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

[Geert Uytterhoeven]

Hi Arnd,

On Wed, Jan 26, 2022 at 1:26 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Wed, Jan 26, 2022 at 10:16 AM Geert Uytterhoeven
> <geert@xxxxxxxxxxxxxx> wrote:
> > On Wed, Jan 26, 2022 at 9:54 AM Kees Cook <keescook@xxxxxxxxxxxx> 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.

Indeed, size matters not here.

> 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.

Thanks for investigating! So let's ignore this.

FTR, my gcc-9 is 9.4.0, i.e. >= 9.2.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds