selftests: rds: pin RDS sockets to their intended transport

The RDS selftests create AF_RDS sockets but never selects a transport,
so the transport is chosen implicitly based on network topology when
the socket is bound.  If underlying connection establishment fails, RDS
can fall back to another transport (e.g. loopback) and the test still
passes, silently bypassing the intended datapath it is meant to
exercise.

Set SO_RDS_TRANSPORT to the proper RDS_TRANS_IB or RDS_TRANS_TCP before
they are bound, so the test fails loudly if the intended transport is
unavailable rather than passing on a different path.

Signed-off-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260602050657.26389-3-achender@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/tools/testing/selftests/net/rds/test.py b/tools/testing/selftests/net/rds/test.py
index 08f2a846a8ab..9e4df01cb0d4 100755
--- a/tools/testing/selftests/net/rds/test.py
+++ b/tools/testing/selftests/net/rds/test.py
@@ -59,6 +59,14 @@ rdma_addrs = [
 OP_FLAG_TCP     = 0x1
 OP_FLAG_RDMA    = 0x2
 
+# from include/uapi/linux/rds.h: SO_RDS_TRANSPORT pins a socket to a
+# specific RDS transport so connection setup cannot silently fall back
+# to another (e.g. loopback) transport.
+SOL_RDS          = 276
+SO_RDS_TRANSPORT = 8
+RDS_TRANS_TCP    = 2
+RDS_TRANS_IB     = 0
+
 signal_handler_label = ""
 
 tap_idx = 0
@@ -214,11 +222,21 @@ def snd_rcv_packets(env):
             netns_socket(netns_list[0], socket.AF_RDS, socket.SOCK_SEQPACKET),
             netns_socket(netns_list[1], socket.AF_RDS, socket.SOCK_SEQPACKET),
         ]
+
+        # Pin the sockets to the TCP transport so it doesn't fail over to a
+        # different transport during this test
+        for s in sockets:
+            s.setsockopt(SOL_RDS, SO_RDS_TRANSPORT, RDS_TRANS_TCP)
     elif flags & OP_FLAG_RDMA:
         sockets = [
             socket.socket(socket.AF_RDS, socket.SOCK_SEQPACKET),
             socket.socket(socket.AF_RDS, socket.SOCK_SEQPACKET),
         ]
+
+        # Pin the sockets to the RDMA transport so it doesn't fail over to a
+        # different transport during this test
+        for s in sockets:
+            s.setsockopt(SOL_RDS, SO_RDS_TRANSPORT, RDS_TRANS_IB)
     else:
         raise RuntimeError(f"Invalid transport flag sets no transports: {flags}")