linux-kernel - Re: objtool warning "uses BP as a scratch register" with clang-9

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Date:   Tue, 27 Aug 2019 09:51:02 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: objtool warning "uses BP as a scratch register" with clang-9

On Tue, Aug 27, 2019 at 02:30:06PM +0200, Arnd Bergmann wrote:
> I upgraded to the latest clang-9 snapshot from http://apt.llvm.org/ today.
> Many problems are fixed, but I still get tons of warnings like
> 
> arch/x86/kernel/cpu/mtrr/generic.o: warning: objtool:
> mtrr_type_lookup_variable uses BP as a scratch register
> arch/x86/kernel/process.o: warning: objtool: get_tsc_mode()+0x21: call
> without frame pointer save/setup
> arch/x86/kernel/early_printk.o: warning: objtool: early_vga_write uses
> BP as a scratch register
> arch/x86/kernel/sysfb_simplefb.o: warning: objtool: parse_mode uses BP
> as a scratch register
> arch/x86/kernel/head64.o: warning: objtool: __startup_64 uses BP as a
> scratch register
> kernel/time/timeconv.o: warning: objtool: time64_to_tm uses BP as a
> scratch register
> kernel/trace/ring_buffer.o: warning: objtool:
> ring_buffer_discard_commit uses BP as a scratch register
> ...
> 
> I created a reduced test case:
> 
> $ cat crc32.i
> typedef unsigned u32;
> long a, c;
> u32 b, f;
> u32 *d, *e;
> void fn1() {
>   u32 *g = &f, *h = e, *i = d;
>   for (; a < c; a++)
>     b = i[b >> 8 & 255] ^ h[b] ^ g[5];
> }
> $ clang-9 -c  crc32.i  -O2   ; objtool check  crc32.o
> crc32.o: warning: objtool: fn1 uses BP as a scratch register

I assume your config has CONFIG_UNWINDER_FRAME_POINTER=y, since objtool
is doing frame pointer checking.  Though I'm not sure about that, since
I don't see -fno-omit-frame-pointer on the reduced clang command line.
Do you still see this warning with -fno-omit-frame-pointer (assuming
clang has that option)?

-- 
Josh