Re: [patch 05/11] syslets: core code

[Andi Kleen]

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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