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Date: Wed, 28 Oct 2020 14:39:03 -0700 From: Alexei Starovoitov <alexei.starovoitov@...il.com> To: Ard Biesheuvel <ardb@...nel.org> Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, bpf@...r.kernel.org, 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>, Geert Uytterhoeven <geert@...ux-m68k.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 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@...nel.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. __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.
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