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

[Ard Biesheuvel <ardb@kernel.org>]

On Wed, 28 Oct 2020 at 22:39, Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Wed, Oct 28, 2020 at 06:15:05PM +0100, Ard Biesheuvel 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>
> > ---
> >  include/linux/compiler-gcc.h   | 2 --
> >  include/linux/compiler_types.h | 4 ----
> >  kernel/bpf/Makefile            | 6 +++++-
> >  kernel/bpf/core.c              | 2 +-
> >  4 files changed, 6 insertions(+), 8 deletions(-)
> >
> > diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> > index d1e3c6896b71..5deb37024574 100644
> > --- a/include/linux/compiler-gcc.h
> > +++ b/include/linux/compiler-gcc.h
> > @@ -175,5 +175,3 @@
> >  #else
> >  #define __diag_GCC_8(s)
> >  #endif
> > -
> > -#define __no_fgcse __attribute__((optimize("-fno-gcse")))
>
> See my reply in the other thread.
> I prefer
> -#define __no_fgcse __attribute__((optimize("-fno-gcse")))
> +#define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer")))
>
> Potentially with -fno-asynchronous-unwind-tables.
>

So how would that work? arm64 has the following:

KBUILD_CFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables

ifeq ($(CONFIG_SHADOW_CALL_STACK), y)
KBUILD_CFLAGS += -ffixed-x18
endif

and it adds -fpatchable-function-entry=2 for compilers that support
it, but only when CONFIG_FTRACE is enabled.

Also, as Nick pointed out, -fno-gcse does not work on Clang.

Every architecture will have a different set of requirements here. And
there is no way of knowing which -f options are disregarded when you
use the function attribute.

So how on earth are you going to #define __no-fgcse correctly for
every configuration imaginable?

> __attribute__((optimize("")) is not as broken as you're claiming to be.
> It has quirky gcc internal logic, but it's still widely used
> in many software projects.

So it's fine because it is only a little bit broken? I'm sorry, but
that makes no sense whatsoever.

If you insist on sticking with this broken construct, can you please
make it GCC/x86-only at least?