linux-kernel - Re: [PATCH v8 08/10] x86, mpx: add prctl commands PR_MPX_REGISTER, PR_MPX

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Date:	Mon, 15 Sep 2014 01:00:25 +0100
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Qiaowei Ren <qiaowei.ren@...el.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>, x86@...nel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 08/10] x86, mpx: add prctl commands PR_MPX_REGISTER,
 PR_MPX_UNREGISTER

> The base of the bounds directory is set into mm_struct during
> PR_MPX_REGISTER command execution. This member can be used to
> check whether one application is mpx enabled.

Not really because by the time you ask the question another thread might
have decided to unregister it.


> +int mpx_register(struct task_struct *tsk)
> +{
> +	struct mm_struct *mm = tsk->mm;
> +
> +	if (!cpu_has_mpx)
> +		return -EINVAL;
> +
> +	/*
> +	 * runtime in the userspace will be responsible for allocation of
> +	 * the bounds directory. Then, it will save the base of the bounds
> +	 * directory into XSAVE/XRSTOR Save Area and enable MPX through
> +	 * XRSTOR instruction.
> +	 *
> +	 * fpu_xsave() is expected to be very expensive. In order to do
> +	 * performance optimization, here we get the base of the bounds
> +	 * directory and then save it into mm_struct to be used in future.
> +	 */
> +	mm->bd_addr = task_get_bounds_dir(tsk);
> +	if (!mm->bd_addr)
> +		return -EINVAL;

What stops two threads calling this in parallel ?
> +
> +	return 0;
> +}
> +
> +int mpx_unregister(struct task_struct *tsk)
> +{
> +	struct mm_struct *mm = current->mm;
> +
> +	if (!cpu_has_mpx)
> +		return -EINVAL;
> +
> +	mm->bd_addr = NULL;

or indeed calling this in parallel

What are the semantics across execve() ?

Alan
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