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Message-ID: <20170814153213.GU6321@e103592.cambridge.arm.com>
Date: Mon, 14 Aug 2017 16:32:13 +0100
From: Dave Martin <Dave.Martin@....com>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc: kernel-hardening@...ts.openwall.com,
Mark Rutland <mark.rutland@....com>,
Kees Cook <keescook@...omium.org>, Arnd Bergmann <arnd@...db.de>,
Nicolas Pitre <nico@...aro.org>,
Marc Zyngier <marc.zyngier@....com>,
Russell King <linux@...linux.org.uk>,
Tony Lindgren <tony@...mide.com>,
Matt Fleming <matt@...eblueprint.co.uk>,
Thomas Garnier <thgarnie@...gle.com>,
linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 02/30] ARM: assembler: introduce adr_l, ldr_l and str_l
macros
On Mon, Aug 14, 2017 at 01:53:43PM +0100, Ard Biesheuvel wrote:
> Like arm64, ARM supports position independent code sequences that
> produce symbol references with a greater reach than the ordinary
> adr/ldr instructions.
>
> Currently, we use open coded instruction sequences involving literals
> and arithmetic operations. Instead, we can use movw/movt pairs on v7
> CPUs, circumventing the D-cache entirely. For older CPUs, we can emit
> the literal into a subsection, allowing it to be emitted out of line
> while retaining the ability to perform arithmetic on label offsets.
>
> E.g., on pre-v7 CPUs, we can emit a PC-relative reference as follows:
>
> ldr <reg>, 222f
> 111: add <reg>, <reg>, pc
> .subsection 1
> 222: .long <sym> - (111b + 8)
> .previous
>
> This is allowed by the assembler because, unlike ordinary sections,
> subsections are combined into a single section into the object file,
> and so the label references are not true cross-section references that
> are visible as relocations. Note that we could even do something like
>
> add <reg>, pc, #(222f - 111f) & ~0xfff
> ldr <reg>, [<reg>, #(222f - 111f) & 0xfff]
> 111: add <reg>, <reg>, pc
> .subsection 1
> 222: .long <sym> - (111b + 8)
> .previous
>
> if it turns out that the 4 KB range of the ldr instruction is insufficient
> to reach the literal in the subsection, although this is currently not a
> problem (of the 98 objects built from .S files in a multi_v7_defconfig
> build, only 11 have .text sections that are over 1 KB, and the largest one
> [entry-armv.o] is 3308 bytes)
>
> Subsections have been available in binutils since 2004 at least, so
> they should not cause any issues with older toolchains.
>
> So use the above to implement the macros mov_l, adr_l, adrm_l (using ldm
I don't see adrm_l in this patch.
> to load multiple literals at once), ldr_l and str_l, all of which will
> use movw/movt pairs on v7 and later CPUs, and use PC-relative literals
> otherwise.
Also...
By default, I'd assume that we should port _all_ uses of :upper16:/
:lower16: to use these. Does this series consciously do that? Are
there any exceptions?
[...]
Cheers
---Dave
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