linux-kernel - Re: [RFC] [PATCH] Pre-emption control for userspace

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Date:	Thu, 06 Mar 2014 06:34:32 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Khalid Aziz <khalid.aziz@...cle.com>,
	Ingo Molnar <mingo@...nel.org>, akpm@...ux-foundation.org,
	andi.kleen@...el.com, rob@...dley.net, viro@...iv.linux.org.uk,
	oleg@...hat.com, venki@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC] [PATCH] Pre-emption control for userspace

The no checking is omitting access_ok(), no? Either way, disabling page faults have to be done explicitly.

On March 6, 2014 6:33:04 AM PST, Thomas Gleixner <tglx@...utronix.de> wrote:
>
>
>On Thu, 6 Mar 2014, Peter Zijlstra wrote:
>
>> On Thu, Mar 06, 2014 at 02:45:00PM +0100, Rasmus Villemoes wrote:
>> > Peter Zijlstra <peterz@...radead.org> writes:
>> > 
>> > > On Thu, Mar 06, 2014 at 02:24:43PM +0100, Rasmus Villemoes wrote:
>> > >> Is it possible to implement non-sleeping versions of
>{get,put}_user()?
>> > >
>> > > __{get,put}_user()
>> > 
>> > Huh?
>> > 
>> > arch/x86/include/asm/uaccess.h:
>> > 
>> > /**
>> >  * __get_user: - Get a simple variable from user space, with less
>checking.
>> >  * @x:   Variable to store result.
>> >  * @ptr: Source address, in user space.
>> >  *
>> >  * Context: User context only.  This function may sleep.
>> > 
>> > What am I missing?
>> 
>> __get_user() -> __get_user_nocheck() -> __get_user_size() ->
>__get_user_asm()
>> 
>> And __get_user_asm() seems to generate the required .fixup section
>for
>> this to work in pagefault_disable() context.
>
>Well, it still might sleep if you're using it in page fault enabled
>context. So the documentation of that function sucks.
>
>Thanks,
>
>	tglx

-- 
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