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

[Nick Desaulniers]

On Wed, Oct 28, 2020 at 10:15 AM Ard Biesheuvel <ardb@xxxxxxxxxx> 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@xxxxxxxxxxxxxx/
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---
>  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")))
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 6e390d58a9f8..ac3fa37a84f9 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -247,10 +247,6 @@ struct ftrace_likely_data {
>  #define asm_inline asm
>  #endif
>
> -#ifndef __no_fgcse
> -# define __no_fgcse
> -#endif
> -
>  /* Are two types/vars the same type (ignoring qualifiers)? */
>  #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
>
> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> index bdc8cd1b6767..c1b9f71ee6aa 100644
> --- a/kernel/bpf/Makefile
> +++ b/kernel/bpf/Makefile
> @@ -1,6 +1,10 @@
>  # SPDX-License-Identifier: GPL-2.0
>  obj-y := core.o
> -CFLAGS_core.o += $(call cc-disable-warning, override-init)
> +ifneq ($(CONFIG_BPF_JIT_ALWAYS_ON),y)
> +# ___bpf_prog_run() needs GCSE disabled on x86; see 3193c0836f203 for details
> +cflags-nogcse-$(CONFIG_X86)$(CONFIG_CC_IS_GCC) := -fno-gcse
> +endif
> +CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-nogcse-yy)

Writing multiple conditions in a conditional block in GNU make is
painful, hence the double `y` trick.  I feel like either 3 nested
conditionals (one for CONFIG_BPF_JIT_ALWAYS_ON, CONFIG_X86, and
CONFIG_CC_IS_GCC) would have been clearer, or using three `y`, rather
than mixing and matching `if`s with multiple `y`s, but regardless of
what color I think we should paint the bikeshed:

Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

This also doesn't resolve all issues here, but is a step in the right
direction, IMO.

>
>  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
> --
> 2.17.1
>


-- 
Thanks,
~Nick Desaulniers