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Message-ID: <20170713141142.GE3966@e103592.cambridge.arm.com>
Date: Thu, 13 Jul 2017 15:11:43 +0100
From: Dave Martin <Dave.Martin@....com>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc: Mark Rutland <mark.rutland@....com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will.deacon@....com>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
Laura Abbott <labbott@...oraproject.org>
Subject: Re: [RFC PATCH 07/10] arm64: kernel: switch to register x18 as a
task struct pointer
On Thu, Jul 13, 2017 at 01:27:50PM +0100, Ard Biesheuvel wrote:
> On 13 July 2017 at 11:41, Dave Martin <Dave.Martin@....com> wrote:
> > On Wed, Jul 12, 2017 at 03:44:20PM +0100, Ard Biesheuvel wrote:
> >> In order to free up sp_el0, which we will need to deal with faulting
> >> stack accesses when using virtually mapped stacks, switch to register
> >> x18 as the task struct register. This is permitted by the AAPCS64 ABI,
> >> and simplifies many references to 'current', given that they no longer
> >> involve a MSR instruction to access SP_EL0.
> >>
> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> >
> > [...]
> >
> >> diff --git a/arch/arm64/include/asm/current.h b/arch/arm64/include/asm/current.h
> >> index f6580d4afb0e..b4e3acff699c 100644
> >> --- a/arch/arm64/include/asm/current.h
> >> +++ b/arch/arm64/include/asm/current.h
> >> @@ -13,11 +13,9 @@ struct task_struct;
> >> */
> >> static __always_inline struct task_struct *get_current(void)
> >> {
> >> - unsigned long sp_el0;
> >> + register unsigned long tsk asm ("x18");
> >>
> >> - asm ("mrs %0, sp_el0" : "=r" (sp_el0));
> >> -
> >> - return (struct task_struct *)sp_el0;
> >> + return (struct task_struct *)tsk;
> >
> > Nit:
> >
> > You're explicitly returning an uninitialised variable here: the asm
> > annotation doesn't change the fact that tsk lifetime is that of the
> > function. Sufficiently aggressive GCC can probably optimise the whole
> > thing (and any caller) away as undefined behaviour.
> >
> > The GCC docs say
> >
> > "The only supported use for [specifying registers for local variables]
> > is to specify registers for input and output operands when calling
> > Extended 'asm'".
> >
>
> Ah ok, so it needs to live outside of the function, just like
> current_stack_pointer.
>
> >
> > As an alternative, you could make tsk a global register variable. I
> > don't know whether it should be volatile or not in that case --
> > probably not, since it's constant for a given thread.
> >
> > Alternatively, the following should work:
> >
> > unsigned long ret;
> >
> > asm ("mrs %0, x18" : "=r" (ret));
> >
> > return ret;
> >
> > (with -ffixed-x18, naturally).
> >
>
> Indeed (assuming you meant mov not mrs)
Yes (oops).
Cheers
---Dave
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