linux-kernel - Re: [PATCH] MIPS: Allow FPU emulator to use non-stack area.

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Date:	Tue, 7 Oct 2014 09:43:39 +0100
From:	James Hogan <james.hogan@...tec.com>
To:	David Daney <ddaney.cavm@...il.com>, <Zubair.Kakakhel@...tec.com>,
	<peterz@...radead.org>, <paul.gortmaker@...driver.com>,
	<davidlohr@...com>, <macro@...ux-mips.org>, <chenhc@...ote.com>,
	<zajec5@...il.com>, <keescook@...omium.org>,
	<alex@...x-smith.me.uk>, <tglx@...utronix.de>,
	<blogic@...nwrt.org>, <jchandra@...adcom.com>,
	<paul.burton@...tec.com>, <qais.yousef@...tec.com>,
	<ralf@...ux-mips.org>, <markos.chandras@...tec.com>,
	<manuel.lauss@...il.com>, <akpm@...ux-foundation.org>,
	<lars.persson@...s.com>, <torvalds@...ux-foundation.org>
CC:	<linux-kernel@...r.kernel.org>, <linux-mips@...ux-mips.org>,
	<libc-alpha@...rceware.org>, David Daney <david.daney@...ium.com>
Subject: Re: [PATCH] MIPS: Allow FPU emulator to use non-stack area.

Hi David,

On 06/10/14 21:11, David Daney wrote:
> Any userspace thread desiring a non-executable stack,
> must allocate a 4-byte aligned area at least 8 bytes long

<snip>

> +SYSCALL_DEFINE1(set_fpuemul_xol_area, unsigned long, addr)
> +{
> +	struct thread_info *ti = task_thread_info(current);
> +
> +	ti->fpu_emul_xol = addr;
> +	return 0;
> +}
> +

<snip>

> -	/* Ensure that the two instructions are in the same cache line */
> -	fr = (struct emuframe __user *)
> -		((regs->regs[29] - sizeof(struct emuframe)) & ~0x7);
> +	if (ti->fpu_emul_xol != ~0ul)
> +		fr = (struct emuframe *)ti->fpu_emul_xol;
> +	else
> +		/* Ensure that the two instructions are in the same cache line */
> +		fr = (struct emuframe __user *)
> +			((regs->regs[29] - sizeof(struct emuframe)) & ~0x7);

I know your patch was more an RFC, but on a technical note, this
comment/code seems to imply that the address should be 8 byte aligned
(rather than 4 byte) so they both land in the same cache line. Also, I
think the alignment should be validated in the syscall.

Cheers
James
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