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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 27 Oct 2020 13:36:20 -0700
Message-ID: <CAKwvOd=ufk3G8moNb8Z1Ruw9hm1YkynZ5mQNf2k1FsmkXfCJiw@mail.gmail.com>
Subject: Re: [PATCH v6 13/29] arm64/build: Assert for unwanted sections
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Ard Biesheuvel <ardb@kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Kees Cook <keescook@chromium.org>,
        Ingo Molnar <mingo@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Peter Collingbourne <pcc@google.com>,
        James Morse <james.morse@arm.com>,
        Borislav Petkov <bp@suse.de>, Ingo Molnar <mingo@redhat.com>,
        Russell King <linux@armlinux.org.uk>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Arnd Bergmann <arnd@arndb.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Linux-Arch <linux-arch@vger.kernel.org>,
        linux-efi <linux-efi@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        linux-toolchains@vger.kernel.org
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On Tue, Oct 27, 2020 at 1:32 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> On Tue, Oct 27, 2020 at 01:28:02PM -0700, Nick Desaulniers wrote:
> > > commit 3193c0836f203a91bef96d88c64cccf0be090d9c
> > > Author: Josh Poimboeuf <jpoimboe@redhat.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.

Ah, sorry, in that case default-CONFIG_RETPOLINE+gcc-9.3:
kernel/bpf/core.o: warning: objtool: ___bpf_prog_run()+0x8d4: sibling
call from callable instruction with modified stack frame
-- 
Thanks,
~Nick Desaulniers