On Tue, Oct 27, 2020 at 01:17:55PM -0700, Nick Desaulniers wrote:

On Tue, Oct 27, 2020 at 1:15 PM Ard Biesheuvel [off-list ref] wrote:

quoted

On Tue, 27 Oct 2020 at 21:12, Nick Desaulniers [off-list ref] wrote:

quoted

On Tue, Oct 27, 2020 at 12:25 PM Geert Uytterhoeven
[off-list ref] wrote:

quoted

Hi Nick,

CC Josh

On Mon, Oct 26, 2020 at 6:49 PM Nick Desaulniers
[off-list ref] wrote:

quoted

On Mon, Oct 26, 2020 at 10:44 AM Geert Uytterhoeven
[off-list ref] wrote:

quoted

On Mon, Oct 26, 2020 at 6:39 PM Ard Biesheuvel [off-list ref] wrote:

quoted

On Mon, 26 Oct 2020 at 17:01, Geert Uytterhoeven [off-list ref] wrote:

quoted

On Mon, Oct 26, 2020 at 2:29 PM Geert Uytterhoeven [off-list ref] wrote:

quoted

On Mon, Oct 26, 2020 at 1:29 PM Geert Uytterhoeven [off-list ref] wrote:

quoted

I.e. including the ".eh_frame" warning. I have tried bisecting that
warning (i.e. with be2881824ae9eb92 reverted), but that leads me to
commit b3e5d80d0c48c0cc ("arm64/build: Warn on orphan section
placement"), which is another red herring.

kernel/bpf/core.o is the only file containing an eh_frame section,
causing the warning.

When I see .eh_frame, I think -fno-asynchronous-unwind-tables is
missing from someone's KBUILD_CFLAGS.
But I don't see anything curious in kernel/bpf/Makefile, unless
cc-disable-warning is somehow broken.

I tracked it down to kernel/bpf/core.c:___bpf_prog_run() being tagged
with __no_fgcse aka __attribute__((optimize("-fno-gcse"))).

Even if the function is trivially empty ("return 0;"), a ".eh_frame" section
is generated.  Removing the __no_fgcse tag fixes that.

That's weird.  I feel pretty strongly that unless we're working around
a well understood compiler bug with a comment that links to a
submitted bug report, turning off rando compiler optimizations is a
terrible hack for which one must proceed straight to jail; do not pass
go; do not collect $200.  But maybe I'd feel differently for this case
given the context of the change that added it.  (Ard mentions
retpolines+orc+objtool; can someone share the relevant SHA if you have
it handy so I don't have to go digging?)

commit 3193c0836f203a91bef96d88c64cccf0be090d9c
Author: Josh Poimboeuf [off-list ref]
Date:   Wed Jul 17 20:36:45 2019 -0500

    bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()

has

Fixes: e55a73251da3 ("bpf: Fix ORC unwinding in non-JIT BPF code")

and mentions objtool and CONFIG_RETPOLINE.

quoted

 (I feel the same about there
being an empty asm(); statement in the definition of asm_volatile_goto
for compiler-gcc.h).  Might be time to "fix the compiler."

(It sounds like Arvind is both in agreement with my sentiment, and has
the root cause).

I agree that the __no_fgcse hack is terrible. Does Clang support the
following pragmas?

#pragma GCC push_options
#pragma GCC optimize ("-fno-gcse")
#pragma GCC pop_options

?

Put it in godbolt.org.  Pretty sure it's `#pragma clang` though.
`#pragma GCC` might be supported in clang or silently ignored, but
IIRC pragmas were a bit of a compat nightmare.  I think Arnd wrote
some macros to set pragmas based on toolchain.  (Uses _Pragma, for
pragmas in macros, IIRC).

-- 
Thanks,
~Nick Desaulniers

https://gcc.gnu.org/onlinedocs/gcc/Function-Specific-Option-Pragmas.html#Function-Specific-Option-Pragmas

#pragma GCC optimize is equivalent to the function attribute, so does
that actually help?

Btw, the bug mentioned in asm_volatile_goto seems like its been fixed in
4.9, so the hack could be dropped now?