linux-kernel - Re: [PATCH] s390: disable -Warray-bounds

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Message-ID: <CAHk-=wjahn2fZtMxmc4D9UOUGWVws1wZ=X46jrTV=_tEpzXeoA@mail.gmail.com>
Date:   Wed, 8 Jun 2022 13:07:05 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Sven Schnelle <svens@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] s390: disable -Warray-bounds

Coming back to this, because my rc2 week tends to be quiet as people
take a breather, and as such a good time for me to do system upgrades.

And gcc-12 dropped in Fedora 36, and shows problems on x86 too.

So I suspect we'll have to disable -Warray-bounds globally on gcc-12,
not just on s390.

Unless Kees has patches ready to go already.

Some of the warnings look potentially simple, ie

   struct mbus_dram_target_info;

in <linux/mbus.h> has the comment

         *   [..]  Peripherals are
         * required to support at least 4 decode windows.

and then as a result has

        int             num_cs;
        struct mbus_dram_window {
           [..]
        } cs[4];

and that "cs[4]" looks just bogus - it can be a much larger array, the
'4' is just a minimum. The real limit is that 'num_cs' one.

But there's a *lot* of warnings, and many of them are due to this, and
while some are obvious, others aren't.

There are other things too in gcc-12 that seem half-baked. I was
interested to see the new '-Wdangling-pointer' thing, but then when I
looked at it, the two cases I looked at were just bogus, so ..

                     Linus

On Fri, Apr 22, 2022 at 10:54 AM Kees Cook <keescook@...omium.org> wrote:
>
> On Fri, Apr 22, 2022 at 03:43:08PM +0200, Sven Schnelle wrote:
> > gcc-12 shows a lot of array bound warnings on s390. This is caused
> > by our S390_lowcore macro:
> >
> > which uses an hardcoded address of 0. Wrapping that with
> > absolute_pointer() works, but gcc no longer knows that a 12 bit
> > instruction is sufficient to access lowcore. So it emits instructions
> > like 'lghi %r1,0; l %rx,xxx(%r1)' instead of a single load/store
> > instruction. As s390 stores variables often read/written in lowcore,
> > this is considered problematic. Therefore disable -Warray-bounds on
> > s390 for now until there is a better real solution.
> >
> > Signed-off-by: Sven Schnelle <svens@...ux.ibm.com>
>
> It looks like the source of this problem (the literal-values-treated-as-NULL)
> is gcc-12 specific. From the discussions, it sounded like Jacob was
> going to fix this "correctly" in gcc-13. It might be a good idea to make
> this version-checked? (i.e. only disable on gcc-12)