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

[Nick Desaulniers]

On Mon, Oct 26, 2020 at 10:44 AM Geert Uytterhoeven
<geert@xxxxxxxxxxxxxx> wrote:
>
> Hi Ard,
>
> 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.

> > > > If I compile core.c with "-g" added, like arm64 defconfig does, the
> > > > eh_frame section is no longer emitted.
> > > >
> > > > Hence setting CONFIG_DEBUG_INFO=y, cfr. arm64 defconfig, the warning
> > > > is gone, but I'm back to the the "Unexpected GOT/PLT entries" below...
> > > >
> > > > > Note that even on plain be2881824ae9eb92, I get:
> > > > >
> > > > >     aarch64-linux-gnu-ld: Unexpected GOT/PLT entries detected!
> > > > >     aarch64-linux-gnu-ld: Unexpected run-time procedure linkages detected!
> > > > >
> > > > > The parent commit obviously doesn't show that (but probably still has
> > > > > the problem).
> > >
> > > Reverting both
> > > b3e5d80d0c48c0cc ("arm64/build: Warn on orphan section placement")
> > > be2881824ae9eb92 ("arm64/build: Assert for unwanted sections")
> > > seems to solve my problems, without any ill effects?
> > >
> >
> > I cannot reproduce the issue here with my distro GCC+binutils (Debian 8.3.0)
> >
> > The presence of .data.rel.ro and .got.plt sections suggests that the
> > toolchain is using -fpie and/or -z relro to build shared objects
> > rather than a fully linked bare metal binary.
> >
> > Which toolchain are you using? Does adding -fno-pie to the compiler
>
> gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)  from Ubuntu 20.04LTS.
>
> > command line and/or adding -z norelro to the linker command line make
> > any difference?
>
> I'll give that a try later...

This patch just got picked up into the for-next branch of the arm64
tree; it enables `-z norelro` regardless of configs.
https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/commit/?h=for-next/core&id=3b92fa7485eba16b05166fddf38ab42f2ff6ab95
If you apply that, that should help you test `-z norelro` quickly.

-- 
Thanks,
~Nick Desaulniers