On Wednesday, 25 January 2006 03:46, Benjamin LaHaise wrote:
> On Wed, Jan 25, 2006 at 12:35:38AM +0100, Rafael J. Wysocki wrote:
> > > > + if (!access_ok(VERIFY_WRITE, (unsigned long __user *)arg, _IOC_SIZE(cmd))) {
> > > > + error = -EINVAL;
> > > > + break;
> > > > + }
> > >
> > > Why do we need an access_ok() here?
> >
> > Because we use __put_user() down the road?
> >
> > The problem is if the address is wrong we should not try to call
> > alloc_swap_page() at all. If we did, we wouldn't be able to return the result
> > and we would leak a swap page.
>
> Then access_ok() is not the droid you are looking for... since it won't
> catch several cases (out of memory being the most obvious).
Thanks, I haven't thought about it.
> Doing an early put_user() wouldn't hurt and reduces the chance of later failure
> even further. __put_user() should never be used outside of a select few
> performance critical code paths.
Do you mean to use a fake put_user() instead of access_ok()? And then
put_user() once again or is it reasonable to call __put_user() with the same
arg?
Rafael
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