On Wed, Sep 19, 2018 at 7:38 AM, Tycho Andersen <tycho@xxxxxxxx> wrote: > 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 :) I would be very surprised if the locking works. How can you run a thread in a process when another thread has allocated but not installed an fd and is blocked for an arbitrarily long time? > >> 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'm still not sure I see the problem. Suppose I'm implementing a user notifier for a nasty syscall like recvmsg(). If I'm the tracer, by the time I decide to install an fd, I've committed to returning something other than -EINTR, even if a non-fatal signal is sent before I finish. No rollback should be necessary. In the (unlikely?) event that some tracer needs to be able to rollback an fd installation to return -EINTR, a SECCOMP_NOTIF_CLOSE_FD operation should be good enough, I think. Or maybe PUT_FD can put -1 to delete an fd. --Andy
