On Wed, Sep 19, 2018 at 07:19:56AM -0700, Andy Lutomirski wrote: > > > > On Sep 19, 2018, at 2:55 AM, Tycho Andersen <tycho@xxxxxxxx> wrote: > > > >> On Wed, Sep 12, 2018 at 04:52:38PM -0700, Andy Lutomirski wrote: > >>> On Thu, Sep 6, 2018 at 8:28 AM, Tycho Andersen <tycho@xxxxxxxx> wrote: > >>> The idea here is that the userspace handler should be able to pass an fd > >>> back to the trapped task, for example so it can be returned from socket(). > >>> > >>> I've proposed one API here, but I'm open to other options. In particular, > >>> this only lets you return an fd from a syscall, which may not be enough in > >>> all cases. For example, if an fd is written to an output parameter instead > >>> of returned, the current API can't handle this. Another case is that > >>> netlink takes as input fds sometimes (IFLA_NET_NS_FD, e.g.). If netlink > >>> ever decides to install an fd and output it, we wouldn't be able to handle > >>> this either. > >> > >> An alternative could be to have an API (an ioctl on the listener, > >> perhaps) that just copies an fd into the tracee. There would be the > >> obvious set of options: do we replace an existing fd or allocate a new > >> one, and is it CLOEXEC. Then the tracer could add an fd and then > >> return it just like it's a regular number. > >> > >> I feel like this would be more flexible and conceptually simpler, but > >> maybe a little slower for the common cases. What do you think? > > > > I'm just implementing this now, and there's one question: when do we > > actually do the fd install? Should we do it when the user calls > > SECCOMP_NOTIF_PUT_FD, or when the actual response is sent? It feels > > like we should do it when the response is sent, instead of doing it > > right when SECCOMP_NOTIF_PUT_FD is called, since if there's a > > subsequent signal and the tracer decides to discard the response, > > we'll have to implement some delete mechanism to delete the fd, but it > > would have already been visible to the process, etc. So I'll go > > forward with this unless there are strong objections, but I thought > > I'd point it out just to avoid another round trip. > > > > > > Can you do that non-racily? That is, you need to commit to an fd *number* right away, but what if another thread uses the number before you actually install the fd? I was thinking we could just do an __alloc_fd() and then do the fd_install() when the response is sent or clean up the case that the listener or task dies. I haven't actually tried to run the code yet, so it's possible the locking won't work :) > Do we really allow non-“kill” signals to interrupt the whole process? It might be the case that we don’t really need to clean up from signals if there’s a guarantee that the thread dies. Yes, we do, because of this: https://lkml.org/lkml/2018/3/15/1122 I could change that to just be a killable wait, though; I don't have strong opinions about it and several people have commented that the code is kind of weird. Tycho
