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arm64: relocatable: suppress R_AARCH64_ABS64 relocations in vmlinux

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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>
This commit is contained in:
Ard Biesheuvel 2016-07-24 14:00:13 +02:00 committed by Will Deacon
parent d6732fc402
commit 08cc55b2af
3 changed files with 5 additions and 29 deletions
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2
arch/arm64/Makefile
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@ -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

21
arch/arm64/kernel/head.S
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@ -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

11
arch/arm64/kernel/vmlinux.lds.S
View File

@ -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 = .;