On Thu, Aug 30, 2018 at 10:10 PM Willem de Bruijn
[off-list ref] wrote:

On Wed, Aug 29, 2018 at 9:05 AM Arnd Bergmann [off-list ref] wrote:

quoted

The SIOCGSTAMP/SIOCGSTAMPNS ioctl commands are implemented by many
socket protocol handlers, and all of those end up calling the same
sock_get_timestamp()/sock_get_timestampns() helper functions, which
results in a lot of duplicate code.

With the introduction of 64-bit time_t on 32-bit architectures, this
gets worse, as we then need four different ioctl commands in each
socket protocol implementation.

To simplify that, let's add a new .gettstamp() operation in
struct proto_ops, and move ioctl implementation into the common
sock_ioctl()/compat_sock_ioctl_trans() functions that these all go
through.

We can reuse the sock_get_timestamp() implementation, but generalize
it so it can deal with both native and compat mode, as well as
timeval and timespec structures.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>

This also will simplify fixing a recently reported race condition with
sock_get_timestamp [1]. That calls sock_enable_timestamp, which
modifies sk->sk_flags, without taking the socket lock. Currently some
callers of sock_get_timestamp hold the lock (ax25, netrom, qrtr), many
don't. See also how this patch removes the lock_sock in the netrom
case. Moving the call to sock_gettstamp outside the protocol handlers
will allow taking the lock inside the function.

I suppose it would be best to always take that lock then, rather than
removing the lock as my patch does at the moment.

If this is the only valid implementation of .gettstamp, the indirect
call could be avoided in favor of a simple branch.

I thought about that as well, but I could not come up with a
good way to encode the difference between socket protocols
that allow timestamping and those that don't.

I think ideally we would just call sock_gettstamp() unconditonally
on every socket, and have that function decide whether timestamps
make sense or not. The part I did not understand is which ones
actually want the timestamps or not. Most protocols that
implement the ioctls also assign skb->tstamp, but there are some
protocols in which I could not see skb->tstamp ever being set,
and some that set it but don't seem to have the ioctls.

Looking at it again, it seems that sock_gettstamp() should
actually deal with this gracefully: it will return a -EINVAL
error condition if the timestamp remains at the
SK_DEFAULT_STAMP initial value, which is probably
just as appropriate (or better) as the current -ENOTTY
default, and if we are actually recording timestamps, we
might just as well report them.

Acked-by: Willem de Bruijn <willemb@google.com>

Thanks,

      Arnd