Re: Dead code in v4l2-mem2mem.c?

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Shaobo,

On Friday 17 Feb 2017 11:42:25 Shaobo wrote:
> Hi Laurent,
> 
> Thanks a lot for your reply.
> 
> I would like to also point out the inconsistency of using `v4l2_m2m_get_vq`
> inside drivers/media/v4l2-core/v4l2-mem2mem.c and inside other files. It
> appears to me almost all call sites of `v4l2_m2m_get_vq` in
> drivers/media/v4l2-core/v4l2-mem2mem.c does not have NULL check afterwards
> while in other files (e.g., drivers/media/platform/mx2_emmaprp.c) they do. I
> was wondering if there is special assumption on this function in mem2mem.c.

I don't see any case where the function could reasonably be called with a NULL 
context other than a severe driver bug. This being said, we need to audit the 
callers to make sure that's really the case. Would you like to do so and 
submit a patch ? :-)

> -----Original Message-----
> From: Laurent Pinchart [mailto:laurent.pinchart@ideasonboard.com]
> Sent: 2017年2月17日 3:26
> To: Shaobo <shaobo@cs.utah.edu>
> Cc: linux-media@vger.kernel.org; mchehab@kernel.org; hverkuil@xs4all.nl;
> sakari.ailus@linux.intel.com; ricardo.ribalda@gmail.com
> Subject: Re: Dead code in v4l2-mem2mem.c?
> 
> Hi Shaobo,
> 
> First of all, could you please make sure you send future mails to the linux-
> media mailing list in plain text only (no HTML) ? The mailing list server
> rejects HTML e-mails.
> 
> On Thursday 16 Feb 2017 16:08:25 Shaobo wrote:
> > Hi there,
> > 
> > My name is Shaobo He and I am a graduate student at University of
> > Utah. I am applying a static analysis tool to the Linux device
> > drivers, looking for NULL pointer dereference and accidentally found a
> > plausible dead code location in v4l2-mem2mem.c due to undefined behavior.
> > 
> > The following is the problematic code segment,
> > 
> > static struct v4l2_m2m_queue_ctx *get_queue_ctx(struct v4l2_m2m_ctx
> > *m2m_ctx,
> > 
> > 						  enum v4l2_buf_type type)
> > 
> > {
> > 
> > 	if (V4L2_TYPE_IS_OUTPUT(type))
> > 	
> > 		return &m2m_ctx->out_q_ctx;
> > 	
> > 	else
> > 	
> > 		return &m2m_ctx->cap_q_ctx;
> > 
> > }
> > 
> > struct vb2_queue *v4l2_m2m_get_vq(struct v4l2_m2m_ctx *m2m_ctx,
> > 
> > 				    enum v4l2_buf_type type)
> > 
> > {
> > 
> > 	struct v4l2_m2m_queue_ctx *q_ctx;
> > 	
> > 	q_ctx = get_queue_ctx(m2m_ctx, type);
> > 	if (!q_ctx)
> > 	
> > 		return NULL;
> > 	
> > 	return &q_ctx->q;
> > 
> > }
> > 
> > `get_queue_ctx` returns a pointer value that is an addition of the
> > base pointer address (`m2m_ctx`) to a non-zero offset. The following
> > is the definition of struct v4l2_m2m_ctx,
> > 
> > struct v4l2_m2m_ctx {
> > 
> > 	/* optional cap/out vb2 queues lock */
> > 	struct mutex			*q_lock;
> > 	
> > 	/* internal use only */
> > 	struct v4l2_m2m_dev		*m2m_dev;
> > 	
> > 	struct v4l2_m2m_queue_ctx	cap_q_ctx;
> > 	
> > 	struct v4l2_m2m_queue_ctx	out_q_ctx;
> > 	
> > 	/* For device job queue */
> > 	struct list_head		queue;
> > 	unsigned long			job_flags;
> > 	wait_queue_head_t		finished;
> > 	
> > 	void				*priv;
> > 
> > };
> > 
> > There is a NULL test in a caller of `get_queue_ctx` (line 85), which
> > appears problematic to me. I'm not sure if it is defined or feasible
> > under the context of Linux kernel. This blog
> > (https://wdtz.org/undefined-behavior-in-binutils-causes-segfault.html)
> > suggests that the NULL check can be optimized away because the only
> > case that the return value can be NULL triggers pointer overflow,
> > which is undefined.
> > 
> > Please let me know if it makes sense or not. Thanks for your time and
> > I am looking forward to your reply.
> 
> The NULL check is indeed wrong. I believe that the m2m_ctx argument passed
> to the v4l2_m2m_get_vq() function should never be NULL. We will however need
> to audit drivers to make sure that's the case. The NULL check could then be
> removed. Alternatively we could check m2m_ctx above the get_queue_ctx()
> call, which wouldn't require auditing drivers. It's a safe option, but
> would likely result in an unneeded NULL check.
> 
> --
> Regards,
> 
> Laurent Pinchart

-- 
Regards,

Laurent Pinchart