On Mon, Nov 2, 2020 at 8:50 PM Sargun Dhillon <sar...@sargun.me> wrote: > On Mon, Nov 2, 2020 at 11:45 AM Michael Kerrisk (man-pages) > <mtk.manpa...@gmail.com> wrote: > > Caveats regarding blocking system calls > > Suppose that the target performs a blocking system call (e.g., > > accept(2)) that the supervisor should handle. The supervisor > > might then in turn execute the same blocking system call. > > > > In this scenario, it is important to note that if the target's > > system call is now interrupted by a signal, the supervisor is not > > informed of this. If the supervisor does not take suitable steps > > to actively discover that the target's system call has been > > canceled, various difficulties can occur. Taking the example of > > accept(2), the supervisor might remain blocked in its accept(2) > > holding a port number that the target (which, after the > > interruption by the signal handler, perhaps closed its listening > > socket) might expect to be able to reuse in a bind(2) call. > > > > Therefore, when the supervisor wishes to emulate a blocking system > > call, it must do so in such a way that it gets informed if the > > target's system call is interrupted by a signal handler. For > > example, if the supervisor itself executes the same blocking > > system call, then it could employ a separate thread that uses the > > SECCOMP_IOCTL_NOTIF_ID_VALID operation to check if the target is > > still blocked in its system call. Alternatively, in the accept(2) > > example, the supervisor might use poll(2) to monitor both the > > notification file descriptor (so as as to discover when the > > target's accept(2) call has been interrupted) and the listening > > file descriptor (so as to know when a connection is available). > > > > If the target's system call is interrupted, the supervisor must > > take care to release resources (e.g., file descriptors) that it > > acquired on behalf of the target. > > > > Does that seem okay? > > > This is far clearer than my explanation. The one thing is that *just* > poll is not good enough, you would poll, with some timeout, and when > that timeout is hit, check if all the current notifications are valid, > as poll isn't woken up when an in progress notification goes off > AFAIK.
Arguably that's so terrible that it qualifies for being in the BUGS section of the manpage. If you want this to be fixed properly, I recommend that someone implements my proposal from <https://lore.kernel.org/lkml/cag48ez1o2h5hdikpo-_o-toxtheu8gnzot9wogdsnrnjqsw...@mail.gmail.com/>, unless you can come up with something better.
