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From:   Evgeny Novikov <novikov@ispras.ru>
Envelope-From: eugenenovikov@yandex.ru
To:     "Lad, Prabhakar" <prabhakar.csengg@gmail.com>
Cc:     Mauro Carvalho Chehab <mchehab@kernel.org>,
        linux-media <linux-media@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "ldv-project@linuxtesting.org" <ldv-project@linuxtesting.org>
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Subject: Re: [PATCH] media: davinci: vpif_capture: fix potential double free
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Date:   Sun, 02 Aug 2020 21:38:31 +0300
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Hi Lad,

25.07.2020, 01:06, "Lad, Prabhakar" <prabhakar.csengg@gmail.com>:
> Hi Evgeny,
>
> On Fri, Jul 24, 2020 at 9:00 PM Evgeny Novikov <novikov@ispras.ru> wrote:
>>  Hi Lad,
>>
>>  Yet again I can not demonstrate you a nice error trace corresponding to the bug fixed by the patch. Indeed, there is a branch in vpif_probe() that explicitly invokes vpif_probe_complete() and the patch targets the possible issue that can happen during this.
>>
>>  When I developed the patch I saw on vpif_display.ko. It looks very similar regarding things touched by the patch. In particular, it does not free vpif_obj.sd in its vpif_probe_complete(). But now I see that it does this in vpif_remove()!
>
> Makes sense.
>
>>  Do you think that vpif_capture.ko should do the same? If so, I guess that I should fix the patch appropriately since likely it just replaces one (very rare) bug with another one (on a typical execution path).
>
> Yes it should.

vpif_remove() from vpif_capture.ko already both frees vpif_obj.sd and unregisters the V4L2 device. So, there is no need to fix the patch.

BTW, other drivers, e.g. drivers/media/platform/renesas-ceu.c, do not clean up memory allocated by probe within v4l2_async_notifier_operations.complete as well. 

--
Best regards,
Evgeny Novikov

> Cheers,
> --Prabhakar
>
>>  --
>>  Evgeny Novikov
>>  Linux Verification Center, ISP RAS
>>  http://linuxtesting.org
>>
>>  24.07.2020, 17:17, "Lad, Prabhakar" <prabhakar.csengg@gmail.com>:
>>  > Hi Evgeny,
>>  >
>>  > Thank you for the patch.
>>  >
>>  > On Thu, Jul 23, 2020 at 6:04 PM Evgeny Novikov <novikov@ispras.ru> wrote:
>>  >> In case of errors vpif_probe_complete() releases memory for vpif_obj.sd
>>  >> and unregisters the V4L2 device. But then this is done again by
>>  >> vpif_probe() itself. The patch removes the cleaning from
>>  >> vpif_probe_complete().
>>  >>
>>  >> Found by Linux Driver Verification project (linuxtesting.org).
>>  >>
>>  >> Signed-off-by: Evgeny Novikov <novikov@ispras.ru>
>>  >> ---
>>  >> drivers/media/platform/davinci/vpif_capture.c | 2 --
>>  >> 1 file changed, 2 deletions(-)
>>  >>
>>  >> diff --git a/drivers/media/platform/davinci/vpif_capture.c b/drivers/media/platform/davinci/vpif_capture.c
>>  >> index d9ec439faefa..72a0e94e2e21 100644
>>  >> --- a/drivers/media/platform/davinci/vpif_capture.c
>>  >> +++ b/drivers/media/platform/davinci/vpif_capture.c
>>  >> @@ -1482,8 +1482,6 @@ static int vpif_probe_complete(void)
>>  >> /* Unregister video device */
>>  >> video_unregister_device(&ch->video_dev);
>>  >> }
>>  >> - kfree(vpif_obj.sd);
>>  >> - v4l2_device_unregister(&vpif_obj.v4l2_dev);
>>  >
>>  > vpif_probe_complete() is a async callback and probe() should have
>>  > already completed by then.
>>  >
>>  > Cheers,
>>  > --Prabhakar
>>  >
>>  >> return err;
>>  >> }
>>  >> --
>>  >> 2.16.4