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…
