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ctdb-common: Set immediate mode for pcap capture

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Fix a problem where ctdb_killtcp (almost always) fails to capture
packets with --enable-pcap and libpcap ≥ 1.9.1.  The problem is due to
a gradual change in libpcap semantics when using
pcap_get_selectable_fd(3PCAP) to get a file descriptor and then using
that file descriptor in non-blocking mode.

pcap_set_immediate_mode(3PCAP) says:

  pcap_set_immediate_mode() sets whether immediate mode should be set
  on a capture handle when the handle is activated.  In immediate
  mode, packets are always delivered as soon as they arrive, with no
  buffering.

and

  On Linux, with previous releases of libpcap, capture devices are
  always in immediate mode; however, in 1.5.0 and later, they are, by
  default, not in immediate mode, so if pcap_set_immediate_mode() is
  available, it should be used.

However, it wasn't until libpcap commit
2ade7676101366983bd4f86bc039ffd25da8c126 (before libpcap 1.9.1) that
it became a requirement to use pcap_set_immediate_mode(), even with a
timeout of 0.

More explanation in this libpcap issue comment:

  https://github.com/the-tcpdump-group/libpcap/issues/860#issuecomment-541204548

Do a configure check for pcap_set_immediate_mode() even though it has
existed for 10 years.  It is easy enough.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15451

Signed-off-by: Martin Schwenke <mschwenke@ddn.com>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>

Autobuild-User(master): Amitay Isaacs <amitay@samba.org>
Autobuild-Date(master): Tue Aug 15 10:53:52 UTC 2023 on atb-devel-224

(cherry picked from commit dc7b48c404337891b5105df4d6751cf549a533eb)

Autobuild-User(v4-18-test): Jule Anger <janger@samba.org>
Autobuild-Date(v4-18-test): Tue Aug 29 12:27:35 UTC 2023 on atb-devel-224
This commit is contained in:
Martin Schwenke 2023-08-15 12:34:20 +10:00 committed by Jule Anger
parent 2b5512712e
commit 5fac5b7b2f
2 changed files with 8 additions and 0 deletions
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7
ctdb/common/system_socket.c
View File

@ -1007,6 +1007,13 @@ int ctdb_sys_open_capture_socket(const char *iface, void **private_data)
DBG_ERR("Failed to set timeout for pcap capture\n");
goto fail;
}
#ifdef HAVE_PCAP_SET_IMMEDIATE_MODE
ret = pcap_set_immediate_mode(pt, 1);
if (ret < 0) {
DBG_ERR("Failed to set immediate mode for pcap capture\n");
goto fail;
}
#endif
ret = pcap_activate(pt);
if (ret < 0) {
DBG_ERR("Failed to activate pcap capture\n");

1
ctdb/wscript
View File

@ -221,6 +221,7 @@ def configure(conf):
if not conf.CHECK_FUNCS_IN('pcap_open_live', 'pcap', headers='pcap.h'):
Logs.error('Need libpcap')
sys.exit(1)
conf.CHECK_FUNCS_IN('pcap_set_immediate_mode', 'pcap', headers='pcap.h')
if not conf.CHECK_FUNCS_IN('backtrace backtrace_symbols', 'execinfo',
checklibc=True, headers='execinfo.h'):