From: Mark Rutland mark.rutland@arm.com
[ Upstream commit 9abe390e689f4f5c23c5f507754f8678431b4f72 ]
Certain portions of code always need to be position-independent regardless of CONFIG_RELOCATABLE, including code which is executed in an idmap or which is executed before relocations are applied. In some kernel configurations the LLD linker generates position-dependent veneers for such code, and when executed these result in early boot-time failures.
Marc Zyngier encountered a boot failure resulting from this when building a (particularly cursed) configuration with LLVM, as he reported to the list:
https://lore.kernel.org/linux-arm-kernel/86wmjwvatn.wl-maz@kernel.org/
In Marc's kernel configuration, the .head.text and .rodata.text sections end up more than 128MiB apart, requiring a veneer to branch between the two:
| [mark@lakrids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w _text | ffff800080000000 g .head.text 0000000000000000 _text | [mark@lakrids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w primary_entry | ffff8000889df0e0 g .rodata.text 000000000000006c primary_entry,
... consequently, LLD inserts a position-dependent veneer for the branch from _stext (in .head.text) to primary_entry (in .rodata.text):
| ffff800080000000 <_text>: | ffff800080000000: fa405a4d ccmp x18, #0x0, #0xd, pl // pl = nfrst | ffff800080000004: 14003fff b ffff800080010000 <__AArch64AbsLongThunk_primary_entry> ... | ffff800080010000 <__AArch64AbsLongThunk_primary_entry>: | ffff800080010000: 58000050 ldr x16, ffff800080010008 <__AArch64AbsLongThunk_primary_entry+0x8> | ffff800080010004: d61f0200 br x16 | ffff800080010008: 889df0e0 .word 0x889df0e0 | ffff80008001000c: ffff8000 .word 0xffff8000
... and as this is executed early in boot before the kernel is mapped in TTBR1 this results in a silent boot failure.
Fix this by passing '--pic-veneer' to the linker, which will cause the linker to use position-independent veneers, e.g.
| ffff800080000000 <_text>: | ffff800080000000: fa405a4d ccmp x18, #0x0, #0xd, pl // pl = nfrst | ffff800080000004: 14003fff b ffff800080010000 <__AArch64ADRPThunk_primary_entry> ... | ffff800080010000 <__AArch64ADRPThunk_primary_entry>: | ffff800080010000: f004e3f0 adrp x16, ffff800089c8f000 <__idmap_text_start> | ffff800080010004: 91038210 add x16, x16, #0xe0 | ffff800080010008: d61f0200 br x16
I've opted to pass '--pic-veneer' unconditionally, as:
* In addition to solving the boot failure, these sequences are generally nicer as they require fewer instructions and don't need to perform data accesses.
* While the position-independent veneer sequences have a limited +/-2GiB range, this is not a new restriction. Even kernels built with CONFIG_RELOCATABLE=n are limited to 2GiB in size as we have several structues using 32-bit relative offsets and PPREL32 relocations, which are similarly limited to +/-2GiB in range. These include extable entries, jump table entries, and alt_instr entries.
* GNU LD defaults to using position-independent veneers, and supports the same '--pic-veneer' option, so this change is not expected to adversely affect GNU LD.
I've tested with GNU LD 2.30 to 2.42 inclusive and LLVM 13.0.1 to 19.1.0 inclusive, using the kernel.org binaries from:
* https://mirrors.edge.kernel.org/pub/tools/crosstool/ * https://mirrors.edge.kernel.org/pub/tools/llvm/
Signed-off-by: Mark Rutland mark.rutland@arm.com Reported-by: Marc Zyngier maz@kernel.org Cc: Ard Biesheuvel ardb@kernel.org Cc: Nathan Chancellor nathan@kernel.org Cc: Nick Desaulniers ndesaulniers@google.com Cc: Will Deacon will@kernel.org Acked-by: Ard Biesheuvel ardb@kernel.org Reviewed-by: Nathan Chancellor nathan@kernel.org Link: https://lore.kernel.org/r/20240927101838.3061054-1-mark.rutland@arm.com Signed-off-by: Catalin Marinas catalin.marinas@arm.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/arm64/Makefile | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile index f6bc3da1ef110..c8e237b20ef29 100644 --- a/arch/arm64/Makefile +++ b/arch/arm64/Makefile @@ -10,7 +10,7 @@ # # Copyright (C) 1995-2001 by Russell King
-LDFLAGS_vmlinux :=--no-undefined -X +LDFLAGS_vmlinux :=--no-undefined -X --pic-veneer
ifeq ($(CONFIG_RELOCATABLE), y) # Pass --no-apply-dynamic-relocs to restore pre-binutils-2.27 behaviour