linux-kernel - Re: New warnings with gcc-11

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Message-ID: <CAHk-=whJsh4FOcMQ+eDx=f4joa-CCH1pmYtrsw0H7L0HV_GhJg@mail.gmail.com>
Date:   Sat, 8 May 2021 11:51:11 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jani Nikula <jani.nikula@...el.com>
Cc:     Dave Airlie <airlied@...il.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        LKML <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: New warnings with gcc-11

I have heard nothing about this, and it remains the only warning from
my allmodconfig build (I have another one for drm compiled with clang,
but there I at least heard back that a fix exists).

Since I am going to release rc1 tomorrow, and I don't want to release
it with an ugly compiler warning, I took it upon myself to just fix
the code:

  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fec4d42724a1bf3dcba52307e55375fdb967b852

HOWEVER.

That commit fixes the warning, and is at worst harmless. At best it
fixes an access to random stack memory. But it does smell like
somebody who actually knows how these arrays work should look at that
code.

IOW, maybe the code should actually have read 16 bytes from the Event
Status Indicator? Maybe offset 10 was wrong? Maybe
drm_dp_channel_eq_ok() should never have taken six bytes to begin
with?

It's a mystery, and I haven't heard anything otherwise, so there it is.

              Linus

On Wed, Apr 28, 2021 at 12:27 AM Jani Nikula <jani.nikula@...el.com> wrote:
>
> On Tue, 27 Apr 2021, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > I've updated to Fedora 34 on one of my machines, and it causes a lot
> > of i915 warnings like
> >
> >   drivers/gpu/drm/i915/intel_pm.c: In function ‘ilk_setup_wm_latency’:
> >   drivers/gpu/drm/i915/intel_pm.c:3059:9: note: referencing argument 3
> > of type ‘const u16 *’ {aka ‘const short unsigned int *’}
> >   drivers/gpu/drm/i915/intel_pm.c:2994:13: note: in a call to function
> > ‘intel_print_wm_latency’
> >
> > and the reason is that gcc now seems to look at the argument array
> > size more, and notices that
>
> Arnd Bergmann reported some of these a while back. I think we have some
> of them fixed in our -next already, but not all. Thanks for the
> reminder.