Re: [PATCH v6 13/29] arm64/build: Assert for unwanted sections

[Nick Desaulniers]

(+ right linux-toolchains mailing list, apologies for adding the wrong
one, I'm forever doomed to have gmail autocomplete to the wrong one
now that I've sent to it before)

On Tue, Oct 27, 2020 at 1:15 PM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Tue, 27 Oct 2020 at 21:12, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
> >
> > On Tue, Oct 27, 2020 at 12:25 PM Geert Uytterhoeven
> > <geert@xxxxxxxxxxxxxx> wrote:
> > >
> > > Hi Nick,
> > >
> > > CC Josh
> > >
> > > On Mon, Oct 26, 2020 at 6:49 PM Nick Desaulniers
> > > <ndesaulniers@xxxxxxxxxx> wrote:
> > > > On Mon, Oct 26, 2020 at 10:44 AM Geert Uytterhoeven
> > > > <geert@xxxxxxxxxxxxxx> wrote:
> > > > > On Mon, Oct 26, 2020 at 6:39 PM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> > > > > > On Mon, 26 Oct 2020 at 17:01, Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > > > > > > On Mon, Oct 26, 2020 at 2:29 PM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > > > > > > > On Mon, Oct 26, 2020 at 1:29 PM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > > > > > > > > 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 <jpoimboe@xxxxxxxxxx>
> 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.

Thanks for the context.  It might be time to revisit the above commit.
If I revert it (small conflict that's easy to fixup),
kernel/bpf/core.o builds cleanly with defconfig+GCC-9.3, so maybe
obtool did get smart enough to handle that case?  Probably regresses
the performance of that main dispatch loop for BPF, but not sure what
folks are expecting when retpolines are enabled.
-- 
Thanks,
~Nick Desaulniers