linux-kernel - Re: warning: objtool: fn1 uses BP as a scratch register

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Date:   Fri, 19 Jul 2019 20:44:07 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: warning: objtool: fn1 uses BP as a scratch register

On Fri, Jul 19, 2019 at 8:31 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> On Fri, Jul 19, 2019 at 11:23:16AM -0700, Nick Desaulniers wrote:
> > On Fri, Jul 19, 2019 at 11:10 AM Arnd Bergmann <arnd@...db.de> wrote:
> > >
> > > A lot of objtool fixes showed up in linux-next, so I looked at some
> > > remaining ones.
> > > This one comes a lot up in some configurations
> > >
> > > https://godbolt.org/z/ZZLVD-
> > >
> > > struct ov7670_win_size {
> > >   int width;
> > >   int height;
> > > };
> > > struct ov7670_devtype {
> > >   struct ov7670_win_size *win_sizes;
> > >   unsigned n_win_sizes;
> > > };
> > > struct ov7670_info {
> > >   int min_width;
> > >   int min_height;
> > >   struct ov7670_devtype devtype;
> > > } a;
> > > int b;
> > > int fn1() {
> > >   struct ov7670_info c = a;
> > >   int i = 0;
> > >   for (; i < c.devtype.n_win_sizes; i++) {
> > >     struct ov7670_win_size d = c.devtype.win_sizes[i];
> > >     if (c.min_width && d.width < d.height < c.min_height)
> > >       if (b)
> > >         return 0;
> > >   }
> > >   return 2;
> > > }
> > >
> > > $ clang-8 -O2 -fno-omit-frame-pointer -fno-strict-overflow -c ov7670.i
> > > $ objtool check  --no-unreachable --uaccess ov7670.o
> > > ov7670.o: warning: objtool: fn1 uses BP as a scratch register
> >
> > Thanks for the report and reduced test case.  From the godbolt link, I
> > don't see %rbp, %ebp, %bp, or %bpl being referenced (other that %rbp
> > in the typical epilogue).  Am I missing something? Is objtool maybe
> > not reporting the precise function at fault?
>
> I haven't looked, but it could very well be an objtool bug (surprise).

Actually the reproducer may be wrong. I reduced the test case using
9.0.0-svn363902-1~exp1+0~20190620001509.2315~1.gbp76e756,
and this contains a link

         testl %ebp, %ebp

I get the same thing with clang-8, but godbolt.org shows it only
with clang-8 (see https://godbolt.org/z/g1lZO0) , not with trunk.

       Arnd