linux-kernel - [PATCH 2/2] LoongArch: Pass annotate-tablejump option if LTO is enabled

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Message-ID: <20250812132716.1465-3-yangtiezhu@loongson.cn>
Date: Tue, 12 Aug 2025 21:27:16 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Josh Poimboeuf <jpoimboe@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Huacai Chen <chenhuacai@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>
Cc: loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] LoongArch: Pass annotate-tablejump option if LTO is enabled

When compiling with LLVM and CONFIG_LTO_CLANG is set, there exist
many objtool warnings "sibling call from callable instruction with
modified stack frame".

For this special case, the related object file shows that there is
no generated relocation section '.rela.discard.tablejump_annotate'
for the table jump instruction jirl, thus objtool can not know that
what is the actual destination address.

It needs to do something on the LLVM side to make sure that there is
the relocation section '.rela.discard.tablejump_annotate' if LTO is
enabled, but in order to maintain compatibility for the current LLVM
compiler, this can be done in the kernel Makefile for now. Ensure it
is aware of linker with LTO, '--loongarch-annotate-tablejump' needs
to be passed via '-mllvm' to ld.lld.

Note that it should also pass the compiler option -mannotate-tablejump
rather than only pass '-mllvm --loongarch-annotate-tablejump' to ld.lld
if LTO is enabled, otherwise there are no jump info for some table jump
instructions.

Reported-by: Nathan Chancellor <nathan@...nel.org>
Closes: https://lore.kernel.org/loongarch/20250731175655.GA1455142@ax162/
Fixes: e20ab7d454ee ("LoongArch: Enable jump table for objtool")
Co-developed-by: WANG Rui <wangrui@...ngson.cn>
Signed-off-by: WANG Rui <wangrui@...ngson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
---
 arch/loongarch/Makefile | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
index b0703a4e02a2..6cdbcbbe730e 100644
--- a/arch/loongarch/Makefile
+++ b/arch/loongarch/Makefile
@@ -103,6 +103,12 @@ KBUILD_CFLAGS			+= $(call cc-option,-mthin-add-sub) $(call cc-option,-Wa$(comma)
 ifdef CONFIG_OBJTOOL
 ifdef CONFIG_CC_HAS_ANNOTATE_TABLEJUMP
 KBUILD_CFLAGS			+= -mannotate-tablejump
+# The annotate-tablejump option can not be passed to LLVM backend when LTO is enabled.
+# Ensure it is aware of linker with LTO, '--loongarch-annotate-tablejump' needs to be
+# passed via '-mllvm' to ld.lld.
+ifdef CONFIG_LTO_CLANG
+KBUILD_LDFLAGS			+= -mllvm --loongarch-annotate-tablejump
+endif
 else
 KBUILD_CFLAGS			+= -fno-jump-tables # keep compatibility with older compilers
 endif
-- 
2.42.0