Re: [PATCH v3 11/21] objtool: Allow empty alternatives
From: Josh Poimboeuf <jpoimboe@kernel.org>
Date: 2026-08-08 00:11:45
Also in:
linux-arm-kernel, lkml
Subsystem:
arm64 port (aarch64 architecture), the rest · Maintainers:
Catalin Marinas, Will Deacon, Linus Torvalds
On Fri, Aug 07, 2026 at 04:40:34PM -0700, Josh Poimboeuf wrote:
On Wed, May 13, 2026 at 09:37:11AM +0200, Peter Zijlstra wrote:quoted
On Tue, May 12, 2026 at 08:33:45PM -0700, Josh Poimboeuf wrote:quoted
arm64 can have empty alternatives, which are effectively no-ops. Ignore them. While at it, fix a memory leak.How does this happen?Yeah, that commit log sucks... The case I stumbled on was the dcache_by_myline_op_nosync macro: alternative_if ARM64_WORKAROUND_4311569 .ifnc \op, cvau mov \start, \tmp mov \tmp, xzr cbnz \start, .Ldcache_op\@ .endif alternative_else_nop_endif That's actually a bit weird, the .ifnc should probably be outside the alternative_if. Let me try fixing that instead.
I'll probably drop the above patch (leaving empty alternatives as a hard
error) and replace it with this one:
commit 5dd83bdddff38f4df2829b05f876e57215b2e850
Author: Josh Poimboeuf [off-list ref]
Date: Fri Aug 7 16:41:10 2026 -0700
arm64: Remove unnecessary empty alternatives
The code in arch/arm64/mm/cache.S creates three empty alternatives, all
from calling the dcache_by_myline_op_nosync asm macro. If \op == cvau,
it creates an empty alternative for ARM64_WORKAROUND_4311569.
Since orig_len == 0, the alternative patching code mostly ignores it,
though it does do an unnecessary clean_dcache_range_nopatch(). It also
triggers an objtool error ("empty alternative entry").
Clean it up by moving the .ifnc check outside the alternative emit.
Signed-off-by: Josh Poimboeuf [off-list ref]
diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index effae53e9739d..6e059efced5bb 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h@@ -404,13 +404,13 @@ alternative_else_nop_endif add \start, \start, \linesz cmp \start, \end b.lo .Ldcache_op\@ -alternative_if ARM64_WORKAROUND_4311569 .ifnc \op, cvau +alternative_if ARM64_WORKAROUND_4311569 mov \start, \tmp mov \tmp, xzr cbnz \start, .Ldcache_op\@ - .endif alternative_else_nop_endif + .endif _cond_uaccess_extable .Ldcache_op\@, \fixup .endm