Re: [PATCH] bpf: Tweak BPF jump table optimizations for objtool compatibility

[Josh Poimboeuf]

On Tue, May 05, 2020 at 04:59:39PM -0700, Alexei Starovoitov wrote:
> As far as workaround I prefer the following:
> From 94bbc27c5a70d78846a5cb675df4cf8732883564 Mon Sep 17 00:00:00 2001
> From: Alexei Starovoitov <ast@xxxxxxxxxx>
> Date: Tue, 5 May 2020 16:52:41 -0700
> Subject: [PATCH] bpf,objtool: tweak interpreter compilation flags to help objtool
> 
> tbd
> 
> Fixes: 3193c0836f20 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()")
> Reported-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
> Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> ---
>  include/linux/compiler-gcc.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index d7ee4c6bad48..05104c3cc033 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -171,4 +171,4 @@
>  #define __diag_GCC_8(s)
>  #endif
> 
> -#define __no_fgcse __attribute__((optimize("-fno-gcse")))
> +#define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer")))
> --
> 2.23.0
> 
> I've tested it with gcc 8,9,10 and clang 11 with FP=y and with ORC=y.
> All works.
> I think it's safer to go with frame pointers even for ORC=y considering
> all the pain this issue had caused. Even if objtool gets confused again
> in the future __bpf_prog_run() will have frame pointers and kernel stack
> unwinding can fall back from ORC to FP for that frame.
> wdyt?

It seems dangerous to me.  The GCC manual recommends against it.

And how do we know what other flags are getting removed for various
arches (now or in the future)?

-- 
Josh