Re: [PATCH v4 2/5] userfaultfd: add /dev/userfaultfd for fine grained access control

[Axel Rasmussen <axelrasmussen@google.com>]

On Wed, Jul 20, 2022 at 10:42 AM Nadav Amit <namit@vmware.com> wrote:
>
> On Jul 19, 2022, at 7:32 PM, Peter Xu <peterx@redhat.com> wrote:
>
> > ⚠ External Email
> >
> > On Tue, Jul 19, 2022 at 11:55:21PM +0000, Nadav Amit wrote:
> >> Anyhow, I do want to clarify a bit about the “cross-process support”
> >> userfaultfd situation. Basically, you can already get cross-process support
> >> today, by using calling userfaultfd() on the controlled process and calling
> >> pidfd_open() from another process. It does work and I do not remember any
> >> issues that it introduced (in contrast, for instance, to io-uring, that
> >> would break if you use userfaultfd+iouring+fork today).
> >
> > Do you mean to base it on pidof_getfd()?
>
> autocorrect? :)
>
> I did refer to pidfd_getfd() as a syscall that can be used today by one
> process to control the address space of another process. I did not intend to
> use it for the actual implementation.
>
> > Just want to mention that this will still need collaboration of the target
> > process as userfaultfd needs to be created explicitly there.  From that POV
> > it's still more similar to general SCM_RIGHTS trick to pass over the fd but
> > just to pass it in a different way.
>
> There are also some tricks you can do with ptrace in order not to need the
> collaboration, but they are admittedly fragile.
>
> > IMHO the core change about having /proc/pid/userfaultfd is skipping that
> > only last step to create the handle.
>
> Yes. The point that I was trying to make is that there are no open issues
> with adding support for remote process control through
> /proc/pid/userfaultfd. This is in contrast, for example, for using io-uring
> with userfaultfd. For instance, if you try to use io-uring TODAY with
> userfaultfd (without the async support that I need to add), and you try to
> monitor the fork event, things would break (the new userfaultfd file
> descriptor after fork would be installed on the io-worker thread).
>
> This is all to say that it is really simple to add support for one process
> monitoring userfaultfd of another process, since I understood that Axel had
> concerned that this might be utterly broken…

Mostly I was worried it would be nontrivial to implement, and it isn't
a use case I plan to use so I was hoping to ignore it and defer it to
some future patches. ;)

But, if it "just works" I'm happy to include it in v5.