linux-kernel - Re: [PATCH v6 13/29] arm64/build: Assert for unwanted sections

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Message-ID: <20201027203210.GB1833548@rani.riverdale.lan>
Date:   Tue, 27 Oct 2020 16:32:10 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Collingbourne <pcc@...gle.com>,
        James Morse <james.morse@....com>,
        Borislav Petkov <bp@...e.de>, Ingo Molnar <mingo@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        linux-toolchains@...r.kernel.org
Subject: Re: [PATCH v6 13/29] arm64/build: Assert for unwanted sections

On Tue, Oct 27, 2020 at 01:28:02PM -0700, Nick Desaulniers wrote:
> > commit 3193c0836f203a91bef96d88c64cccf0be090d9c
> > Author: Josh Poimboeuf <jpoimboe@...hat.com>
> > Date:   Wed Jul 17 20:36:45 2019 -0500
> >
> >     bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()
> >
> > has
> >
> > Fixes: e55a73251da3 ("bpf: Fix ORC unwinding in non-JIT BPF code")
> >
> > and mentions objtool and CONFIG_RETPOLINE.
> 
> Thanks for the context.  It might be time to revisit the above commit.
> If I revert it (small conflict that's easy to fixup),
> kernel/bpf/core.o builds cleanly with defconfig+GCC-9.3, so maybe
> obtool did get smart enough to handle that case?  Probably regresses
> the performance of that main dispatch loop for BPF, but not sure what
> folks are expecting when retpolines are enabled.
> -- 
> Thanks,
> ~Nick Desaulniers

The objtool issue was with RETPOLINE disabled.