linux-kernel - Re: [PATCH v1 2/3] objtool/LoongArch: Fix unreachable instruction warnings about EFISTUB

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Message-ID: <4thrzifl6ntk7kdf65egt4srzkbrxqoqf7yzmasblwvaq3qwmt@vigfgpbxzjkq>
Date: Thu, 4 Sep 2025 14:46:33 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: Tiezhu Yang <yangtiezhu@...ngson.cn>, 
	Peter Zijlstra <peterz@...radead.org>, Nathan Chancellor <nathan@...nel.org>, loongarch@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/3] objtool/LoongArch: Fix unreachable instruction
 warnings about EFISTUB

On Thu, Sep 04, 2025 at 10:39:30AM -0700, Josh Poimboeuf wrote:
> On Thu, Sep 04, 2025 at 11:59:30AM +0800, Huacai Chen wrote:
> > This is from RISC-V code.
> > 
> > __HEAD
> > SYM_CODE_START(_start)
> >         /*
> >          * Image header expected by Linux boot-loaders. The image header data
> >          * structure is described in asm/image.h.
> >          * Do not modify it without modifying the structure and all bootloaders
> >          * that expects this header format!!
> >          */
> > #ifdef CONFIG_EFI
> >         /*
> >          * This instruction decodes to "MZ" ASCII required by UEFI.
> >          */
> >         c.li s4,-13
> >         j _start_kernel
> > #else
> >         /* jump to start kernel */
> >         j _start_kernel
> >         /* reserved */
> >         .word 0
> > #endif
> > 
> > The HEAD section has instructions, if you change it into a data
> > section then it loses the "x" attribute.

Actually, the "x" attribute isn't needed for vmlinux.  The vmlinux
linker script places it in the text region regardless.

Moving the data to a data section should be really simple, something
like the below.

And yes, even the above RISC-V code can be in a data section.  Those
instructions are part of the 'struct riscv_image_header' data structure.

diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
index e3865e92a917a..c42500d9fad81 100644
--- a/arch/loongarch/kernel/head.S
+++ b/arch/loongarch/kernel/head.S
@@ -17,7 +17,7 @@
 
 #include "efi-header.S"
 
-	__HEAD
+	__HEADDATA
 
 _head:
 	.word	IMAGE_DOS_SIGNATURE	/* "MZ", MS-DOS header */
diff --git a/arch/loongarch/kernel/vmlinux.lds.S b/arch/loongarch/kernel/vmlinux.lds.S
index 08ea921cdec16..fc35ef349aba6 100644
--- a/arch/loongarch/kernel/vmlinux.lds.S
+++ b/arch/loongarch/kernel/vmlinux.lds.S
@@ -38,6 +38,7 @@ SECTIONS
 	. = VMLINUX_LOAD_ADDRESS;
 
 	_text = .;
+	HEAD_DATA_SECTION
 	HEAD_TEXT_SECTION
 
 	. = ALIGN(PECOFF_SEGMENT_ALIGN);
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 6b2311fa41393..c74492e1baa5a 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -629,6 +629,11 @@
 		*(.static_call.text)					\
 		__static_call_text_end = .;
 
+#define HEAD_DATA_SECTION						\
+	.head.data : AT(ADDR(.head.data) - LOAD_OFFSET) {		\
+		KEEP(*(.head.data))					\
+	}
+
 /* Section used for early init (in .S files) */
 #define HEAD_TEXT  KEEP(*(.head.text))
 
diff --git a/include/linux/init.h b/include/linux/init.h
index 331886205049e..fcb02ab3faae2 100644
--- a/include/linux/init.h
+++ b/include/linux/init.h
@@ -98,6 +98,7 @@
 
 /* For assembly routines */
 #define __HEAD		.section	".head.text","ax"
+#define __HEADDATA	.section	".head.data","aw"
 #define __INIT		.section	".init.text","ax"
 #define __FINIT		.previous