Date: Mon, 14 Aug 2017 16:50:02 +0100 From: Dave Martin <Dave.Martin@....com> To: Ard Biesheuvel <ard.biesheuvel@...aro.org> Cc: 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>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Russell King <linux@...linux.org.uk>, Tony Lindgren <tony@...mide.com>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, Thomas Garnier <thgarnie@...gle.com>, Matt Fleming <matt@...eblueprint.co.uk> Subject: Re: [PATCH 02/30] ARM: assembler: introduce adr_l, ldr_l and str_l macros On Mon, Aug 14, 2017 at 04:38:02PM +0100, Ard Biesheuvel wrote: > On 14 August 2017 at 16:29, Dave Martin <Dave.Martin@....com> wrote: > > 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 > > > > This is reinventing ldr= > > > > I seem to remember ldr= barfing on things that .long happily accepts > > though, was this the reason? > > > > Yes. ldr = does not accept expressions involving symbols, only plain > symbols or expressions that evaluate to constants. > > So something like > > ldr <reg>, =<sym> - <label> > > is rejected while the equivalent > > ldr <reg>, 0f > 0: .long <sym> - <label> > > does work. I wouldn't bother trying to rationalise gas' behaviour here. I think it's an accident of implementation rather than there being some fundamental reason for it. AFAICT gas could quite happily resolve ldr= in exactly the same way as .long and thus not have this problem. But we can't rewrite history. [...] > >> + .macro __adldst_l, op, reg, sym, tmp, c > >> + .if __LINUX_ARM_ARCH__ < 7 > >> + ldr\c \tmp, 111f > >> + .subsection 1 > >> + .align 2 > >> +111: .long \sym - (222f + ARM_PC_BIAS) > > > > See above comment about ldr=. > > > >> + .previous > >> + .else > >> + W(movw\c\()) \tmp, #:lower16:\sym - (222f + ARM_PC_BIAS) > >> + W(movt\c\()) \tmp, #:upper16:\sym - (222f + ARM_PC_BIAS) > > > > Why W()? > > > > There are no narrow forms of these instructions anyway -- if there were > > then they couldn't accommodate a 16-bit immediate. > > > > That's a trick, actually, which I failed to add a comment for. > > We use .arm sections in the thumb2 kernel, and using these macros > there would result in the wrong offset to be used. Adding the .w > suffix forces an error in the assembler which even results in a fairly > meaningful error message complaining about using .w in ARM code. Ewww... I think it'd be best to add a comment explaining that. There's a fair change someone will trip over this at some point (or worse, "fix" the assembly errors). > > >> + .endif > >> +222: > >> + .ifc \op, add > >> + add\c \reg, \tmp, pc > >> + .elseif CONFIG_THUMB2_KERNEL == 1 > >> + add \tmp, \tmp, pc > >> + \op\c \reg, [\tmp] > > > > Shame > > \op\c \reg, [pc, \tmp] > > doesn't work. > > > > But it doesn't, apparently. > > > > No, thumb2 does not allow that Meh. Oh well. [...] Cheers ---Dave
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