[PATCH] bpf,objtool: tweak interpreter compilation flags to help objtool

[Alexei Starovoitov]

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?