Re: [PATCH v4 3.0-rc2-tip 2/22] 2: uprobes: Breakground page replacement.

Oleg Nesterov
2011-06-14T15:43:08Z

On 06/14, Peter Zijlstra wrote:
>
> On Tue, 2011-06-14 at 16:27 +0200, Oleg Nesterov wrote:
> > On 06/14, Peter Zijlstra wrote:
> > >
> > > On Mon, 2011-06-13 at 19:00 +0200, Oleg Nesterov wrote:
> > > >
> > > > Also. This is called under down_read(mmap_sem), can't we race with
> > > > access_process_vm() modifying the same memory?
> > >
> > > Shouldn't matter COW and similar things are serialized using the pte
> > > lock.
> >
> > Yes, but afaics this doesn't matter. Suppose that write_opcode() is
> > called when access_process_vm() does copy_to_user_page(). We can cow
> > the page before memcpy() completes.
>
> access_process_vm() will end up doing a FOLL_WRITE itself when
> copy_to_user_page() is called since write=1 in that case.
>
> At that point we have a COW-race, someone wins, but the other will then
> return the same page.
>
> At this point further PTRACE pokes can indeed race with the memcpy in
> write_opcode().

Currently it can't, write_opcode() does another cow. But that cow can,
and this is the same, yes.

> A possible fix would be to lock_page() around
> copy_to_user_page() (its already done in set_page_dirty_lock(), so
> pulling it out shouldn't matter much).

Yes, or write_opcode() could take mmap_sem for writing as Srikar suggests.

But do we really care? Whatever we do we can race with the other updates
to this memory. Say, someone can write to vma->vm_file.

Oleg.