL);
>> and the second one triggered
> This confirms my suspicion. Could you please try the attached patch ?

Yup, that seems to fix it.

I wonder if there are more instances of this error in other subsystems.

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