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

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Date:   Tue, 27 Oct 2020 16:00:33 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        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>
Subject: Re: [PATCH v6 13/29] arm64/build: Assert for unwanted sections

On Tue, Oct 27, 2020 at 08:33:00PM +0100, Ard Biesheuvel wrote:
> On Tue, 27 Oct 2020 at 20:25, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > >
> > > When I see .eh_frame, I think -fno-asynchronous-unwind-tables is
> > > missing from someone's KBUILD_CFLAGS.
> > > But I don't see anything curious in kernel/bpf/Makefile, unless
> > > cc-disable-warning is somehow broken.
> >
> > I tracked it down to kernel/bpf/core.c:___bpf_prog_run() being tagged
> > with __no_fgcse aka __attribute__((optimize("-fno-gcse"))).
> >
> > Even if the function is trivially empty ("return 0;"), a ".eh_frame" section
> > is generated.  Removing the __no_fgcse tag fixes that.
> >
> 
> 
> Given that it was added for issues related to retpolines, ORC and
> objtool, it should be safe to make that annotation x86-only.

The optimize attribute is not meant for production use. I had mentioned
this at the time but it got lost: the optimize attribute apparently does
not add options, it replaces them completely. So I'm guessing this one
is dropping the -fno-asynchronous-unwind-tables and causing the eh_frame
sections, though I don't know why that doesn't cause eh_frame on x86?

https://lore.kernel.org/lkml/alpine.LSU.2.21.2004151445520.11688@wotan.suse.de/