linux-kernel - Re: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in another pointer

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Date:   Tue, 9 Feb 2021 07:50:53 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>, msuchanek@...e.de,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in another pointer

On Tue, Feb 09, 2021 at 12:36:20PM +1000, Nicholas Piggin wrote:
> What if you did this?

> +static inline struct task_struct *get_current(void)
> +{
> +	register struct task_struct *task asm ("r2");
> +
> +	return task;
> +}

Local register asm variables are *only* guaranteed to live in that
register as operands to an asm.  See
  https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables
("The only supported use" etc.)

You can do something like

static inline struct task_struct *get_current(void)
{
	register struct task_struct *task asm ("r2");

	asm("" : "+r"(task));

	return task;
}

which makes sure that "task" actually is in r2 at the point of that asm.


Segher