linux-kernel - Re: [PATCH] Makefile: Introduce CONFIG_ZERO_CALL_USED

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Date:   Tue, 11 May 2021 14:59:20 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-hardening@...r.kernel.org, Qing Zhao <qing.zhao@...cle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH] Makefile: Introduce CONFIG_ZERO_CALL_USED_REGS

On Mon, May 10, 2021 at 03:01:48PM -0700, Kees Cook wrote:
> On Mon, May 10, 2021 at 02:45:03PM +0100, Mark Rutland wrote:

[...]

> > That's 441301 new MOVs, and the equivalent of 442511 new instructions
> > overall. There are 135728 new MOVs to x16 and x17 specifically, which
> > account for ~31% of that.
> 
> I assume the x16/x17 case could be addressed by the compiler if it
> examined the need for PLTs, or is that too late (in the sense that the
> linker is doing that phase)?

Most (all?) PLTs will be created at link time, and IIUC the compiler
simply has to assume any non-static function might have a PLT, since the
AAPCS permits that. Maybe some of the smaller memory size models don't
permit PLTs, but I have no real knowledge of that area and I'm already
out on a limb.

LTO could probably help with visiblity, but otherwise I don't see a way
the compiler could be sure a PLT won't exist.

> Regardless, I will update the documentation on this feature. :)

Great; thanks!

Mark.