selftests: rds: Add SUDO_USER env variable

This patch modifies rds selftests to use the environment variable
SUDO_USER for tcpdumps if it is set.  This is needed to avoid chown
operations on the vng 9pfs which is not supported.  Passing a user
listed in sudoers avoids the tcpdump privilege drop which may
otherwise create empty pcaps

Signed-off-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260504054143.4027538-7-achender@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/rds/README.txt

@@ -37,6 +37,12 @@ ENV VARIABLES:
 			the specified --rwdir path for logs to persist on
 			the host.
 
+	SUDO_USER	The user name that should be used for tcpdump
+			--relinquish-privileges.  Set this to a user
+			belonging to the sudoers group to avoid drop
+			privilege errors with the vng 9p filesystem
+			which may result in empty pcaps
+
 EXAMPLE:
 
     # Create a suitable gcov enabled .config
@@ -54,6 +60,7 @@ EXAMPLE:
     # launch the tests in a VM
     vng -v --rwdir ./ --run . --user root --cpus 4 -- \
         "export PYTHONPATH=tools/testing/selftests/net/; \
+         export SUDO_USER=example_user; \
          export RDS_LOG_DIR=tools/testing/selftests/net/rds/rds_logs; \
          tools/testing/selftests/net/rds/run.sh"
 

tools/testing/selftests/net/rds/test.py

@@ -134,10 +134,15 @@ if logdir is not None:
     for net in [NET0, NET1]:
         pcap = logdir+'/'+net+'.pcap'
         fd, pcap_tmp = tempfile.mkstemp(suffix=".pcap", prefix=f"{net}-", dir="/tmp")
+
+        tcpdump_cmd = ['ip', 'netns', 'exec', net, '/usr/sbin/tcpdump']
+        sudo_user = os.environ.get('SUDO_USER')
+        if sudo_user:
+            tcpdump_cmd.extend(['-Z', sudo_user])
+        tcpdump_cmd.extend(['-i', 'any', '-w', pcap_tmp])
+
         # pylint: disable-next=consider-using-with
-        p = subprocess.Popen(
-            ['ip', 'netns', 'exec', net,
-             '/usr/sbin/tcpdump', '-i', 'any', '-w', pcap_tmp])
+        p = subprocess.Popen(tcpdump_cmd)
         tcpdump_procs.append((p, pcap_tmp, pcap, fd))
 
 # simulate packet loss, duplication and corruption