Em Wed, May 28, 2014 at 03:17:40PM +0000, David Laight escreveu:

From: Arnaldo Carvalho de Melo
...

quoted

quoted

But, another question...

In the case that the call is interrupted by a signal handler and some
datagrams have already been received, then the call succeeds, and
returns the number of datagrams received, and 'timeout' is updated with
the remaining time. Maybe that's the right behavior, but I just want to

quoted

Note that what the comment in the existing code says should apply here,
namely that the next recv (m or mmsg) syscall on this socket will return
what is in sock->sk->sk_err, that is the signal:

...

quoted

So, yes, the user _can_ process the packets already copied to userspace,
i.e. no packet loss, and then, on the next call, will receive the signal
notification.

The application shouldn't need to see an EINTR response, any signal handler
should be run when the system call returns to user (regardless of the
system call result code).
If that doesn't happen Linux is badly broken!

quoted

From an application point of view this is exactly the same as the signal

occurring just before/after the kernel entry/exit for the system call.

The call should just return early with success status.
No need to preserve the EINTR response for later.

The same might be appropriate for other errors - maybe including EFAULT
copying non-initial messages to userspace.
Put the message being processed back on the socket queue and return
success with the (non-zero) partial message count.

We don't need to put anything back, if we get an EFAULT for a datagram,
then we stop processing that packet, _dropping_ it (and that is just
like recvmsg works, look at __skb_recv_datagram, the skb_unlink there,
and udp_recvmsg, what happens if skb_copy_and_csum_datagram_iovec fails)
and stop the batch, and if no datagrams were received, return the error
straight away.

But if some datagrams were successfully received, and at that point
_already_ removed from queues and sent successfully to userspace,
recvmmsg will return the number of successfully copied datagrams and
store the error so that it can return on the next syscall,

Please refer to the original discussion on how to report how many
successfully copied datagrams and also report that it stopped before the
timeout and the number of requested datagrams in a batch:

http://lkml.kernel.org/r/200905221022.48790.remi.denis-courmont@nokia.com

What is being discussed here is how to return the EFAULT that may happen
_after_ datagram processing, be it interrupted by an EFAULT, signal, or
plain returning all that was requested, with no errors.

This EFAULT _after_ datagram processing may happen when updating the
remaining timeout, because then how can userspace both receive the
number of successfully copied datagrams (in any of the cases mentioned
in the previous paragraph) and know that that timeout can't be used
because there was a problem while trying to copy it to userspace
(EFAULT)?

- Arnaldo