vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment
PATCH 1/1 vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment
Due to undocumented, hysterical raisins on x86, the CFI jump-table sections in .text are needlessly aligned to PMD_SIZE in the vmlinux linker script. When compiling a CFI-enabled arm64 kernel with a 64KiB page-size, a PMD maps 512MiB of virtual memory and so the .text section increases to a whopping 940MiB and blows the final Image up to 960MiB. Others report a link failure.
Since the CFI jump-table requires only instruction alignment, reduce the alignment directives to function alignment for parity with other parts of the .text section. This reduces the size of the .text section for the aforementioned 64KiB page size arm64 kernel to 19MiB for a much more reasonable total Image size of 39MiB.
--- include/asm-generic/vmlinux.lds.h | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
include/asm-generic/vmlinux.lds.h +1 -2
@@ -543,10 +543,9 @@*/ #ifdef CONFIG_CFI_CLANG #define TEXT_CFI_JT \ - . = ALIGN(PMD_SIZE); \ + ALIGN_FUNCTION(); \ __cfi_jt_start = .; \ *(.text..L.cfi.jumptable .text..L.cfi.jumptable.*) \ - . = ALIGN(PMD_SIZE); \ __cfi_jt_end = .;#else #define TEXT_CFI_JT
Signature · 1 line
2.37.3.998.g577e59143f-goog
Quoted 32 lines · Will Deacon
Due to undocumented, hysterical raisins on x86, the CFI jump-table sections in .text are needlessly aligned to
PMD_SIZEin the vmlinux linker script. When compiling a CFI-enabled arm64 kernel with a 64KiB page-size, a PMD maps 512MiB of virtual memory and so the .text section increases to a whopping 940MiB and blows the final Image up to 960MiB. Others report a link failure.Since the CFI jump-table requires only instruction alignment, reduce the alignment directives to function alignment for parity with other parts of the .text section. This reduces the size of the .text section for the aforementioned 64KiB page size arm64 kernel to 19MiB for a much more reasonable total Image size of 39MiB.
Cc Sami Tolvanen <[email protected]>Cc Mark Rutland <[email protected]>Cc "Mohan Rao .vanimina" <[email protected]>Cc Kees Cook <[email protected]>Cc Nathan Chancellor <[email protected]>Link https://lore.kernel.org/all/CAL_GTzigiNOMYkOPX1KDnagPhJtFNqSK=1USNbS0wUL4PW6-Uw@mail.gmail.com/ open hereFixes cf68fffb66d60 ("add support for Clang CFI")Signed-off-by Will Deacon <[email protected]>--- include/asm-generic/vmlinux.lds.h | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)include/asm-generic/vmlinux.lds.h +0
-0@@ -543,10 +543,9 @@*/
include/asm-generic/vmlinux.lds.h line 551
Quoted 10 lines
include/asm-generic/vmlinux.lds.h +1
-2#ifdef CONFIG_CFI_CLANG #define TEXT_CFI_JT \ - . = ALIGN(PMD_SIZE); \ + ALIGN_FUNCTION(); \ __cfi_jt_start = .; \ *(.text..L.cfi.jumptable .text..L.cfi.jumptable.*) \ - . = ALIGN(PMD_SIZE); \ __cfi_jt_end = .;#else #define TEXT_CFI_JT
Thanks for sending the patch!
Sami
Quoted 6 lines · Will Deacon
Due to undocumented, hysterical raisins on x86, the CFI jump-table sections in .text are needlessly aligned to
PMD_SIZEin the vmlinux linker script. When compiling a CFI-enabled arm64 kernel with a 64KiB page-size, a PMD maps 512MiB of virtual memory and so the .text section increases to a whopping 940MiB and blows the final Image up to 960MiB. Others report a link failure.
Heh. "That's not an image ... THIS is an image."[1]
Quoted 5 lines
Since the CFI jump-table requires only instruction alignment, reduce the alignment directives to function alignment for parity with other parts of the .text section. This reduces the size of the .text section for the aforementioned 64KiB page size arm64 kernel to 19MiB for a much more reasonable total Image size of 39MiB.
❤️
-Kees
[1] https://www.youtube.com/watch?v=rne8pOxGuwM#t=27
Signature · 1 line
Kees Cook
Quoted 13 lines · Will Deacon
Due to undocumented, hysterical raisins on x86, the CFI jump-table sections in .text are needlessly aligned to
PMD_SIZEin the vmlinux linker script. When compiling a CFI-enabled arm64 kernel with a 64KiB page-size, a PMD maps 512MiB of virtual memory and so the .text section increases to a whopping 940MiB and blows the final Image up to 960MiB. Others report a link failure.Since the CFI jump-table requires only instruction alignment, reduce the alignment directives to function alignment for parity with other parts of the .text section. This reduces the size of the .text section for the aforementioned 64KiB page size arm64 kernel to 19MiB for a much more reasonable total Image size of 39MiB.
Quoted 8 lines
Cc Sami Tolvanen <[email protected]>Cc Mark Rutland <[email protected]>Cc "Mohan Rao .vanimina" <[email protected]>Cc Kees Cook <[email protected]>Cc Nathan Chancellor <[email protected]>Link https://lore.kernel.org/all/CAL_GTzigiNOMYkOPX1KDnagPhJtFNqSK=1USNbS0wUL4PW6-Uw@mail.gmail.com/ open hereFixes cf68fffb66d60 ("add support for Clang CFI")Signed-off-by Will Deacon <[email protected]>
Thanks for this!
I gave this a spin atop v6.-rc3 defconfig with:
* CONFIG_ARM64_64K_PAGES=y
* CONFIG_LTO_CLANG_THIN=y
* CONFIG_CFI_CLANG=y
The sizes clearly speak for themselves:
| % ls -al *-v6.0-rc3-64K-CFI* | -rwxr-xr-x 1 mark mark 959693312 Sep 23 13:13 Image-v6.0-rc3-64K-CFI | -rwxr-xr-x 1 mark mark 38781440 Sep 23 13:19 Image-v6.0-rc3-64K-CFI-patched | -rwxr-xr-x 1 mark mark 1378631080 Sep 23 13:13 vmlinux-v6.0-rc3-64K-CFI | -rwxr-xr-x 1 mark mark 453015376 Sep 23 13:19 vmlinux-v6.0-rc3-64K-CFI-patched
... and before and after applying the patch, the kernel booted fine under QEMU (in TCG mode, as Cortex-A53).
I see that in scripts/module.lds.S we place the CFI bits into the .text section, and that's only aligned to PAGE_SIZE, so we don't have a similar issue for modules.
FWIW:
Mark.
Quoted 23 lines
--- include/asm-generic/vmlinux.lds.h | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)include/asm-generic/vmlinux.lds.h +1
-2@@ -543,10 +543,9 @@*/ #ifdef CONFIG_CFI_CLANG #define TEXT_CFI_JT \ - . = ALIGN(PMD_SIZE); \ + ALIGN_FUNCTION(); \ __cfi_jt_start = .; \ *(.text..L.cfi.jumptable .text..L.cfi.jumptable.*) \ - . = ALIGN(PMD_SIZE); \ __cfi_jt_end = .;#else #define TEXT_CFI_JT Signature · 1 line
2.37.3.998.g577e59143f-goog
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Will Deacon
AppliedQuoted 8 lines · Will Deacon
Due to undocumented, hysterical raisins on x86, the CFI jump-table sections in .text are needlessly aligned to
PMD_SIZEin the vmlinux linker script. When compiling a CFI-enabled arm64 kernel with a 64KiB page-size, a PMD maps 512MiB of virtual memory and so the .text section increases to a whopping 940MiB and blows the final Image up to 960MiB. Others report a link failure.[...]
Applied to arm64 (for-next/fixes), thanks!
[1/1] vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment
https://git.kernel.org/arm64/c/13b056696291
Cheers,
Signature · 5 lines
Will https://fixes.arm64.dev https://next.arm64.dev https://will.arm64.dev