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

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Date:   Fri, 19 Jul 2019 11:23:16 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: warning: objtool: fn1 uses BP as a scratch register

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?
-- 
Thanks,
~Nick Desaulniers