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SubjectRe: [PATCH 4/4] x86/boot: Check that there are no runtime relocations
On 2020-05-25, Ard Biesheuvel wrote:
>On Sun, 24 May 2020 at 23:28, Arvind Sankar <nivedita@alum.mit.edu> 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@alum.mit.edu>
>> ---
>> 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

The output section name does not matter: it will be discarded by the linker.

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

If for some reasons there is at least one SHT_REL and at least one
SHT_RELA, LLD will error "section type mismatch for .rel.dyn", while the
intended diagnostic is the assert below.

>(or even
>
>.rel.dyn : { *(.rel.* }
>.rela.dyn : { *(.rela.*) }
>
>if the output section name matters, and always assert that both are empty)?

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

looks good to me.


FWIW I intend to add -z rel and -z rela to LLD: https://reviews.llvm.org/D80496#inline-738804
(binutils thread https://sourceware.org/pipermail/binutils/2020-May/111244.html)

In case someone builds the x86-64 kernel with -z rel, your suggested
input section description will work out of the box...


>> .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
>>

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