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. Best, Shaobo -----Original Message----- From: Laurent Pinchart [mailto:laurent.pinchart@xxxxxxxxxxxxxxxx] Sent: 2017年2月17日 3:26 To: Shaobo <shaobo@xxxxxxxxxxx> Cc: linux-media@xxxxxxxxxxxxxxx; mchehab@xxxxxxxxxx; hverkuil@xxxxxxxxx; sakari.ailus@xxxxxxxxxxxxxxx; ricardo.ribalda@xxxxxxxxx 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
