Re: install_special_mapping && vm_pgoff (Was: vvar, gup && coredump)

[Oleg Nesterov <oleg@redhat.com>]

On 03/17, Andy Lutomirski wrote:
>
> On Tue, Mar 17, 2015 at 6:43 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > But at least the bug exposed by the test-case looks clear:
> >
> >         do_linear_fault:
> >
> >                 vmf->pgoff = (((address & PAGE_MASK) - vma->vm_start) >> PAGE_SHIFT)
> >                                 + vma->vm_pgoff;
> >                 ...
> >
> >                 special_mapping_fault:
> >
> >                         pgoff = vmf->pgoff - vma->vm_pgoff;
> >
> >
> > So special_mapping_fault() can only work if this mapping starts from the
> > first page in ->pages[].
> >
> > So perhaps we need _something like_ the (wrong/incomplete) patch below...
> >
> > Or, really, perhaps we can create vdso_mapping ? So that map_vdso() could
> > simply mmap the anon_inode file...
>
> That's slightly tricky, I think, because it could start showing up in
> /proc/PID/map_files or whatever it's called, and I don't think we want
> that.

Hmm. To me this looke liks improvement. And again, with this change
uprobe-in-vdso can work.

OK, this is off-topic right now, lets forget this for the moment.

> Your patch does look like a considerable improvement, though.  Let me
> see if I can find some time to fold it in with the rest of my special
> mapping rework over the next few days.

I'll try to recheck... Perhaps I'll send this (changed) patch for review.
This is a bugfix, even if the bug is minor.

And note that with this change vvar->access() becomes trivial. I think it
makes sense to fix "gup() fails in vvar" too. Gdb developers have enough
other problems with the poor kernel interfaces ;)

Oleg.