linux-kernel - Re: Remaining randconfig objtool warnings, linux-next-20200428

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Date:   Tue, 28 Apr 2020 22:19:46 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: Remaining randconfig objtool warnings, linux-next-20200428

On Tue, Apr 28, 2020 at 6:10 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Tue, Apr 28, 2020 at 04:49:15PM +0200, Arnd Bergmann wrote:
> > ==> build/x86/0xFD7B7323_defconfig/log <==
> > arch/x86/entry/entry_64.o: warning: objtool: .entry.text+0x991: unreachable instruction
>
> This warning looks correct, did you make a change to entry_64.S?

I bisected my local patches and found that I had a local hack that turned
off CONFIG_RETPOLINE for testing something unrelated. I can reproduce
it on linux-next with this patch:

--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -447,8 +447,7 @@ config GOLDFISH
        depends on X86_GOLDFISH

 config RETPOLINE
-       bool "Avoid speculative indirect branches in kernel"
-       default y
+       def_bool n
        select STACK_VALIDATION if HAVE_STACK_VALIDATION
        help
          Compile kernel with the retpoline compiler options to guard against

> > ==> build/x86/0xE0F2ACFF_defconfig/log <==
> > kernel/time/posix-stubs.o: warning: objtool: __x64_sys_timer_create()+0x23: sibling call from callable instruction with modified stack frame
> >
> > ==> build/x86/0x2EA4CE4F_defconfig/log <==
> > kernel/kcov.o: warning: objtool: write_comp_data()+0x1b: call to check_kcov_mode() with UACCESS enabled
> > kernel/kcov.o: warning: objtool: __sanitizer_cov_trace_pc()+0x15: call to check_kcov_mode() with UACCESS enabled
> >
> > ==> build/x86/0x500B1B82_defconfig/log <==
> > kernel/bpf/core.o: warning: objtool: ___bpf_prog_run()+0x1269: call
> > without frame pointer save/setup
>
> I'll look into these.

Thanks!

    Arnd