nwfilter: increase pcap buffer size to be compatible with TPACKET_V3

When an nwfilter rule sets the parameter CTRL_IP_LEARNING to "dhcp",
this turns on the "dhcpsnoop" thread, which uses libpcap to monitor
traffic on the domain's tap device and extract the IP address from the
DHCP response.

If libpcap on the host is built with HAVE_TPACKET3 defined (to enable
support for TPACKET_V3), the dhcpsnoop code's initialization of the
libpcap socket would fail with the following error:

  virNWFilterSnoopDHCPOpen:1134 : internal error: pcap_setfilter: can't remove kernel filter: Bad file descriptor

It turns out that this was because TPACKET_V3 requires a larger buffer
size than libvirt was setting (we were setting it to 128k). Changing
the buffer size to 256k eliminates the error, and the dhcpsnoop thread
once again works properly.

A fuller explanation of why TPACKET_V3 requires such a large buffer,
for future git spelunkers:

libpcap calls setsockopt(... SOL_PACKET, PACKET_RX_RING...) to setup a
ring buffer for receiving packets; two of the attributes sent to this
API are called tp_frame_size, and tp_frame_nr. If libpcap was built
with HAVE_TPACKET3 defined, tp_trame_size is set to MAXIMUM_SNAPLEN
(defined in libpcap sources as 262144) and tp_frame_nr is set to:

 [the buffer size we set, i.e. PCAP_BUFFERSIZE i.e. 262144] / tp_frame_size.

So if PCAP_BUFFERSIZE < MAXIMUM_SNAPLEN, then tp_frame_nr (the number
of frames in the ring buffer) is 0, which is nonsensical. This same
value is later used as a multiplier to determine the size for a call
to malloc() (which would also fail).

(NB: if HAVE_TPACKET3 is *not* defined, then tp_frame_size is set to
the snaplen set by the user (in our case 576) plus a small amount to
account for ethernet headers, so 256k is far more than adequate)

Since the TPACKET_V3 code in libpcap actually reads multiple packets
into each frame, it's not a problem to have only a single frame
(especially when we are monitoring such infrequent traffic), so it's
okay to set this relatively small buffer size (in comparison to the
default, which is 2MB), which is important since every guest using
dhcp snooping in a nwfilter rule will hold 2 of these buffers for the
entire life of the guest.

Thanks to Christian Ehrhardt for discovering that buffer size was the
problem (this was not at all obvious from the error that was logged!)

Resolves: https://bugzilla.redhat.com/1547237
Fixes: https://bugs.launchpad.net/libvirt/+bug/1758037

Signed-off-by: Laine Stump <laine@laine.org>
Reviewed-by: Christian Ehrhardt <christian.ehrhardt@canonical.com> (V1)
Reviewed-by: John Ferlan <jferlan@redhat.com>
Tested-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>

src/nwfilter/nwfilter_dhcpsnoop.c

@@ -256,10 +256,21 @@ struct _virNWFilterDHCPDecodeJob {
 # define DHCP_BURST_INTERVAL_S  10 /* sec */
 
 /*
- * libpcap 1.5 requires a 128kb buffer
+ * NB: Any libpcap built with HAVE_TPACKET3 will require
- * 128 kb is bigger than (DHCP_PKT_BURST * PCAP_PBUFSIZE / 2)
+ * PCAP_BUFFERSIZE to be at least 262144 (although
+ * pcap_set_buffer_size() with a lower value will succeed, and the
+ * error will only show up later when pcap_setfilter() is called).
+ *
+ * It is possible that in the future libpcap could increase the
+ * minimum size even further, but due to the fact that each guest
+ * using dhcp snooping keeps 2 pcap sockets open (and thus 2 buffers
+ * allocated) for the life of the guest, we want to minimize the
+ * length of the buffer, so instead of leaving it at the default size
+ * (2MB), we are setting it to the minimum viable size and including
+ * this clue in the source to help quickly resolve the problem when/if
+ * it reoccurs.
  */
-# define PCAP_BUFFERSIZE        (128 * 1024)
+# define PCAP_BUFFERSIZE        (256 * 1024)
 
 # define MAX_QUEUED_JOBS        (DHCP_PKT_BURST + 2 * DHCP_PKT_RATE)
 
@@ -1114,6 +1125,11 @@ virNWFilterSnoopDHCPOpen(const char *ifname, virMacAddr *mac,
         goto cleanup_nohandle;
     }
 
+    /* IMPORTANT: If there is any failure of *any* pcap_* function
+     * during setup of the socket, look to the comment where
+     * PCAP_BUFFERSIZE is defined. It may be too small, even if the
+     * generated error doesn't imply that.
+     */
     if (pcap_set_snaplen(handle, PCAP_PBUFSIZE) < 0 ||
         pcap_set_buffer_size(handle, PCAP_BUFFERSIZE) < 0 ||
         pcap_activate(handle) < 0) {