Re: [RFC PATCH] media: venus: Fix NULL pointer dereference in core selection

[Doug Anderson]

Hi,

On Mon, Jun 1, 2020 at 3:03 PM Douglas Anderson <dianders@xxxxxxxxxxxx> wrote:
>
> The newly-introduced function min_loaded_core() iterates over all of
> the venus instances an tries to figure out how much load each instance
> is putting on each core.  Not all instances, however, might be fully
> initialized.  Specifically the "codec_freq_data" is initialized as
> part of vdec_queue_setup(), but an instance may already be in the list
> of all instances before that time.
>
> Let's band-aid this by checking to see if codec_freq_data is NULL
> before dereferencing.
>
> NOTE: without this fix I was running into a crash.  Specifically there
> were two venus instances.  One was doing start_streaming.  The other
> was midway through queue_setup but hadn't yet gotten to initting
> "codec_freq_data".
>
> Fixes: eff82f79c562 ("media: venus: introduce core selection")
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
> I'm not massively happy about this commit but it's the best I could
> come up with without being much more of an expert in the venus codec.
> If someone has a better patch then please just consider this one to be
> a bug report and feel free to submit a better fix!  :-)
>
> In general I wonder a little bit about whether it's safe to be peeking
> at all the instances without grabbing the "inst->lock" on each one.  I
> guess it is since we do it both here and in load_scale_v4() but I
> don't know why.
>
> One thought I had was that we could fully avoid accessing the other
> instances, at least in min_loaded_core(), by just keeping track of
> "core1_load" and "core2_load" in "struct venus_core".  Whenever we add
> a new instance we could add to the relevant variables and whenever we
> release an instance we could remove.  Such a change seems cleaner but
> would require someone to test to make sure we didn't miss any case
> (AKA we always properly added/removed our load from the globals).
>
>  drivers/media/platform/qcom/venus/pm_helpers.c | 2 ++
>  1 file changed, 2 insertions(+)

This fixes the same crash as the patch:

https://lore.kernel.org/r/1588314480-22409-1-git-send-email-mansur@xxxxxxxxxxxxxx