linux-kernel - Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA

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Message-ID: <202102251216.935EFD7F@keescook>
Date:   Thu, 25 Feb 2021 12:16:40 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>, Mark Brown <broonie@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Kristina Martsenko <kristina.martsenko@....com>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Scull <ascull@...gle.com>,
        David Brazdil <dbrazdil@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

On Thu, Feb 25, 2021 at 12:20:56PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> When looking at kernel size optimizations, I found that arm64
> does not currently support HAVE_LD_DEAD_CODE_DATA_ELIMINATION,
> which enables the --gc-sections flag to the linker.
> 
> I see that for a defconfig build with llvm, there are some
> notable improvements from enabling this, in particular when
> combined with the recently added CONFIG_LTO_CLANG_THIN
> and CONFIG_TRIM_UNUSED_KSYMS:
> 
>    text    data     bss     dec     hex filename
> 16570322 10998617 506468 28075407 1ac658f defconfig/vmlinux
> 16318793 10569913 506468 27395174 1a20466 trim_defconfig/vmlinux
> 16281234 10984848 504291 27770373 1a7be05 gc_defconfig/vmlinux
> 16029705 10556880 504355 27090940 19d5ffc gc+trim_defconfig/vmlinux
> 17040142 11102945 504196 28647283 1b51f73 thinlto_defconfig/vmlinux
> 16788613 10663201 504196 27956010 1aa932a thinlto+trim_defconfig/vmlinux
> 16347062 11043384 502499 27892945 1a99cd1 gc+thinlto_defconfig/vmlinux
> 15759453 10532792 502395 26794640 198da90 gc+thinlto+trim_defconfig/vmlinux
> 
> I needed a small change to the linker script to get clean randconfig
> builds, but I have not done any meaningful boot testing on it to
> see if it works. If there are no regressions, I wonder whether this
> should be autmatically done for LTO builds, given that it improves
> both kernel size and compile speed.
> 
> Link: https://lore.kernel.org/lkml/CAK8P3a05VZ9hSKRzVTxTn+1nf9E+gqebJWTj6N23nfm+ELHt9A@mail.gmail.com/
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook