Messages in this thread | | | Subject | Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE | From | Daniel Borkmann <> | Date | Wed, 28 Oct 2020 00:04:25 +0100 |
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On 10/27/20 9:57 PM, Ard Biesheuvel wrote: > Commit 3193c0836f203 ("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,
Looks like there are couple more as well aside from __no_fgcse, are you also planning to fix them?
arch/powerpc/kernel/setup.h:14:#define __nostackprotector __attribute__((__optimize__("no-stack-protector"))) tools/include/linux/compiler-gcc.h:37:#define __no_tail_call __attribute__((optimize("no-optimize-sibling-calls")))
> it causes -fno-asynchronous-unwind-tables to be disregarded, > resulting in .eh_frame info to be emitted for the function > inadvertently.
Would have been useful to add a pointer to the original discussion with Link tag.
Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/
> This reverts commit 3193c0836f203, and instead, it disables the -fgcse > optimization for the entire source file, but only when building for > X86. > > Cc: Nick Desaulniers <ndesaulniers@google.com> > Cc: Arvind Sankar <nivedita@alum.mit.edu> > Cc: Randy Dunlap <rdunlap@infradead.org> > Cc: Josh Poimboeuf <jpoimboe@redhat.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Alexei Starovoitov <ast@kernel.org> > Cc: Daniel Borkmann <daniel@iogearbox.net> > Cc: Peter Zijlstra (Intel) <peterz@infradead.org> > Cc: Geert Uytterhoeven <geert@linux-m68k.org> > Cc: Kees Cook <keescook@chromium.org> > Fixes: 3193c0836f203 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()") > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> [...] > diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile > index bdc8cd1b6767..02b58f44c479 100644 > --- a/kernel/bpf/Makefile > +++ b/kernel/bpf/Makefile > @@ -1,6 +1,8 @@ > # SPDX-License-Identifier: GPL-2.0 > obj-y := core.o > -CFLAGS_core.o += $(call cc-disable-warning, override-init) > +# ___bpf_prog_run() needs GCSE disabled on x86; see 3193c0836f203 for details > +cflags-core-$(CONFIG_X86) := -fno-gcse > +CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-core-y)
Also, this needs to be guarded behind !CONFIG_RETPOLINE and !CONFIG_BPF_JIT_ALWAYS_ON in particular the latter since only in this case interpreter is compiled in ... most distros have the CONFIG_BPF_JIT_ALWAYS_ON set these days for x86.
Do you have an analysis for the commit log on what else this penalizes in core.c if it's now for the entire translation unit?
> obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o > obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c > index 9268d77898b7..55454d2278b1 100644 > --- a/kernel/bpf/core.c > +++ b/kernel/bpf/core.c > @@ -1369,7 +1369,7 @@ u64 __weak bpf_probe_read_kernel(void *dst, u32 size, const void *unsafe_ptr) > * > * Decode and execute eBPF instructions. > */ > -static u64 __no_fgcse ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack) > +static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack) > { > #define BPF_INSN_2_LBL(x, y) [BPF_##x | BPF_##y] = &&x##_##y > #define BPF_INSN_3_LBL(x, y, z) [BPF_##x | BPF_##y | BPF_##z] = &&x##_##y##_##z >
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