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

[Ard Biesheuvel]

On Tue, 27 Oct 2020 at 11:20, Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
>
> Hi Jean-Philippe,
>
> On Tue, Oct 27, 2020 at 11:09 AM Jean-Philippe Brucker
> <jean-philippe@xxxxxxxxxx> wrote:
> > On Mon, Oct 26, 2020 at 06:38:46PM +0100, Ard Biesheuvel wrote:
> > > > > > 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)
> >
> > I have the same problem with one of my debug configs and Linux v5.10-rc1,
> > and can reproduce with the Debian 8.3.0 toolchain, by using the arm64
> > defconfig and disabling CONFIG_MODULES:
> >
> > ld -EL -maarch64elf --no-undefined -X -z norelro -shared -Bsymbolic -z notext --no-apply-dynamic-relocs --fix-cortex-a53-843419 --orphan-handling=warn --build-id=sha1 --strip-debug -o .tmp_vmlinux.kallsyms1 -T ./arch/arm64/kernel/vmlinux.lds --whole-archive arch/arm64/kernel/head.o init/built-in.a usr/built-in.a arch/arm64/built-in.a kernel/built-in.a certs/built-in.a mm/built-in.a fs/built-in.a ipc/built-in.a security/built-in.a crypto/built-in.a block/built-in.a arch/arm64/lib/built-in.a lib/built-in.a drivers/built-in.a sound/built-in.a net/built-in.a virt/built-in.a --no-whole-archive --start-group arch/arm64/lib/lib.a lib/lib.a ./drivers/firmware/efi/libstub/lib.a --end-group
> > ld: Unexpected GOT/PLT entries detected!
> > ld: Unexpected run-time procedure linkages detected!
> >
> > Adding -fno-pie to this command doesn't fix the problem.
> >
> > Note that when cross-building with a GCC 10.2 and binutils 2.35.1 I also
> > get several "aarch64-linux-gnu-ld: warning: -z norelro ignored" in
> > addition to the error, but I don't get that warning with the 8.3.0
> > toolchain.
>
> Thanks, my config (renesas_defconfig) also had CONFIG_MODULES disabled.
> Enabling that fixes the link error due to unexpected entries, but the
> .eh_frame orphan section warning is still there.
>

Looks like this is caused by the VFIO driver doing nasty things with
symbol_get(), resulting in weak symbol references being emitted. Since
taking the address of a weak symbol can yield NULL, the only way for
the linker to accommodate this is to use GOT indirection for the
direct symbol reference, so that the GOT entry can be set to NULL if
the reference is not satisfied at link time.