linux/net/unix
Matthew Dawson a0917e0bc6 datagram: When peeking datagrams with offset < 0 don't skip empty skbs
Due to commit e6afc8ace6 ("udp: remove
headers from UDP packets before queueing"), when udp packets are being
peeked the requested extra offset is always 0 as there is no need to skip
the udp header.  However, when the offset is 0 and the next skb is
of length 0, it is only returned once.  The behaviour can be seen with
the following python script:

from socket import *;
f=socket(AF_INET6, SOCK_DGRAM | SOCK_NONBLOCK, 0);
g=socket(AF_INET6, SOCK_DGRAM | SOCK_NONBLOCK, 0);
f.bind(('::', 0));
addr=('::1', f.getsockname()[1]);
g.sendto(b'', addr)
g.sendto(b'b', addr)
print(f.recvfrom(10, MSG_PEEK));
print(f.recvfrom(10, MSG_PEEK));

Where the expected output should be the empty string twice.

Instead, make sk_peek_offset return negative values, and pass those values
to __skb_try_recv_datagram/__skb_try_recv_from_queue.  If the passed offset
to __skb_try_recv_from_queue is negative, the checked skb is never skipped.
__skb_try_recv_from_queue will then ensure the offset is reset back to 0
if a peek is requested without an offset, unless no packets are found.

Also simplify the if condition in __skb_try_recv_from_queue.  If _off is
greater then 0, and off is greater then or equal to skb->len, then
(_off || skb->len) must always be true assuming skb->len >= 0 is always
true.

Also remove a redundant check around a call to sk_peek_offset in af_unix.c,
as it double checked if MSG_PEEK was set in the flags.

V2:
 - Moved the negative fixup into __skb_try_recv_from_queue, and remove now
redundant checks
 - Fix peeking in udp{,v6}_recvmsg to report the right value when the
offset is 0

V3:
 - Marked new branch in __skb_try_recv_from_queue as unlikely.

Signed-off-by: Matthew Dawson <matthew@mjdsystems.ca>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
2017-08-18 15:12:54 -07:00
..
af_unix.c datagram: When peeking datagrams with offset < 0 don't skip empty skbs 2017-08-18 15:12:54 -07:00
diag.c unix_diag: fix incorrect sign extension in unix_lookup_by_ino 2016-02-19 23:49:23 -05:00
garbage.c net: unix: properly re-increment inflight counter of GC discarded candidates 2017-03-21 15:25:10 -07:00
Kconfig net: Default UDP and UNIX diag to 'n'. 2012-01-07 12:13:06 -08:00
Makefile unix_diag: Write it into kbuild 2011-12-16 13:48:29 -05:00
sysctl_net_unix.c net: Convert uses of typedef ctl_table to struct ctl_table 2013-06-13 02:36:09 -07:00