linux-kernel - [PATCH v2 43/62] x86/alternative: Define ELF section entry size for alternatives

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Message-ID: <68a8126c21f4ee054d6c4d6262d0f465b5babd89.1746821544.git.jpoimboe@kernel.org>
Date: Fri,  9 May 2025 13:17:07 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
	Petr Mladek <pmladek@...e.com>,
	Miroslav Benes <mbenes@...e.cz>,
	Joe Lawrence <joe.lawrence@...hat.com>,
	live-patching@...r.kernel.org,
	Song Liu <song@...nel.org>,
	laokz <laokz@...mail.com>,
	Jiri Kosina <jikos@...nel.org>,
	Marcos Paulo de Souza <mpdesouza@...e.com>,
	Weinan Liu <wnliu@...gle.com>,
	Fazla Mehrab <a.mehrab@...edance.com>,
	Chen Zhongjin <chenzhongjin@...wei.com>,
	Puranjay Mohan <puranjay@...nel.org>
Subject: [PATCH v2 43/62] x86/alternative: Define ELF section entry size for alternatives

In preparation for the objtool klp diff subcommand, define the entry
size for the .altinstructions section in its ELF header.  This will
allow tooling to extract individual entries.

Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
 arch/x86/include/asm/alternative.h | 7 +++++--
 arch/x86/kernel/alternative.c      | 2 ++
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index e18cdaa1573c..212761eec886 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -15,6 +15,8 @@
 #define ALT_DIRECT_CALL(feature) ((ALT_FLAG_DIRECT_CALL << ALT_FLAGS_SHIFT) | (feature))
 #define ALT_CALL_ALWAYS		ALT_DIRECT_CALL(X86_FEATURE_ALWAYS)
 
+#define ALTINSTR_SIZE		14
+
 #ifndef __ASSEMBLER__
 
 #include <linux/stddef.h>
@@ -165,7 +167,8 @@ static inline int alternatives_text_reserved(void *start, void *end)
 	"773:\n"
 
 #define ALTINSTR_ENTRY(ft_flags)					      \
-	".pushsection .altinstructions,\"a\"\n"				      \
+	".pushsection .altinstructions, \"aM\", @progbits, "		      \
+		      __stringify(ALTINSTR_SIZE) "\n"			      \
 	" .long 771b - .\n"				/* label           */ \
 	" .long 774f - .\n"				/* new instruction */ \
 	" .4byte " __stringify(ft_flags) "\n"		/* feature + flags */ \
@@ -328,7 +331,7 @@ void nop_func(void);
 741:									\
 	.skip -(((744f-743f)-(741b-740b)) > 0) * ((744f-743f)-(741b-740b)),0x90	;\
 742:									\
-	.pushsection .altinstructions,"a" ;				\
+	.pushsection .altinstructions, "aM", @progbits, ALTINSTR_SIZE ;	\
 	altinstr_entry 740b,743f,flag,742b-740b,744f-743f ;		\
 	.popsection ;							\
 	.pushsection .altinstr_replacement,"ax"	;			\
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index ec220e53cb52..d6064dd87dde 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -425,6 +425,8 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 	u8 *instr, *replacement;
 	struct alt_instr *a, *b;
 
+	BUILD_BUG_ON(ALTINSTR_SIZE != sizeof(struct alt_instr));
+
 	DPRINTK(ALT, "alt table %px, -> %px", start, end);
 
 	/*
-- 
2.49.0