On 2018-09-12, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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? When we first discussed this I sent a (probably somewhat broken) patch for "dup4" which would let you inject a file descriptor into a different process -- I still think that having a method for injecting a file descriptor without needing ptrace (and SCM_RIGHTS) shenanigans would be generally useful. (With "dup4" you have a more obvious API for flags and whether you allocate a new fd or use a specific one.) -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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