> 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? 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.
