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

[Sami Tolvanen]

On Thu, Sep 22, 2022 at 2:57 PM Will Deacon <will@xxxxxxxxxx> 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>
> ---
>  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@xxxxxxxxxx>

Sami