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From:   Wenwen Wang <wang6495@umn.edu>
Date:   Tue, 8 May 2018 08:04:54 -0500
Message-ID: <CAAa=b7dZFqKxFg=ez5Q7H=94HvqrO16pWH6sEmwrWgx60xCxNw@mail.gmail.com>
Subject: Re: [PATCH] media: staging: atomisp: fix a potential missing-check bug
To:     Dan Carpenter <dan.carpenter@oracle.com>
Cc:     "open list:STAGING SUBSYSTEM" <devel@driverdev.osuosl.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kangjie Lu <kjlu@umn.edu>,
        "open list:STAGING - ATOMISP DRIVER" <linux-media@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        Hans Verkuil <hans.verkuil@cisco.com>,
        Sakari Ailus <sakari.ailus@linux.intel.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Alan Cox <alan@linux.intel.com>, Wenwen Wang <wang6495@umn.edu>
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On Tue, May 8, 2018 at 7:16 AM, Dan Carpenter <dan.carpenter@oracle.com> wrote:
> On Wed, May 02, 2018 at 05:38:49PM -0500, Wenwen Wang wrote:
>> At the end of atomisp_subdev_set_selection(), the function
>> atomisp_subdev_get_rect() is invoked to get the pointer to v4l2_rect. Since
>> this function may return a NULL pointer, it is firstly invoked to check
>> the returned pointer. If the returned pointer is not NULL, then the
>> function is invoked again to obtain the pointer and the memory content
>> at the location of the returned pointer is copied to the memory location of
>> r. In most cases, the pointers returned by the two invocations are same.
>> However, given that the pointer returned by the function
>> atomisp_subdev_get_rect() is not a constant, it is possible that the two
>> invocations return two different pointers. For example, another thread may
>> race to modify the related pointers during the two invocations.
>
> You're assuming a very serious race condition exists.
>
>
>> In that
>> case, even if the first returned pointer is not null, the second returned
>> pointer might be null, which will cause issues such as null pointer
>> dereference.
>
> And then complaining that if a really serious bug exists then this very
> minor bug would exist too...  If there were really a race condition like
> that then we'd want to fix it instead.  In other words, this is not a
> real life bug fix.
>
> But it would be fine as a readability or static checker fix so that's
> fine.

Thanks for your response. From the performance perspective, this bug
should also be fixed, as the second invocation is redundant if it is
expected to return a same pointer as the first one.

Wenwen