linux-kernel - Re: [patch 05/11] syslets: core code

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Date:	Tue, 13 Feb 2007 23:30:17 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@....com.au>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ulrich Drepper <drepper@...hat.com>,
	Zach Brown <zach.brown@...cle.com>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	"David S. Miller" <davem@...emloft.net>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 05/11] syslets: core code

On Tue, Feb 13, 2007 at 11:24:43PM +0100, Ingo Molnar wrote:
> > > +	memset(atom->args, 0, sizeof(atom->args));
> > > +
> > > +	ret |= __get_user(arg_ptr, &uatom->arg_ptr[0]);
> > > +	if (!arg_ptr)
> > > +		return ret;
> > > +	if (!access_ok(VERIFY_WRITE, arg_ptr, sizeof(*arg_ptr)))
> > > +		return -EFAULT;
> > 
> > It's a little unclear why you do that many individual access_ok()s. 
> > And why is the target constant sized anyways?
> 
> each indirect pointer has to be checked separately, before dereferencing 
> it. (Andrew pointed out that they should be VERIFY_READ, i fixed that in 
> my tree)

But why only constant sized? It could be a variable length object, couldn't it?

If it's an array it could be all checked together

(i must be missing something here) 

> > If it's only a few pages you don't need any resource accounting. If 
> > it's more then it's nasty to steal the users quota. I think plain 
> > gup() would be better.
> 
> get_user_pages() would have to be limited in some way - and i didnt want 

If you only use it for a small ring buffer it is naturally limited.

Also beancounter will fix that eventually.

> a single page is enough for 1024 completion pointers - that's more than 
> enough for most purposes - and the default mlock limit is 40K.

Then limit it to a single page and use gup

-Andi
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