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    SubjectRe: [PATCH 4/4] x86/boot: Check that there are no runtime relocations
    On 2020-05-24, Arvind Sankar 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.*)
    >+ }
    > .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
    >

    `grep -qF .rel.local` from 98f78525371b55ccd1c480207ce10296c72fa340
    may be incorrect.. None of these synthesized dynamic relocation sections is
    called *.rel.local* ...
    (it probably wanted to name .rel.data.rel.ro or .rel.data)


    Reviewed-by: Fangrui Song <maskray@google.com>

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