arm64: relocatable: suppress R_AARCH64_ABS64 relocations in vmlinux (08cc55b2) · Commits · e / devices / android_kernel_fairphone_FP3

The linker routines that we rely on to produce a relocatable PIE binary treat it as a shared ELF object in some ways, i.e., it emits symbol based R_AARCH64_ABS64 relocations into the final binary since doing so would be appropriate when linking a shared library that is subject to symbol preemption. (This means that an executable can override certain symbols that are exported by a shared library it is linked with, and that the shared library *must* update all its internal references as well, and point them to the version provided by the executable.) Symbol preemption does not occur for OS hosted PIE executables, let alone for vmlinux, and so we would prefer to get rid of these symbol based relocations. This would allow us to simplify the relocation routines, and to strip the .dynsym, .dynstr and .hash sections from the binary. (Note that these are tiny, and are placed in the .init segment, but they clutter up the vmlinux binary.) Note that these R_AARCH64_ABS64 relocations are only emitted for absolute references to symbols defined in the linker script, all other relocatable quantities are covered by anonymous R_AARCH64_RELATIVE relocations that simply list the offsets to all 64-bit values in the binary that need to be fixed up based on the offset between the link time and run time addresses. Fortunately, GNU ld has a -Bsymbolic option, which is intended for shared libraries to allow them to ignore symbol preemption, and unconditionally bind all internal symbol references to its own definitions. So set it for our PIE binary as well, and get rid of the asoociated sections and the relocation code that processes them. Signed-off-by:

Ard Biesheuvel <ard.biesheuvel@linaro.org> [will: fixed conflict with __dynsym_offset linker script entry] Signed-off-by:

Will Deacon <will.deacon@arm.com>

arch/arm64/Makefile

+1 −1

Original line number	Diff line number	Diff line
		@@ -15,7 +15,7 @@ CPPFLAGS_vmlinux.lds = -DTEXT_OFFSET=$(TEXT_OFFSET)
		GZFLAGS :=-9

		ifneq ($(CONFIG_RELOCATABLE),)
		LDFLAGS_vmlinux += -pie
		LDFLAGS_vmlinux += -pie -Bsymbolic
		endif

		KBUILD_DEFCONFIG := defconfig

arch/arm64/kernel/head.S

+3 −18

Original line number	Diff line number	Diff line
		@@ -781,40 +781,25 @@ __primary_switch:
		* Iterate over each entry in the relocation table, and apply the
		* relocations in place.
		*/
		ldr w8, =__dynsym_offset // offset to symbol table
		ldr w9, =__rela_offset // offset to reloc table
		ldr w10, =__rela_size // size of reloc table

		mov_q x11, KIMAGE_VADDR // default virtual offset
		add x11, x11, x23 // actual virtual offset
		add x8, x8, x11 // __va(.dynsym)
		add x9, x9, x11 // __va(.rela)
		add x10, x9, x10 // __va(.rela) + sizeof(.rela)

		0: cmp x9, x10
		b.hs 2f
		b.hs 1f
		ldp x11, x12, [x9], #24
		ldr x13, [x9, #-8]
		cmp w12, #R_AARCH64_RELATIVE
		b.ne 1f
		b.ne 0b
		add x13, x13, x23 // relocate
		str x13, [x11, x23]
		b 0b

		1: cmp w12, #R_AARCH64_ABS64
		b.ne 0b
		add x12, x12, x12, lsl #1 // symtab offset: 24x top word
		add x12, x8, x12, lsr #(32 - 3) // ... shifted into bottom word
		ldrsh w14, [x12, #6] // Elf64_Sym::st_shndx
		ldr x15, [x12, #8] // Elf64_Sym::st_value
		cmp w14, #-0xf // SHN_ABS (0xfff1) ?
		add x14, x15, x23 // relocate
		csel x15, x14, x15, ne
		add x15, x13, x15
		str x15, [x11, x23]
		b 0b

		2:
		1:
		#endif
		ldr x8, =__primary_switched
		br x8

arch/arm64/kernel/vmlinux.lds.S

+1 −10

Original line number	Diff line number	Diff line
		@@ -103,6 +103,7 @@ SECTIONS
		*(.discard)
		(.discard.)
		*(.interp .dynamic)
		*(.dynsym .dynstr .hash)
		}

		. = KIMAGE_VADDR + TEXT_OFFSET;
		@@ -174,19 +175,9 @@ SECTIONS
		.rela : ALIGN(8) {
		(.rela .rela)
		}
		.dynsym : ALIGN(8) {
		*(.dynsym)
		}
		.dynstr : {
		*(.dynstr)
		}
		.hash : {
		*(.hash)
		}

		__rela_offset = ABSOLUTE(ADDR(.rela) - KIMAGE_VADDR);
		__rela_size = SIZEOF(.rela);
		__dynsym_offset = ABSOLUTE(ADDR(.dynsym) - KIMAGE_VADDR);

		. = ALIGN(SEGMENT_ALIGN);
		__init_end = .;