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

[Nick Desaulniers]

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?)  (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).

--
Thanks,
~Nick Desaulniers