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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