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ARM: Thumb2: align ALT_UP() sections sufficiently

PATCH In review Ard Biesheuvel 2 participants 4 messages Jan 18, 2022

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Stable branch Arm (32-bit)
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Ard Biesheuvel

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#0 lore raw

PATCH 1/1 ARM: Thumb2: align ALT_UP() sections sufficiently

When building for Thumb2, the .alt.smp.init sections that are emitted by the ALT_UP() patching code may not be 32-bit aligned, even though the fixup_smp_on_up() routine expects that. This results in alignment faults at module load time, which need to be fixed up by the fault handler.

So let's align those sections explicitly, and avoid this from occurring.

Cc <[email protected]>
Signed-off-by Ard Biesheuvel <[email protected]>
---
 arch/arm/include/asm/assembler.h | 2 ++
 arch/arm/include/asm/processor.h | 1 +
 2 files changed, 3 insertions(+)
arch/arm/include/asm/assembler.h +2 -0
index 5a530e25ea1a..51ae4674ca49 100644
@@ -261,6 +261,7 @@ THUMB(	fpreg	.req	r7	)
  */
 #define ALT_UP(instr...)					\
 	.pushsection ".alt.smp.init", "a"			;\
+	.align	2						;\
 	.long	9998b - .					;\
 9997:	instr							;\
 	.if . - 9997b == 2					;\

@@ -272,6 +273,7 @@ THUMB(	fpreg	.req	r7	)
 	.popsection
 #define ALT_UP_B(label)					\
 	.pushsection ".alt.smp.init", "a"			;\
+	.align	2						;\
 	.long	9998b - .					;\
 	W(b)	. + (label - 9998b)					;\
 	.popsection
arch/arm/include/asm/processor.h +1 -0
index 6af68edfa53a..bdc35c0e8dfb 100644
@@ -96,6 +96,7 @@ unsigned long __get_wchan(struct task_struct *p);
 #define __ALT_SMP_ASM(smp, up)						\
 	"9998:	" smp "\n"						\
 	"	.pushsection \".alt.smp.init\", \"a\"\n"		\
+	"	.align	2\n"						\
 	"	.long	9998b - .\n"					\
 	"	" up "\n"						\
 	"	.popsection\n"
Signature · 1 line
2.30.2
maintainer
#1 lore raw
Quoted 6 lines · Ard Biesheuvel

When building for Thumb2, the .alt.smp.init sections that are emitted by the ALT_UP() patching code may not be 32-bit aligned, even though the fixup_smp_on_up() routine expects that. This results in alignment faults at module load time, which need to be fixed up by the fault handler.

So let's align those sections explicitly, and avoid this from occurring.

Are you seeing a problem that this patch fixes?

This really should not matter. .alt.smp.init contents are always a whole number of 32-bit words. These are gathered by the linker into the .init.smpalt section, so the contents should always be a whole number of 32-bit words.

This follows the .init.tagtable section, which is also a 32-bit word aligned structure built by the linker... which follows the .init.arch.info section and .init.proc.info sections which all have 32-bit alignment requirements.

Signature · 2 lines
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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author
#2 lore raw
Quoted 16 lines · Russell King (Oracle)
Quoted 6 lines · Ard Biesheuvel

When building for Thumb2, the .alt.smp.init sections that are emitted by the ALT_UP() patching code may not be 32-bit aligned, even though the fixup_smp_on_up() routine expects that. This results in alignment faults at module load time, which need to be fixed up by the fault handler.

So let's align those sections explicitly, and avoid this from occurring.

Are you seeing a problem that this patch fixes?

This really should not matter. .alt.smp.init contents are always a whole number of 32-bit words. These are gathered by the linker into the .init.smpalt section, so the contents should always be a whole number of 32-bit words.

Quoted 5 lines

This follows the .init.tagtable section, which is also a 32-bit word aligned structure built by the linker... which follows the .init.arch.info section and .init.proc.info sections which all have 32-bit alignment requirements.

This only affects modules, not the core kernel. The .alt.smp.init section in a module is visible to the module loader, which means the module loader will make no attempt to position it at a 32-bit aligned address if the ELF alignment is only 16 bits, which appears to be the default in my Thumb2 build [gcc version 10.3.1 20211117 (Debian 10.3.0-13)]

I only spotted this because do_fixup_smp_on_up() was shown as the most recent in-kernel fixup location in /proc/cpu/alignment.

maintainer
#3 lore raw
Quoted 31 lines · Ard Biesheuvel
Quoted 16 lines · Russell King (Oracle)
Quoted 6 lines · Ard Biesheuvel

When building for Thumb2, the .alt.smp.init sections that are emitted by the ALT_UP() patching code may not be 32-bit aligned, even though the fixup_smp_on_up() routine expects that. This results in alignment faults at module load time, which need to be fixed up by the fault handler.

So let's align those sections explicitly, and avoid this from occurring.

Are you seeing a problem that this patch fixes?

This really should not matter. .alt.smp.init contents are always a whole number of 32-bit words. These are gathered by the linker into the .init.smpalt section, so the contents should always be a whole number of 32-bit words.

Quoted 5 lines

This follows the .init.tagtable section, which is also a 32-bit word aligned structure built by the linker... which follows the .init.arch.info section and .init.proc.info sections which all have 32-bit alignment requirements.

This only affects modules, not the core kernel. The .alt.smp.init section in a module is visible to the module loader, which means the module loader will make no attempt to position it at a 32-bit aligned address if the ELF alignment is only 16 bits, which appears to be the default in my Thumb2 build [gcc version 10.3.1 20211117 (Debian 10.3.0-13)]

Quoted 2 lines

I only spotted this because do_fixup_smp_on_up() was shown as the most recent in-kernel fixup location in /proc/cpu/alignment.

Ok, thanks for the explanation.

Signature · 2 lines
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!