linux-kernel - Re: [PATCH v6 13/25] kbuild: lto: merge module sections

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Message-ID: <202010141548.47CB1BC@keescook>
Date:   Wed, 14 Oct 2020 15:49:10 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH v6 13/25] kbuild: lto: merge module sections

On Mon, Oct 12, 2020 at 05:31:51PM -0700, Sami Tolvanen wrote:
> LLD always splits sections with LTO, which increases module sizes. This
> change adds linker script rules to merge the split sections in the final
> module.
> 
> Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> ---
>  scripts/module.lds.S | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/scripts/module.lds.S b/scripts/module.lds.S
> index 69b9b71a6a47..037120173a22 100644
> --- a/scripts/module.lds.S
> +++ b/scripts/module.lds.S
> @@ -25,5 +25,33 @@ SECTIONS {
>  	__jump_table		0 : ALIGN(8) { KEEP(*(__jump_table)) }
>  }
>  
> +#ifdef CONFIG_LTO_CLANG

In looking at this again -- is this ifdef needed? Couldn't this be done
unconditionally? (Which would make it an independent change...)

-Kees

> +/*
> + * With CONFIG_LTO_CLANG, LLD always enables -fdata-sections and
> + * -ffunction-sections, which increases the size of the final module.
> + * Merge the split sections in the final binary.
> + */
> +SECTIONS {
> +	__patchable_function_entries : { *(__patchable_function_entries) }
> +
> +	.bss : {
> +		*(.bss .bss.[0-9a-zA-Z_]*)
> +		*(.bss..L*)
> +	}
> +
> +	.data : {
> +		*(.data .data.[0-9a-zA-Z_]*)
> +		*(.data..L*)
> +	}
> +
> +	.rodata : {
> +		*(.rodata .rodata.[0-9a-zA-Z_]*)
> +		*(.rodata..L*)
> +	}
> +
> +	.text : { *(.text .text.[0-9a-zA-Z_]*) }
> +}
> +#endif
> +
>  /* bring in arch-specific sections */
>  #include <asm/module.lds.h>
> -- 
> 2.28.0.1011.ga647a8990f-goog
> 

-- 
Kees Cook