On Wed, Jul 20, 2022 at 10:42 AM Nadav Amit <namit@xxxxxxxxxx> wrote: > > 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… 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.
