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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Tue, 27 Oct 2020 20:25:26 +0100
Message-ID: <CAMuHMdUDOzJbzf=0jom9dnSzkC+dkMdkyY_BOBMAivbJfF+Gmg@mail.gmail.com>
Subject: Re: [PATCH v6 13/29] arm64/build: Assert for unwanted sections
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Ard Biesheuvel <ardb@kernel.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>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        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>
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Hi Nick,

CC Josh

On Mon, Oct 26, 2020 at 6:49 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
> On Mon, Oct 26, 2020 at 10:44 AM Geert Uytterhoeven
> <geert@linux-m68k.org> wrote:
> > On Mon, Oct 26, 2020 at 6:39 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> > > On Mon, 26 Oct 2020 at 17:01, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > > On Mon, Oct 26, 2020 at 2:29 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > > > On Mon, Oct 26, 2020 at 1:29 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > > > > On Fri, Aug 21, 2020 at 9:56 PM Kees Cook <keescook@chromium.org> wrote:
> > > > > > > In preparation for warning on orphan sections, discard
> > > > > > > unwanted non-zero-sized generated sections, and enforce other
> > > > > > > expected-to-be-zero-sized sections (since discarding them might hide
> > > > > > > problems with them suddenly gaining unexpected entries).
> > > > > > >
> > > > > > > Suggested-by: Ard Biesheuvel <ardb@kernel.org>
> > > > > > > Signed-off-by: Kees Cook <keescook@chromium.org>
> > > > > >
> > > > > > This is now commit be2881824ae9eb92 ("arm64/build: Assert for unwanted
> > > > > > sections") in v5.10-rc1, and is causing the following error with
> > > > > > renesas_defconfig[1]:
> > > > > >
> > > > > >     aarch64-linux-gnu-ld: warning: orphan section `.eh_frame' from
> > > > > > `kernel/bpf/core.o' being placed in section `.eh_frame'
> > > > > >     aarch64-linux-gnu-ld: Unexpected GOT/PLT entries detected!
> > > > > >     aarch64-linux-gnu-ld: Unexpected run-time procedure linkages detected!
> > > > > >
> > > > > > I cannot reproduce this with the standard arm64 defconfig.
> > > > > >
> > > > > > I bisected the error to the aforementioned commit, but understand this
> > > > > > is not the real reason.  If I revert this commit, I still get:
> > > > > >
> > > > > >     aarch64-linux-gnu-ld: warning: orphan section `.got.plt' from
> > > > > > `arch/arm64/kernel/head.o' being placed in section `.got.plt'
> > > > > >     aarch64-linux-gnu-ld: warning: orphan section `.plt' from
> > > > > > `arch/arm64/kernel/head.o' being placed in section `.plt'
> > > > > >     aarch64-linux-gnu-ld: warning: orphan section `.data.rel.ro' from
> > > > > > `arch/arm64/kernel/head.o' being placed in section `.data.rel.ro'
> > > > > >     aarch64-linux-gnu-ld: warning: orphan section `.eh_frame' from
> > > > > > `kernel/bpf/core.o' being placed in section `.eh_frame'
> > > > > >
> > > > > > I.e. including the ".eh_frame" warning. I have tried bisecting that
> > > > > > warning (i.e. with be2881824ae9eb92 reverted), but that leads me to
> > > > > > commit b3e5d80d0c48c0cc ("arm64/build: Warn on orphan section
> > > > > > placement"), which is another red herring.
> > > > >
> > > > > kernel/bpf/core.o is the only file containing an eh_frame section,
> > > > > causing the warning.
>
> 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.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds