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

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Message-ID: <CAK8P3a0XqxBwRH2mkge0Ah87BnqobNWGkE4rhpmvNxGxjgExww@mail.gmail.com>
Date:   Wed, 28 Aug 2019 16:06:32 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Ilie Halip <ilie.halip@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: objtool warning "uses BP as a scratch register" with clang-9

On Wed, Aug 28, 2019 at 11:00 AM Arnd Bergmann <arnd@...db.de> wrote:
> On Tue, Aug 27, 2019 at 11:22 PM 'Nick Desaulniers' via Clang Built Linux <clang-built-linux@...glegroups.com> wrote:

> http://paste.ubuntu.com/p/XjdDsypRxX/
> 0x5BA1B7A1:arch/x86/ia32/ia32_signal.o: warning: objtool:
> ia32_setup_rt_frame()+0x238: call to memset() with UACCESS enabled
> 0x5BA1B7A1:arch/x86/kernel/signal.o: warning: objtool:
> __setup_rt_frame()+0x5b8: call to memset() with UACCESS enabled
> 0x5BA1B7A1:mm/kasan/common.o: warning: objtool: kasan_report()+0x44:
> call to __stack_chk_fail() with UACCESS enabled
> 0x5BA1B7A1:kernel/trace/trace_selftest_dynamic.o: warning: objtool:
> __llvm_gcov_writeout()+0x13: call without frame pointer save/setup
> 0x5BA1B7A1:kernel/trace/trace_selftest_dynamic.o: warning: objtool:
> __llvm_gcov_flush()+0x0: call without frame pointer save/setup
> 0x5BA1B7A1:kernel/trace/trace_clock.o: warning: objtool:
> __llvm_gcov_writeout()+0x14: call without frame pointer save/setup
> 0x5BA1B7A1:kernel/trace/trace_clock.o: warning: objtool:
> __llvm_gcov_flush()+0x0: call without frame pointer save/setup
> 0x5BA1B7A1:kernel/trace/*: # many more of the same, all in this directory
> 0x5BA1B7A1:kernel/trace/trace_uprobe.o: warning: objtool:
> __llvm_gcov_flush()+0x0: call without frame pointer save/setup

I had a look here and opened
https://bugs.llvm.org/show_bug.cgi?id=43141

It seems that CONFIG_FRAME_POINTER is ignored for the functions
that are generated with CONFIG_GCOV_KERNEL. See also:

$ clang-9 -fprofile-arcs -fno-omit-frame-pointer -c -xc /dev/null   -o null.o
$ ./tools/objtool/objtool check null.o
null.o: warning: objtool: __llvm_gcov_writeout()+0x3e: call without
frame pointer save/setup
null.o: warning: objtool: __llvm_gcov_flush()+0x1: call without frame
pointer save/setup
null.o: warning: objtool: __llvm_gcov_init()+0x15: call without frame
pointer save/setup

       Arnd