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

[Geert Uytterhoeven]

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:
> > > > > > On Fri, Aug 21, 2020 at 9:56 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > > > > > > In preparation for warning on orphan sections, discard
> > > > > > > unwanted non-zero-sized generated sections, and enforce other
> > > > > > > expected-to-be-zero-sized sections (since discarding them might hide
> > > > > > > problems with them suddenly gaining unexpected entries).
> > > > > > >
> > > > > > > Suggested-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > > > > > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > > > >
> > > > > > This is now commit be2881824ae9eb92 ("arm64/build: Assert for unwanted
> > > > > > sections") in v5.10-rc1, and is causing the following error with
> > > > > > renesas_defconfig[1]:
> > > > > >
> > > > > >     aarch64-linux-gnu-ld: warning: orphan section `.eh_frame' from
> > > > > > `kernel/bpf/core.o' being placed in section `.eh_frame'
> > > > > >     aarch64-linux-gnu-ld: Unexpected GOT/PLT entries detected!
> > > > > >     aarch64-linux-gnu-ld: Unexpected run-time procedure linkages detected!
> > > > > >
> > > > > > I cannot reproduce this with the standard arm64 defconfig.
> > > > > >
> > > > > > I bisected the error to the aforementioned commit, but understand this
> > > > > > is not the real reason.  If I revert this commit, I still get:
> > > > > >
> > > > > >     aarch64-linux-gnu-ld: warning: orphan section `.got.plt' from
> > > > > > `arch/arm64/kernel/head.o' being placed in section `.got.plt'
> > > > > >     aarch64-linux-gnu-ld: warning: orphan section `.plt' from
> > > > > > `arch/arm64/kernel/head.o' being placed in section `.plt'
> > > > > >     aarch64-linux-gnu-ld: warning: orphan section `.data.rel.ro' from
> > > > > > `arch/arm64/kernel/head.o' being placed in section `.data.rel.ro'
> > > > > >     aarch64-linux-gnu-ld: warning: orphan section `.eh_frame' from
> > > > > > `kernel/bpf/core.o' being placed in section `.eh_frame'
> > > > > >
> > > > > > 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.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds