linux-kernel - Re: [PATCH] [RFC v2] qnx: avoid -Wstringop-overread warning, again

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Date:   Tue, 21 Sep 2021 10:18:59 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Anders Larsen <al@...rsen.net>, Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [RFC v2] qnx: avoid -Wstringop-overread warning, again

On Mon, Sep 20, 2021 at 7:26 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Mon, Sep 20, 2021 at 5:12 AM Arnd Bergmann <arnd@...nel.org> wrote:
> >
> > +               /*
> > +                * work around gcc-11.x using the first field it observes
> > +                * to determing the actual length
> > +                * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99578
> > +                */
> > +               char __empty[0];
> > +               char de_name[];
>
> Ugh. That looks _really_ hacky.
>
> It sounds like we can avoid the gcc bug if we just always use
> "de->de_name[]". Then we don't need to depend on magical behavior
> about one particular gcc version and a strange empty array in front of
> it.
>
> IOW, something like the attached simpler thing that just does that
> "always use de_name[]" and has a comment about why we don't do the
> natural thing

Yes, your patch is much nicer than my attempt. I checked it overnight
and it addresses all the randconfig issues I found.

> Also, just what version of gcc is the broken one? You say "gcc-11",
> but I certainly don't see it with _my_ version of gcc-11, so can we
> (just for that comment) document more precisely what version you have
> (or possibly what config you use to trigger it).

I'm using the gcc-11.1.0 that I uploaded to
https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/11.1.0/

The gcc bug is still open and currently targets gcc-11.3. To make it
worse, this is definitely configuration dependent, and I do not see it
in an allmodconfig build, but only certain randconfig builds. I originally
had a simpler patch myself and also found that this did not fully
address all kernel configs.
gcc-10.x and early do not show the problem, I think the warning
was only introduced in 11.1.

https://pastebin.com/raw/fXmNiute is a .config for x86 that showed the
problem for me on 5.15-rc2 but not with your new patch.

       Arnd