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    SubjectRe: [Linux-uvc-devel] [BUG] NULL pointer dereference caused by uvcvideo stress test
    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.

    Ta
    Alan


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