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Message-ID: <20241012112619.1762860-5-sashal@kernel.org>
Date: Sat, 12 Oct 2024 07:26:01 -0400
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
stable@...r.kernel.org
Cc: Mark Rutland <mark.rutland@....com>,
Marc Zyngier <maz@...nel.org>,
Ard Biesheuvel <ardb@...nel.org>,
Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Will Deacon <will@...nel.org>,
Catalin Marinas <catalin.marinas@....com>,
Sasha Levin <sashal@...nel.org>,
linux-arm-kernel@...ts.infradead.org,
llvm@...ts.linux.dev
Subject: [PATCH AUTOSEL 6.11 05/16] arm64: Force position-independent veneers
From: Mark Rutland <mark.rutland@....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@...rids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w _text
| ffff800080000000 g .head.text 0000000000000000 _text
| [mark@...rids:~/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@....com>
Reported-by: Marc Zyngier <maz@...nel.org>
Cc: Ard Biesheuvel <ardb@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Will Deacon <will@...nel.org>
Acked-by: Ard Biesheuvel <ardb@...nel.org>
Reviewed-by: Nathan Chancellor <nathan@...nel.org>
Link: https://lore.kernel.org/r/20240927101838.3061054-1-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@....com>
Signed-off-by: Sasha Levin <sashal@...nel.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
--
2.43.0
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