linux-kernel - Re: [PATCH] dispnv50: atom: fix an incorrect NULL check on list iterator

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Date:   Mon, 28 Mar 2022 10:09:02 +0800
From:   Xiaomeng Tong <xiam0nd.tong@...il.com>
To:     emil.l.velikov@...il.com
Cc:     airlied@...ux.ie, bskeggs@...hat.com, daniel@...ll.ch,
        dri-devel@...ts.freedesktop.org, kherbst@...hat.com,
        linux-kernel@...r.kernel.org, lyude@...hat.com,
        nouveau@...ts.freedesktop.org, stable@...r.kernel.org,
        xiam0nd.tong@...il.com, yangyingliang@...wei.com
Subject: Re: [PATCH] dispnv50: atom: fix an incorrect NULL check on list iterator

on Sun, 27 Mar 2022 16:59:28 +0100, Emil Velikov wrote:
> On Sun, 27 Mar 2022 at 08:39, Xiaomeng Tong <xiam0nd.tong@...il.com> wrote:
> >
> > The bug is here:
> >         return encoder;
> >
> > The list iterator value 'encoder' will *always* be set and non-NULL
> > by drm_for_each_encoder_mask(), so it is incorrect to assume that the
> > iterator value will be NULL if the list is empty or no element found.
> > Otherwise it will bypass some NULL checks and lead to invalid memory
> > access passing the check.
> >
> > To fix this bug, just return 'encoder' when found, otherwise return
> > NULL.
> >
> 
> Isn't this covered by the upcoming list* iterator rework [1] or is
> this another iterator glitch?

Actually, it is a part of the upcoming work.

> IMHO we should be looking at fixing the implementation and not the
> hundreds of users through the kernel.
>
> HTH
> -Emil
> [1] https://lwn.net/Articles/887097/

Yes, you are right. This has also been taken into account by the upcoming
list iterator rework to avoid a lot uesr' changes as much as possible.

However, this patch is fixing a potential bug caused by incorrect use of
list iterator outside the loop, which can not be fixed by the implementation
itself.

--
Xiaomeng Tong