Re: [PATCH] bpf: Tweak BPF jump table optimizations for objtool compatibility
From: Josh Poimboeuf <hidden>
Date: 2020-05-06 15:53:53
Also in:
bpf, lkml
On Tue, May 05, 2020 at 04:59:39PM -0700, Alexei Starovoitov wrote:
quoted hunk ↗ jump to hunk
As far as workaround I prefer the following: From 94bbc27c5a70d78846a5cb675df4cf8732883564 Mon Sep 17 00:00:00 2001 From: Alexei Starovoitov <ast@kernel.org> 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 <redacted> Reported-by: Arnd Bergmann <arnd@arndb.de> Signed-off-by: Alexei Starovoitov <ast@kernel.org> --- 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