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

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On Jul 19, 2022, at 7:32 PM, Peter Xu <peterx@xxxxxxxxxx> 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…




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