linux-kernel - Re: kisskb: FAILED linux-next/m68k-allmodconfig/m68k-gcc8 Tue Jan 25, 18:24

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Date:   Wed, 26 Jan 2022 13:25:45 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Yury Norov <yury.norov@...il.com>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: kisskb: FAILED linux-next/m68k-allmodconfig/m68k-gcc8 Tue Jan 25, 18:24

On Wed, Jan 26, 2022 at 10:16 AM Geert Uytterhoeven
<geert@...ux-m68k.org> wrote:
> On Wed, Jan 26, 2022 at 9:54 AM Kees Cook <keescook@...omium.org> wrote:
> > >>
> > >> The code that causes this is drivers/net/ipa/ipa_mem.c:ipa_mem_valid():
> > >>
> > >>         DECLARE_BITMAP(regions, IPA_MEM_COUNT) = { };
> > >>         ...
> > >>         for_each_clear_bit(mem_id, regions, IPA_MEM_COUNT) {
> > >>                 if (ipa_mem_id_required(ipa, mem_id))
> > >>                         dev_err(dev, "required memory region %u missing\n",
> > >>                                 mem_id);
> > >>         }
> > >>
> > >> This only happens with gcc-8, not with gcc-9, so it might be a
> > >> compiler bug. I don't see anything wrong with c:ipa_mem_valid()
> > >> nor with m68k's find_first_zero_bit().
> > >
> > >I don't see any problems about how this code uses bitmap API.
> > >The m68k version of find_first_zero_bit() looks correct as well.
> >
> > The trouble is with "enum ipa_mem_id mem_id;" which is an int, and the bitmap API requires unsigned long. I tried to fix this[1] at the source, but the maintainers want each[2] call site to fix it instead. :(
>
> Sorry, I don't get it. "mem_id" is not used as the bitmap, "regions" is,
> and the latter has the correct type?

I think you are right here, and even if it was an array of 'unsigned
int' instead
of 'unsigned long', this should not change the size of the object on
a 32-bit architecture.

I ran the preprocessed code through cvise[1], bisecting for a reduced
test case that fails on gcc-8 but succeeds on gcc-9. The reduced
case is still fairly complex, and it appears to only happen in the
presence of an inline asm. Narrowing down the compiler versions shows
that anything after gcc-9.2 does not warn, but 9.1 and earlier versions do,
which is further indication that  it was probably a false-positive that got
fixed in gcc.

     Arnd

[1] https://github.com/marxin/cvise
[2] https://www.godbolt.org/z/xMr9v1nPT