On Tue, Apr 13, 2021 at 5:09 PM Nathan Chancellor nathan@kernel.org wrote:

After commit 2decad92f473 ("arm64: mte: Ensure TIF_MTE_ASYNC_FAULT is set atomically"), LLVM's integrated assembler fails to build entry.S:

<instantiation>:5:7: error: expected assembly-time absolute expression .org . - (664b-663b) + (662b-661b) ^ <instantiation>:6:7: error: expected assembly-time absolute expression .org . - (662b-661b) + (664b-663b) ^

The root cause is LLVM's assembler has a one-pass design, meaning it cannot figure out these instruction lengths when the .org directive is outside of the subsection that they are in, which was changed by the .arch_extension directive added in the above commit.

Apply the same fix from commit 966a0acce2fc ("arm64/alternatives: move length validation inside the subsection") to the alternative_endif macro, shuffling the .org directives so that the length validation happen will always happen in the same subsections. alternative_insn has not shown any issue yet but it appears that it could have the same issue in the future so just preemptively change it.

Thanks Nathan. Reviewed-by: Nick Desaulniers ndesaulniers@google.com Tested-by: Nick Desaulniers ndesaulniers@google.com

I did some additional disassembly comparison. In case we ever need it again, I'll copy it below for posterity.

$ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu make LLVM=1 LLVM_IAS=1 -j72 O=/tmp/a defconfig all $ b4 am https://lore.kernel.org/lkml/20210414000803.662534-1-nathan@kernel.org/ -o - | git am -3 $ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu make LLVM=1 LLVM_IAS=1 -j72 O=/tmp/b defconfig all $ for f in $(find /tmp/a/arch/arm64 -name *.o); do llvm-objdump -dr $f > $f.txt; done $ for f in $(find /tmp/b/arch/arm64 -name *.o); do llvm-objdump -dr $f > $f.txt; done $ for f in $(find /tmp/a/arch/arm64 -name *.o); do diff -u $f.txt $(echo $f.txt|sed 's/a/b/'); done | less

For no difference. You can check more sections than .text by changing `-d` to `-D` for llvm-objdump, though you're going to get a lot of noise related to changes in .strtab and relocations referring to debug info (.debug_str). But if I drop your patch, rebuild, and recompare, I see the same differences.

Cc: stable@vger.kernel.org Fixes: f7b93d42945c ("arm64/alternatives: use subsections for replacement sequences") Link: https://github.com/ClangBuiltLinux/linux/issues/1347 Signed-off-by: Nathan Chancellor nathan@kernel.org

---

Apologies if my explanation or terminology is off, I am only now getting more familiar with assembly.

arch/arm64/include/asm/alternative-macros.h | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/alternative-macros.h b/arch/arm64/include/asm/alternative-macros.h index 5df500dcc627..8a078fc662ac 100644 --- a/arch/arm64/include/asm/alternative-macros.h +++ b/arch/arm64/include/asm/alternative-macros.h @@ -97,9 +97,9 @@ .popsection .subsection 1 663: \insn2 -664: .previous

•   .org    . - (664b-663b) + (662b-661b)
  

+664: .org . - (664b-663b) + (662b-661b) .org . - (662b-661b) + (664b-663b)

•   .previous
    .endif
  

.endm

@@ -169,11 +169,11 @@ */ .macro alternative_endif 664:

•   .org    . - (664b-663b) + (662b-661b)
  

•   .org    . - (662b-661b) + (664b-663b)
    .if .Lasm_alt_mode==0
    .previous
    .endif
  

•   .org    . - (664b-663b) + (662b-661b)
  

•   .org    . - (662b-661b) + (664b-663b)
  

.endm

/*

base-commit: 738fa58ee1328481d1d7889e7c430b3401c571b9

2.31.1.272.g89b43f80a5