Re: [PATCH v2 1/2] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

Geert Uytterhoeven
2020-10-29T08:25:49Z

On Wed, Oct 28, 2020 at 6:15 PM Ard Biesheuvel <ardb@kernel.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@kernel.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@glider.be>

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