netdev - Re: [PATCH v2 1/2] bpf: don't rely on GCC __attribute_

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Date:   Thu, 29 Oct 2020 09:25:33 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Randy Dunlap <rdunlap@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2 1/2] bpf: don't rely on GCC __attribute__((optimize))
 to disable GCSE

On Wed, Oct 28, 2020 at 6:15 PM Ard Biesheuvel <ardb@...nel.org> wrote:
> Commit 3193c0836 ("bpf: Disable GCC -fgcse optimization for
> ___bpf_prog_run()") introduced a __no_fgcse macro that expands to a
> function scope __attribute__((optimize("-fno-gcse"))), to disable a
> GCC specific optimization that was causing trouble on x86 builds, and
> was not expected to have any positive effect in the first place.
>
> However, as the GCC manual documents, __attribute__((optimize))
> is not for production use, and results in all other optimization
> options to be forgotten for the function in question. This can
> cause all kinds of trouble, but in one particular reported case,
> it causes -fno-asynchronous-unwind-tables to be disregarded,
> resulting in .eh_frame info to be emitted for the function.
>
> This reverts commit 3193c0836, and instead, it disables the -fgcse
> optimization for the entire source file, but only when building for
> X86 using GCC with CONFIG_BPF_JIT_ALWAYS_ON disabled. Note that the
> original commit states that CONFIG_RETPOLINE=n triggers the issue,
> whereas CONFIG_RETPOLINE=y performs better without the optimization,
> so it is kept disabled in both cases.
>
> Fixes: 3193c0836 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()")
> Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>

(probably you missed by tag on v1 due to kernel.org hickups)

Thanks, this gets rid of the following warning, which you may
want to quote in the patch description:

    aarch64-linux-gnu-ld: warning: orphan section `.eh_frame' from
`kernel/bpf/core.o' being placed in section `.eh_frame'

Tested-by: Geert Uytterhoeven <geert+renesas@...der.be>

Gr{oetje,eeting}s,

                        Geert


--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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