Re: [RFC PATCH 27/28] x86/kernel: Switch to PIE linking for the core kernel

[Ard Biesheuvel <ardb@kernel.org>]

On Wed, 25 Sept 2024 at 22:25, Vegard Nossum <vegard.nossum@oracle.com> wrote:
>
>
> On 25/09/2024 17:01, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@kernel.org>
> >
> > Build the kernel as a Position Independent Executable (PIE). This
> > results in more efficient relocation processing for the virtual
> > displacement of the kernel (for KASLR). More importantly, it instructs
> > the linker to generate what is actually needed (a program that can be
> > moved around in memory before execution), which is better than having to
> > rely on the linker to create a position dependent binary that happens to
> > tolerate being moved around after poking it in exactly the right manner.
> >
> > Note that this means that all codegen should be compatible with PIE,
> > including Rust objects, so this needs to switch to the small code model
> > with the PIE relocation model as well.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > ---
> >   arch/x86/Kconfig                        |  2 +-
> >   arch/x86/Makefile                       | 11 +++++++----
> >   arch/x86/boot/compressed/misc.c         |  2 ++
> >   arch/x86/kernel/vmlinux.lds.S           |  5 +++++
> >   drivers/firmware/efi/libstub/x86-stub.c |  2 ++
> >   5 files changed, 17 insertions(+), 5 deletions(-)
> >
...
>
> This patch causes a build failure here (on 64-bit):
>
>    LD      .tmp_vmlinux2
>    NM      .tmp_vmlinux2.syms
>    KSYMS   .tmp_vmlinux2.kallsyms.S
>    AS      .tmp_vmlinux2.kallsyms.o
>    LD      vmlinux
>    BTFIDS  vmlinux
> WARN: resolve_btfids: unresolved symbol bpf_lsm_key_free
> FAILED elf_update(WRITE): invalid section entry size
> make[5]: *** [scripts/Makefile.vmlinux:34: vmlinux] Error 255
> make[5]: *** Deleting file 'vmlinux'
> make[4]: *** [Makefile:1153: vmlinux] Error 2
> make[3]: *** [debian/rules:74: build-arch] Error 2
> dpkg-buildpackage: error: make -f debian/rules binary subprocess
> returned exit status 2
> make[2]: *** [scripts/Makefile.package:121: bindeb-pkg] Error 2
> make[1]: *** [/home/opc/linux-mainline-worktree2/Makefile:1544:
> bindeb-pkg] Error 2
> make: *** [Makefile:224: __sub-make] Error 2
>
> The parent commit builds fine. With V=1:
>
> + ldflags='-m elf_x86_64 -z noexecstack --pie -z text -z
> call-nop=suffix-nop -z max-page-size=0x200000 --build-id=sha1
> --orphan-handling=warn --script=./arch/x86/kernel/vmlinux.lds
> -Map=vmlinux.map'
> + ld -m elf_x86_64 -z noexecstack --pie -z text -z call-nop=suffix-nop
> -z max-page-size=0x200000 --build-id=sha1 --orphan-handling=warn
> --script=./arch/x86/kernel/vmlinux.lds -Map=vmlinux.map -o vmlinux
> --whole-archive vmlinux.a .vmlinux.export.o init/version-timestamp.o
> --no-whole-archive --start-group --end-group .tmp_vmlinux2.kallsyms.o
> .tmp_vmlinux1.btf.o
> + is_enabled CONFIG_DEBUG_INFO_BTF
> + grep -q '^CONFIG_DEBUG_INFO_BTF=y' include/config/auto.conf
> + info BTFIDS vmlinux
> + printf '  %-7s %s\n' BTFIDS vmlinux
>    BTFIDS  vmlinux
> + ./tools/bpf/resolve_btfids/resolve_btfids vmlinux
> WARN: resolve_btfids: unresolved symbol bpf_lsm_key_free
> FAILED elf_update(WRITE): invalid section entry size
>
> I can send the full config off-list if necessary, but looks like it
> might be enough to set CONFIG_DEBUG_INFO_BTF=y.
>

Thanks for the report. Turns out that adding the GOT to .rodata bumps
the section's sh_entsize to 8, and libelf complains if the section
size is not a multiple of the entry size.

I'll include a fix in the next revision.