linux-kernel - Re: [PATCH v3 20/24] media: imx: Add Camera Interface subdev driver

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Date:   Mon, 6 Feb 2017 17:52:48 -0800
From:   Steve Longerbeam <slongerbeam@...il.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     robh+dt@...nel.org, mark.rutland@....com, shawnguo@...nel.org,
        kernel@...gutronix.de, fabio.estevam@....com, mchehab@...nel.org,
        hverkuil@...all.nl, nick@...anahar.org, markus.heiser@...marIT.de,
        p.zabel@...gutronix.de, laurent.pinchart+renesas@...asonboard.com,
        bparrot@...com, geert@...ux-m68k.org, arnd@...db.de,
        sudipm.mukherjee@...il.com, minghsiu.tsai@...iatek.com,
        tiffany.lin@...iatek.com, jean-christophe.trotin@...com,
        horms+renesas@...ge.net.au, niklas.soderlund+renesas@...natech.se,
        robert.jarzmik@...e.fr, songjun.wu@...rochip.com,
        andrew-ct.chen@...iatek.com, gregkh@...uxfoundation.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org,
        devel@...verdev.osuosl.org,
        Steve Longerbeam <steve_longerbeam@...tor.com>
Subject: Re: [PATCH v3 20/24] media: imx: Add Camera Interface subdev driver


On 02/02/2017 02:35 PM, Russell King - ARM Linux wrote:
> <snip>
> However, "*vfd" contains a struct device, and you _correctly_ set the
> release function for "*vfd" to video_device_release via camif_videodev.
>
> However, if you try to rmmod imx-media, then you end up with a kernel
> warning that you're freeing memory containing a held lock, and later
> chaos ensues because kmalloc has been corrupted.
>
> The root cause of this is embedding the device structure within the
> video_device into the driver's private data.  *Any* structure what so
> ever that contains a kref is reference counted, and that includes
> struct device, and therefore also includes struct video_device.  What
> that means is that its lifetime is _not_ under _your_ control, and
> you may not free it except through its release function (which is
> video_device_release().)  However, that also tries to kfree (with an
> offset of 4) your private data, which results in the warning and the
> corrupted kmalloc free lists.
>
> The solution is simple, make "vfd" a pointer in your private data
> structure and kmalloc() it separately, letting video_device_release()
> kfree() that data when it needs to.

Thanks Russell for tracking this down. I remember doing this
when I was reviewing the code for opportunities to "optimize" :-/,
and carelessly caused this bug by not reviewing how video_device
is freed.

Fixed.

Steve