Re: [PATCH 4/4] x86/boot: Check that there are no runtime relocations

[Ard Biesheuvel]

On Sun, 24 May 2020 at 23:28, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
>
> Add a linker script check that there are no runtime relocations, and
> remove the old one that tries to check via looking for specially-named
> sections in the object files.
>
> Drop the tests for -fPIE compiler option and -pie linker option, as they
> are available in all supported gcc and binutils versions (as well as
> clang and lld).
>
> Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
> ---
>  arch/x86/boot/compressed/Makefile      | 28 +++-----------------------
>  arch/x86/boot/compressed/vmlinux.lds.S | 11 ++++++++++
>  2 files changed, 14 insertions(+), 25 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index d3e882e855ee..679a2b383bfe 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -27,7 +27,7 @@ targets := vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2 vmlinux.bin.lzma \
>         vmlinux.bin.xz vmlinux.bin.lzo vmlinux.bin.lz4
>
>  KBUILD_CFLAGS := -m$(BITS) -O2
> -KBUILD_CFLAGS += -fno-strict-aliasing $(call cc-option, -fPIE, -fPIC)
> +KBUILD_CFLAGS += -fno-strict-aliasing -fPIE
>  KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
>  cflags-$(CONFIG_X86_32) := -march=i386
>  cflags-$(CONFIG_X86_64) := -mcmodel=small
> @@ -49,7 +49,7 @@ UBSAN_SANITIZE :=n
>  KBUILD_LDFLAGS := -m elf_$(UTS_MACHINE)
>  # Compressed kernel should be built as PIE since it may be loaded at any
>  # address by the bootloader.
> -KBUILD_LDFLAGS += $(call ld-option, -pie) $(call ld-option, --no-dynamic-linker)
> +KBUILD_LDFLAGS += -pie $(call ld-option, --no-dynamic-linker)
>  LDFLAGS_vmlinux := -T
>
>  hostprogs      := mkpiggy
> @@ -84,30 +84,8 @@ vmlinux-objs-$(CONFIG_ACPI) += $(obj)/acpi.o
>  vmlinux-objs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
>  vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_thunk_$(BITS).o
>
> -# The compressed kernel is built with -fPIC/-fPIE so that a boot loader
> -# can place it anywhere in memory and it will still run. However, since
> -# it is executed as-is without any ELF relocation processing performed
> -# (and has already had all relocation sections stripped from the binary),
> -# none of the code can use data relocations (e.g. static assignments of
> -# pointer values), since they will be meaningless at runtime. This check
> -# will refuse to link the vmlinux if any of these relocations are found.
> -quiet_cmd_check_data_rel = DATAREL $@
> -define cmd_check_data_rel
> -       for obj in $(filter %.o,$^); do \
> -               $(READELF) -S $$obj | grep -qF .rel.local && { \
> -                       echo "error: $$obj has data relocations!" >&2; \
> -                       exit 1; \
> -               } || true; \
> -       done
> -endef
> -
> -# We need to run two commands under "if_changed", so merge them into a
> -# single invocation.
> -quiet_cmd_check-and-link-vmlinux = LD      $@
> -      cmd_check-and-link-vmlinux = $(cmd_check_data_rel); $(cmd_ld)
> -
>  $(obj)/vmlinux: $(vmlinux-objs-y) FORCE
> -       $(call if_changed,check-and-link-vmlinux)
> +       $(call if_changed,ld)
>
>  OBJCOPYFLAGS_vmlinux.bin :=  -R .comment -S
>  $(obj)/vmlinux.bin: vmlinux FORCE
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index d826ab38a8f9..0ac14feacb24 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -11,9 +11,15 @@ OUTPUT_FORMAT(CONFIG_OUTPUT_FORMAT)
>  #ifdef CONFIG_X86_64
>  OUTPUT_ARCH(i386:x86-64)
>  ENTRY(startup_64)
> +
> +#define REL .rela
> +
>  #else
>  OUTPUT_ARCH(i386)
>  ENTRY(startup_32)
> +
> +#define REL .rel
> +
>  #endif
>
>  SECTIONS
> @@ -42,6 +48,9 @@ SECTIONS
>                 *(.rodata.*)
>                 _erodata = . ;
>         }
> +       REL.dyn : {
> +               *(REL.*)
> +       }

Do we really need the macro here? Could we just do

.rel.dyn : { *(.rel.* .rela.*) }

(or even

.rel.dyn  : { *(.rel.* }
.rela.dyn : { *(.rela.*) }

if the output section name matters, and always assert that both are empty)?

>         .got : {
>                 *(.got)
>         }
> @@ -83,3 +92,5 @@ ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0x18, "Unexpected GOT/PLT en
>  #else
>  ASSERT(SIZEOF(.got.plt) == 0 || SIZEOF(.got.plt) == 0xc, "Unexpected GOT/PLT entries detected!")
>  #endif
> +
> +ASSERT(SIZEOF(REL.dyn) == 0, "Unexpected runtime relocations detected!")
> --
> 2.26.2
>