Re: [PATCH] vmlinux.lds.h: CFI: Reduce alignment of jump-table to function alignment

[Mark Rutland]

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@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: "Mohan Rao .vanimina" <mailtoc.mohanrao@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
> Link: https://lore.kernel.org/all/CAL_GTzigiNOMYkOPX1KDnagPhJtFNqSK=1USNbS0wUL4PW6-Uw@xxxxxxxxxxxxxx/
> Fixes: cf68fffb66d60 ("add support for Clang CFI")
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>

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@xxxxxxx>
Tested-by: Mark Rutland <mark.rutland@xxxxxxx>

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
>