Re: How to transfer memory from PCI memory directly to user space safely and portable?

["H. Peter Anvin" <hpa@transmeta.com>]

Philipp Rumpf wrote:
> 
> On Mon, Nov 27, 2000 at 10:36:34AM -0800, H. Peter Anvin wrote:
> > Followup to:  <20001126151120.V2272@parcelfarce.linux.theplanet.co.uk>
> > By author:    Philipp Rumpf <prumpf@parcelfarce.linux.theplanet.co.uk>
> > In newsgroup: linux.dev.kernel
> > >
> > > I hope count isn't provided by userspace here ?
> > >
> > > > 1. What happens if the user space memory is swapped to disk? Will
> > > > verify_area() make sure that the memory is in physical RAM when it returns,
> > > > or will it return -EFAULT, or will something even worse happen?
> > >
> > > On i386, you'll sleep implicitly waiting for the page fault to be handled;  in
> > > the generic case, anything could happen.
> > >
> >
> > That doesn't sound right.  I would expect it to wait for the page to
> > be brought in on any and all architectures, otherwise it seems rather
> > impossible to write portable Linux kernel code.
> 
> The code in question was
>         memcpy_fromio(user_space_dst, iobase, count);
> 
> Assuming user_space_dst is a userspace pointer, what I said is true;  on some
> architectures we will be dereferencing random pointers in kernel space, and
> we won't get -EFAULT right on any architecture.
> 
> Or did I miss something ?
> 

Yes, the post you responded to was talking about verify_area() [which is,
admittedly, obsolete.]

	-hpa

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