linux-kernel - [PATCH] arm64: alternatives: Move length validation in alternative

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Date:   Tue, 13 Apr 2021 17:08:04 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Jian Cai <jiancai@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Nathan Chancellor <nathan@...nel.org>, stable@...r.kernel.org
Subject: [PATCH] arm64: alternatives: Move length validation in alternative_{insn,endif}

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.

Cc: stable@...r.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@...nel.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