arm64: Align .text section to PAGE_SIZE

PATCH New Jeremy Linton 3 participants 5 messages Oct 26, 2015 arm64

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Jeremy Linton

author

2015-10-26 18:09 UTC #0 lore raw

PATCH v2 1/1 arm64: Align .text section to PAGE_SIZE

v2:
Use ALIGN_DEBUG_RO_MIN() instead of ALIGN() directly.

v1:
It appears that 64k page kernel's die early, in a somewhat random set of locations when built without KVM. Most likely during memblock manipulations (depending on kernel debug options).

Normally when KVM is built into the kernel it has an explicit PAGE_SIZE alignment requirement and that forces the text section to be aligned to PAGE_SIZE. Without it, the alignment granularity is likely to be 4k.

This updates the linker script to assure that the the text section is aligned to a minimum of PAGE_SIZE regardless of build options.

Signed-off-by Jeremy Linton <[email protected]>

---
 arch/arm64/kernel/vmlinux.lds.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

arch/arm64/kernel/vmlinux.lds.S +1 -1

index 8a5d97b..f3fc966 100644
@@ -91,7 +91,7 @@ SECTIONS
 		_text = .;
 		HEAD_TEXT
 	}
-	ALIGN_DEBUG_RO
+	ALIGN_DEBUG_RO_MIN(PAGE_SIZE)
 	.text : {			/* Real text segment		*/
 		_stext = .;		/* Text and read-only data	*/
 			__exception_text_start = .;

Signature · 1 line

2.4.3

Mark Rutland

reviewer

2015-10-26 20:16 UTC #1 lore raw

PATCH v2 1/1 arm64: Align .text section to PAGE_SIZE

Hi,

Quoted 4 lines · Jeremy Linton

v2:
Use ALIGN_DEBUG_RO_MIN() instead of ALIGN() directly.
v1:

Move the v* stuff below the ---, there's no need for that in the permanent commit message.

Quoted 8 lines

It appears that 64k page kernel's die early, in a somewhat random set of locations when built without KVM. Most likely during memblock manipulations (depending on kernel debug options).
Normally when KVM is built into the kernel it has an explicit PAGE_SIZE alignment requirement and that forces the text section to be aligned to PAGE_SIZE. Without it, the alignment granularity is likely to be 4k.

Please replace these with a description of the issue as an effect of CONFIG_DEBUG_RO_DATA && !CONFIG_DEBUG_ALIGN_RODATA, now that Ard figured that out for us, so we don't have to figure it out again in future.

e.g.

A kernel built with DEBUG_RO_DATA && !CONFIG_DEBUG_ALIGN_RODATA doesn't have .text aligned to a page boundary, though fixup_executable works at page-genularity thanks to its use of create_mapping. If .text is not page-aligned, the first page it exists in may be marked non-executable, leading to failures when an attempt is made to execute code in said page.

Quoted 1 line

This updates the linker script to assure that the the text section is

Nit: s/assure/ensure/

Quoted 12 lines

aligned to a minimum of PAGE_SIZE regardless of build options.
Signed-off-by Jeremy Linton <[email protected]>
---
 arch/arm64/kernel/vmlinux.lds.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
arch/arm64/kernel/vmlinux.lds.S +0 -0
index 8a5d97b..f3fc966 100644

arch/arm64/kernel/vmlinux.lds.S SECTIONS · line 94

Quoted 5 lines

arch/arm64/kernel/vmlinux.lds.S +1 -1

@@ -91,7 +91,7 @@ SECTIONS
 		_text = .;
 		HEAD_TEXT
 	}
-	ALIGN_DEBUG_RO
+	ALIGN_DEBUG_RO_MIN(PAGE_SIZE)

I'm in two minds about having a separate macro, as you suggested to save space when !DEBUG_RO_DATA. I won't strongly push for it, though it would be nice to not waste memory here if we don't have to.

Also, couldn't the same issue happen for other sections (e.g. rodata)?

Perhaps it would be better to have something like the following (with PAGE_SHIFT sorted out to be includable in a linker script), so any ALIGN_DEBUG_RO* correctly aligns things to whatever boundary is required, and we don't need to over-align things in the !DEBUG_RO_DATA case.

Thanks,
Mark.

---->8----

arch/arm64/kernel/vmlinux.lds.S +4 -1

index 9807333..4d77757 100644
@@ -60,9 +60,12 @@ PECOFF_FILE_ALIGNMENT = 0x200;
 #define PECOFF_EDATA_PADDING
 #endif
 
-#ifdef CONFIG_DEBUG_ALIGN_RODATA
+#if defined(CONFIG_DEBUG_ALIGN_RODATA)
 #define ALIGN_DEBUG_RO                 . = ALIGN(1<<SECTION_SHIFT);
 #define ALIGN_DEBUG_RO_MIN(min)                ALIGN_DEBUG_RO
+#elif defined(CONFIG_DEBUG_RODATA)
+#define ALIGN_DEBUG_RO                 . = ALIGN(1<<PAGE_SHIFT);
+#define ALIGN_DEBUG_RO_MIN(min)                ALIGN_DEBUG_RO
 #else
 #define ALIGN_DEBUG_RO
 #define ALIGN_DEBUG_RO_MIN(min)                . = ALIGN(min);

Mark Rutland

reviewer

2015-10-26 20:26 UTC #2 lore raw

PATCH v2 1/1 arm64: Align .text section to PAGE_SIZE

Quoted 5 lines

Perhaps it would be better to have something like the following (with PAGE_SHIFT sorted out to be includable in a linker script), so any ALIGN_DEBUG_RO* correctly aligns things to whatever boundary is required, and we don't need to over-align things in the !DEBUG_RO_DATA case.

Ignore the bit about PAGE_SHIFT and linker script. I'd got confused as I had a local typo breaking the build.

With the below diff applied I can boot a defconfig +CONFIG_DEBUG_RODATA +CONFIG_ARM64_64K_PAGES v4.3-rc7 kernel on Juno

Thanks,
Mark.

Quoted 19 lines

---->8----

arch/arm64/kernel/vmlinux.lds.S +4 -1

index 9807333..4d77757 100644
@@ -60,9 +60,12 @@ PECOFF_FILE_ALIGNMENT = 0x200;
 #define PECOFF_EDATA_PADDING
 #endif
 
-#ifdef CONFIG_DEBUG_ALIGN_RODATA
+#if defined(CONFIG_DEBUG_ALIGN_RODATA)
 #define ALIGN_DEBUG_RO                 . = ALIGN(1<<SECTION_SHIFT);
 #define ALIGN_DEBUG_RO_MIN(min)                ALIGN_DEBUG_RO
+#elif defined(CONFIG_DEBUG_RODATA)
+#define ALIGN_DEBUG_RO                 . = ALIGN(1<<PAGE_SHIFT);
+#define ALIGN_DEBUG_RO_MIN(min)                ALIGN_DEBUG_RO
 #else
 #define ALIGN_DEBUG_RO
 #define ALIGN_DEBUG_RO_MIN(min)                . = ALIGN(min);

Laura Abbott

2015-10-26 21:09 UTC #3 lore raw

PATCH v2 1/1 arm64: Align .text section to PAGE_SIZE

Quoted 6 lines · Mark Rutland

Quoted 5 lines
Perhaps it would be better to have something like the following (with PAGE_SHIFT sorted out to be includable in a linker script), so any ALIGN_DEBUG_RO* correctly aligns things to whatever boundary is required, and we don't need to over-align things in the !DEBUG_RO_DATA case.

Quoted 6 lines

Ignore the bit about PAGE_SHIFT and linker script. I'd got confused as I had a local typo breaking the build.
With the below diff applied I can boot a defconfig +CONFIG_DEBUG_RODATA +CONFIG_ARM64_64K_PAGES v4.3-rc7 kernel on Juno

Most of the alignment was found by trial and error so I suspect it was just getting lucky before. I think this should be covering all the cases. Hopefully the protections will make it more restrictive instead of less to catch any more corner cases. For whoever ends up submitting the patch,

Reviewed-by Laura Abbott <[email protected]>

Quoted 22 lines

Thanks,
Mark.

Quoted 19 lines

---->8----

arch/arm64/kernel/vmlinux.lds.S +4 -1

index 9807333..4d77757 100644
@@ -60,9 +60,12 @@ PECOFF_FILE_ALIGNMENT = 0x200;
  #define PECOFF_EDATA_PADDING
  #endif
 
-#ifdef CONFIG_DEBUG_ALIGN_RODATA
+#if defined(CONFIG_DEBUG_ALIGN_RODATA)
  #define ALIGN_DEBUG_RO                 . = ALIGN(1<<SECTION_SHIFT);
  #define ALIGN_DEBUG_RO_MIN(min)                ALIGN_DEBUG_RO
+#elif defined(CONFIG_DEBUG_RODATA)
+#define ALIGN_DEBUG_RO                 . = ALIGN(1<<PAGE_SHIFT);
+#define ALIGN_DEBUG_RO_MIN(min)                ALIGN_DEBUG_RO
  #else
  #define ALIGN_DEBUG_RO
  #define ALIGN_DEBUG_RO_MIN(min)                . = ALIGN(min);

Mark Rutland

reviewer

2015-10-26 21:35 UTC #4 lore raw

PATCH v2 1/1 arm64: Align .text section to PAGE_SIZE

Quoted 10 lines · Laura Abbott

Quoted 9 lines · Mark Rutland
Quoted 5 lines
Perhaps it would be better to have something like the following (with PAGE_SHIFT sorted out to be includable in a linker script), so any ALIGN_DEBUG_RO* correctly aligns things to whatever boundary is required, and we don't need to over-align things in the !DEBUG_RO_DATA case.
Ignore the bit about PAGE_SHIFT and linker script. I'd got confused as I had a local typo breaking the build.

Quoted 8 lines

Quoted 3 lines
With the below diff applied I can boot a defconfig +CONFIG_DEBUG_RODATA +CONFIG_ARM64_64K_PAGES v4.3-rc7 kernel on Juno
Most of the alignment was found by trial and error so I suspect it was just getting lucky before. I think this should be covering all the cases. Hopefully the protections will make it more restrictive instead of less to catch any more corner cases.

Hopefully!

On a somewhat related note, we should probably move .rodata away from .text; currently we give everything between _stext and _etext PAGE_KERNEL_EXEC permissions (i.e. !PXN), and .rodata has no reason to be executable.

Doing so would also make it simpler to add .rodata to the mem_init notice, which I've been meaning to do for a while.

I can take a look at shuffling sections for that tomorrow.

Quoted 3 lines

For whoever ends up submitting the patch,
Reviewed-by Laura Abbott <[email protected]>

Cheers; I'll spin this as a full patch momentarily.

Thanks,
Mark.

Quoted 20 lines

Quoted 19 lines

---->8----

arch/arm64/kernel/vmlinux.lds.S +4 -1

index 9807333..4d77757 100644
@@ -60,9 +60,12 @@ PECOFF_FILE_ALIGNMENT = 0x200;
 #define PECOFF_EDATA_PADDING
 #endif
 
-#ifdef CONFIG_DEBUG_ALIGN_RODATA
+#if defined(CONFIG_DEBUG_ALIGN_RODATA)
 #define ALIGN_DEBUG_RO                 . = ALIGN(1<<SECTION_SHIFT);
 #define ALIGN_DEBUG_RO_MIN(min)                ALIGN_DEBUG_RO
+#elif defined(CONFIG_DEBUG_RODATA)
+#define ALIGN_DEBUG_RO                 . = ALIGN(1<<PAGE_SHIFT);
+#define ALIGN_DEBUG_RO_MIN(min)                ALIGN_DEBUG_RO
 #else
 #define ALIGN_DEBUG_RO
 #define ALIGN_DEBUG_RO_MIN(min)                . = ALIGN(min);