* Re: [PATCH] vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment
2022-09-22 21:57 [PATCH] vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment Will Deacon
@ 2022-09-22 22:00 ` Sami Tolvanen
2022-09-22 22:18 ` Kees Cook
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: Sami Tolvanen @ 2022-09-22 22:00 UTC (permalink / raw)
To: Will Deacon
Cc: LKML, linux-arm-kernel, Mark Rutland, Mohan Rao .vanimina,
Kees Cook, Nathan Chancellor
On Thu, Sep 22, 2022 at 2:57 PM Will Deacon <will@kernel.org> wrote:
>
> 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.
>
> Cc: Sami Tolvanen <samitolvanen@google.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: "Mohan Rao .vanimina" <mailtoc.mohanrao@gmail.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Link: https://lore.kernel.org/all/CAL_GTzigiNOMYkOPX1KDnagPhJtFNqSK=1USNbS0wUL4PW6-Uw@mail.gmail.com/
> Fixes: cf68fffb66d60 ("add support for Clang CFI")
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> include/asm-generic/vmlinux.lds.h | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 7515a465ec03..7c90b1ab3e00 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -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
Thanks for sending the patch!
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Sami
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^ permalink raw reply [flat|nested] 5+ messages in thread* Re: [PATCH] vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment
2022-09-22 21:57 [PATCH] vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment Will Deacon
2022-09-22 22:00 ` Sami Tolvanen
@ 2022-09-22 22:18 ` Kees Cook
2022-09-23 12:24 ` Mark Rutland
2022-09-23 15:40 ` Will Deacon
3 siblings, 0 replies; 5+ messages in thread
From: Kees Cook @ 2022-09-22 22:18 UTC (permalink / raw)
To: Will Deacon
Cc: linux-kernel, linux-arm-kernel, Sami Tolvanen, Mark Rutland,
Mohan Rao .vanimina, Nathan Chancellor
On Thu, Sep 22, 2022 at 10:57:15PM +0100, Will Deacon wrote:
> 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.
Heh. "That's not an image ... THIS is an image."[1]
> 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.
❤️
Reviewed-by: Kees Cook <keescook@chromium.org>
-Kees
[1] https://www.youtube.com/watch?v=rne8pOxGuwM#t=27
--
Kees Cook
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^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment
2022-09-22 21:57 [PATCH] vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment Will Deacon
2022-09-22 22:00 ` Sami Tolvanen
2022-09-22 22:18 ` Kees Cook
@ 2022-09-23 12:24 ` Mark Rutland
2022-09-23 15:40 ` Will Deacon
3 siblings, 0 replies; 5+ messages in thread
From: Mark Rutland @ 2022-09-23 12:24 UTC (permalink / raw)
To: Will Deacon
Cc: linux-kernel, linux-arm-kernel, Sami Tolvanen,
Mohan Rao .vanimina, Kees Cook, Nathan Chancellor
On Thu, Sep 22, 2022 at 10:57:15PM +0100, Will Deacon wrote:
> 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.
>
> Cc: Sami Tolvanen <samitolvanen@google.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: "Mohan Rao .vanimina" <mailtoc.mohanrao@gmail.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Link: https://lore.kernel.org/all/CAL_GTzigiNOMYkOPX1KDnagPhJtFNqSK=1USNbS0wUL4PW6-Uw@mail.gmail.com/
> Fixes: cf68fffb66d60 ("add support for Clang CFI")
> Signed-off-by: Will Deacon <will@kernel.org>
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:
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Mark.
> ---
> include/asm-generic/vmlinux.lds.h | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 7515a465ec03..7c90b1ab3e00 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -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
> --
> 2.37.3.998.g577e59143f-goog
>
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^ permalink raw reply [flat|nested] 5+ messages in thread* Re: [PATCH] vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment
2022-09-22 21:57 [PATCH] vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment Will Deacon
` (2 preceding siblings ...)
2022-09-23 12:24 ` Mark Rutland
@ 2022-09-23 15:40 ` Will Deacon
3 siblings, 0 replies; 5+ messages in thread
From: Will Deacon @ 2022-09-23 15:40 UTC (permalink / raw)
To: linux-kernel, Will Deacon
Cc: catalin.marinas, kernel-team, Mark Rutland, Nathan Chancellor,
Kees Cook, Mohan Rao .vanimina, Sami Tolvanen, linux-arm-kernel
On Thu, 22 Sep 2022 22:57:15 +0100, Will Deacon wrote:
> 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.
>
> [...]
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,
--
Will
https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev
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^ permalink raw reply [flat|nested] 5+ messages in thread