From: Ard Biesheuvel ardb@kernel.org
[ Upstream commit f7b93d42945cc71e1346dd5ae07c59061d56745e ]
When building very large kernels, the logic that emits replacement sequences for alternatives fails when relative branches are present in the code that is emitted into the .altinstr_replacement section and patched in at the original site and fixed up. The reason is that the linker will insert veneers if relative branches go out of range, and due to the relative distance of the .altinstr_replacement from the .text section where its branch targets usually live, veneers may be emitted at the end of the .altinstr_replacement section, with the relative branches in the sequence pointed at the veneers instead of the actual target.
The alternatives patching logic will attempt to fix up the branch to point to its original target, which will be the veneer in this case, but given that the patch site is likely to be far away as well, it will be out of range and so patching will fail. There are other cases where these veneers are problematic, e.g., when the target of the branch is in .text while the patch site is in .init.text, in which case putting the replacement sequence inside .text may not help either.
So let's use subsections to emit the replacement code as closely as possible to the patch site, to ensure that veneers are only likely to be emitted if they are required at the patch site as well, in which case they will be in range for the replacement sequence both before and after it is transported to the patch site.
This will prevent alternative sequences in non-init code from being released from memory after boot, but this is tolerable given that the entire section is only 512 KB on an allyesconfig build (which weighs in at 500+ MB for the entire Image). Also, note that modules today carry the replacement sequences in non-init sections as well, and any of those that target init code will be emitted into init sections after this change.
This fixes an early crash when booting an allyesconfig kernel on a system where any of the alternatives sequences containing relative branches are activated at boot (e.g., ARM64_HAS_PAN on TX2)
Signed-off-by: Ard Biesheuvel ardb@kernel.org Cc: Suzuki K Poulose suzuki.poulose@arm.com Cc: James Morse james.morse@arm.com Cc: Andre Przywara andre.przywara@arm.com Cc: Dave P Martin dave.martin@arm.com Link: https://lore.kernel.org/r/20200630081921.13443-1-ardb@kernel.org Signed-off-by: Will Deacon will@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- arch/arm64/include/asm/alternative.h | 16 ++++++++-------- arch/arm64/kernel/vmlinux.lds.S | 3 --- 2 files changed, 8 insertions(+), 11 deletions(-)
diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h index 4ed869845a23b..1824768fb1ee9 100644 --- a/arch/arm64/include/asm/alternative.h +++ b/arch/arm64/include/asm/alternative.h @@ -68,11 +68,11 @@ void apply_alternatives(void *start, size_t length); ".pushsection .altinstructions,"a"\n" \ ALTINSTR_ENTRY(feature) \ ".popsection\n" \ - ".pushsection .altinstr_replacement, "a"\n" \ + ".subsection 1\n" \ "663:\n\t" \ newinstr "\n" \ "664:\n\t" \ - ".popsection\n\t" \ + ".previous\n\t" \ ".org . - (664b-663b) + (662b-661b)\n\t" \ ".org . - (662b-661b) + (664b-663b)\n" \ ".endif\n" @@ -112,9 +112,9 @@ void apply_alternatives(void *start, size_t length); 662: .pushsection .altinstructions, "a" altinstruction_entry 661b, 663f, \cap, 662b-661b, 664f-663f .popsection - .pushsection .altinstr_replacement, "ax" + .subsection 1 663: \insn2 -664: .popsection +664: .previous .org . - (664b-663b) + (662b-661b) .org . - (662b-661b) + (664b-663b) .endif @@ -155,7 +155,7 @@ void apply_alternatives(void *start, size_t length); .pushsection .altinstructions, "a" altinstruction_entry 663f, 661f, \cap, 664f-663f, 662f-661f .popsection - .pushsection .altinstr_replacement, "ax" + .subsection 1 .align 2 /* So GAS knows label 661 is suitably aligned */ 661: .endm @@ -174,9 +174,9 @@ void apply_alternatives(void *start, size_t length); .macro alternative_else 662: .if .Lasm_alt_mode==0 - .pushsection .altinstr_replacement, "ax" + .subsection 1 .else - .popsection + .previous .endif 663: .endm @@ -187,7 +187,7 @@ void apply_alternatives(void *start, size_t length); .macro alternative_endif 664: .if .Lasm_alt_mode==0 - .popsection + .previous .endif .org . - (664b-663b) + (662b-661b) .org . - (662b-661b) + (664b-663b) diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S index 6edfdf5b061d6..c4e55176f4b6d 100644 --- a/arch/arm64/kernel/vmlinux.lds.S +++ b/arch/arm64/kernel/vmlinux.lds.S @@ -154,9 +154,6 @@ SECTIONS *(.altinstructions) __alt_instructions_end = .; } - .altinstr_replacement : { - *(.altinstr_replacement) - }
. = ALIGN(PAGE_SIZE); __inittext_end = .;