[PATCH net-next v1 3/3] selftests: rds: Fix tcpdump segfault in rds selftests

[Allison Henderson <achender@kernel.org>]

net/rds/test.py sees a segfault in tcpdump when executed through the
ksft runner.

[   21.903713] tcpdump[1469]: segfault at 0 ip 000072100e99126d
sp 00007ffccf740fd0 error 4
[   21.903721]  in libc.so.6[16a26d,7798b149a000+188000]
[   21.905074]  in libc.so.6[16a26d,72100e84f000+188000] likely on
CPU 5 (core 5, socket 0)
[   21.905084] Code: 00 0f 85 a0 00 00 00 48 83 c4 38 89 d8 5b 41 5c
41 5d 41 5e 41 5f 5d c3 0f 1f 44 00 00 48 8b 05 91 8b 09 00 8b 4d ac
64 89 08 <41> 0f b6 07 83 e8 2b a8 fd 0f 84 54 ff ff ff 49 8b 36 4c 89
ff e8
[   21.906760]  likely on CPU 9 (core 9, socket 0)
[   21.913469] Code: 00 0f 85 a0 00 00 00 48 83 c4 38 89 d8 5b 41 5c 41
5d 41 5e 41 5f 5d c3 0f 1f 44 00 00 48 8b 05 91 8b 09 00 8b 4d ac 64 89
08 <41> 0f b6 07 83 e8 2b a8 fd 0f 84 54 ff ff ff 49 8b 36 4c 89 ff e8

The os.fork() call creates extra complexity because it forks the entire
process including the python interpreter.  ip() then calls cmd() which
creates a subprocess.Popen.  We can avoid the extra layering by simply
calling subprocess.Popen directly. Track the process handles directly
and terminate them at cleanup rather than relying on killall. Further
tcpdump's -Z flag attempts to change savefile ownership, which is not
supported by the 9p protocol.  Fix this by writing pcap captures to
"/tmp" during the test and move them to the log directory after tcpdump
exits.

Signed-off-by: Allison Henderson <achender@kernel.org>
---
 tools/testing/selftests/net/rds/test.py | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/tools/testing/selftests/net/rds/test.py b/tools/testing/selftests/net/rds/test.py
index 8256afe6ad6f..93e23e8b256c 100755
--- a/tools/testing/selftests/net/rds/test.py
+++ b/tools/testing/selftests/net/rds/test.py
@@ -11,8 +11,8 @@ import signal
 import socket
 import subprocess
 import sys
-from pwd import getpwuid
-from os import stat
+import tempfile
+import shutil
 
 # Allow utils module to be imported from different directory
 this_dir = os.path.dirname(os.path.realpath(__file__))
@@ -125,14 +125,14 @@ ip(f"-n {NET1} route add {addrs[0][0]}/32 dev {VETH1}")
 ip(f"netns exec {NET0} ping -c 1 {addrs[1][0]}")
 
 # Start a packet capture on each network
+tcpdump_procs = []
 for net in [NET0, NET1]:
-    tcpdump_pid = os.fork()
-    if tcpdump_pid == 0:
-        pcap = logdir+'/'+net+'.pcap'
-        subprocess.check_call(['touch', pcap])
-        user = getpwuid(stat(pcap).st_uid).pw_name
-        ip(f"netns exec {net} /usr/sbin/tcpdump -Z {user} -i any -w {pcap}")
-        sys.exit(0)
+    pcap = logdir+'/'+net+'.pcap'
+    fd, pcap_tmp = tempfile.mkstemp(suffix=".pcap", prefix=f"{net}-", dir="/tmp")
+    p = subprocess.Popen(
+        ['ip', 'netns', 'exec', net,
+         '/usr/sbin/tcpdump', '-i', 'any', '-w', pcap_tmp])
+    tcpdump_procs.append((p, pcap_tmp, pcap, fd))
 
 # simulate packet loss, duplication and corruption
 for net, iface in [(NET0, VETH0), (NET1, VETH1)]:
@@ -248,7 +248,11 @@ for s in sockets:
 print(f"getsockopt(): {nr_success}/{nr_error}")
 
 print("Stopping network packet captures")
-subprocess.check_call(['killall', '-q', 'tcpdump'])
+for p, pcap_tmp, pcap, fd in tcpdump_procs:
+    p.terminate()
+    p.wait()
+    os.close(fd)
+    shutil.move(pcap_tmp, pcap)
 
 # We're done sending and receiving stuff, now let's check if what
 # we received is what we sent.
-- 
2.43.0