Fix sniff timeout when libpcap select returns readable with no packets

Use nonblock_recv() in timed sniff loops when the socket supports it,
so a blocking pcap_next_ex cannot stall past the sniff timeout (#4590).

Regression test simulates libpcap reporting the fd as readable while
nonblock_recv returns no BPF-matched packet.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: T3pp31

scapy/contrib/isotp/isotp_utils.py

@@ -345,11 +345,14 @@ def __init__(self, *args, **kwargs):
             basecls=kwargs.pop("basecls", ISOTP))
         super(ISOTPSession, self).__init__(*args, **kwargs)
 
-    def recv(self, sock: SuperSocket) -> Iterator[Packet]:
+    def recv(self, sock: SuperSocket, nonblock: bool = False) -> Iterator[Packet]:
         """
         Will be called by sniff() to ask for a packet
         """
-        pkt = sock.recv()
+        if nonblock and hasattr(sock, "nonblock_recv"):
+            pkt = sock.nonblock_recv()
+        else:
+            pkt = sock.recv()
         if not pkt:
             return
         self.m.feed(pkt)

scapy/sendrecv.py

@@ -1336,7 +1336,11 @@ def stop_cb():
                     # The session object is passed the socket to call recv() on,
                     # and may perform additional processing (ip defrag, etc.)
                     try:
-                        packets = session.recv(s)
+                        packets = session.recv(
+                            s,
+                            nonblock=timeout is not None and
+                            hasattr(s, "nonblock_recv"),
+                        )
                         # A session can return multiple objects
                         for p in packets:
                             if lfilter and not lfilter(p):

scapy/sessions.py

@@ -51,11 +51,14 @@ def process(self, pkt: Packet) -> Optional[Packet]:
             return self.supersession.process(pkt)
         return pkt
 
-    def recv(self, sock: 'SuperSocket') -> Iterator[Packet]:
+    def recv(self, sock: 'SuperSocket', nonblock: bool = False) -> Iterator[Packet]:
         """
         Will be called by sniff() to ask for a packet
         """
-        pkt = sock.recv()
+        if nonblock and hasattr(sock, "nonblock_recv"):
+            pkt = sock.nonblock_recv()
+        else:
+            pkt = sock.recv()
         if not pkt:
             return
         pkt = self.process(pkt)
@@ -407,11 +410,14 @@ def process(self,
             return pkt
         return None
 
-    def recv(self, sock: 'SuperSocket') -> Iterator[Packet]:
+    def recv(self, sock: 'SuperSocket', nonblock: bool = False) -> Iterator[Packet]:
         """
         Will be called by sniff() to ask for a packet
         """
-        pkt = sock.recv(stop_dissection_after=self.stop_dissection_after)
+        if nonblock and hasattr(sock, "nonblock_recv"):
+            pkt = sock.nonblock_recv()
+        else:
+            pkt = sock.recv(stop_dissection_after=self.stop_dissection_after)
         # Now handle TCP reassembly
         if self.app:
             while pkt is not None:

test/regression.uts

@@ -2061,6 +2061,45 @@ o.send(REFPACKET)
 pkts = sniff(opened_socket=[o], timeout=3)
 assert len(pkts) == 10
 
+= sniff timeout with select-ready but empty pcap socket (#4590)
+
+import time
+from scapy.automaton import ObjectPipe
+from scapy.supersocket import SuperSocket
+
+class PcapLikeSocket(SuperSocket):
+    def __init__(self):
+        self.ins = ObjectPipe(name="pcap_like_socket")
+        self.outs = None
+        self.recv_called = False
+
+    def recv(self, x=65535, **kwargs):
+        self.recv_called = True
+        time.sleep(60)
+        return None
+
+    def nonblock_recv(self, x=65535):
+        return None
+
+    @staticmethod
+    def select(sockets, remain=None):
+        return list(sockets)
+
+    def close(self):
+        if self.closed:
+            return
+        self.closed = True
+        self.ins.close()
+
+sock = PcapLikeSocket()
+t0 = time.monotonic()
+pkts = sniff(opened_socket=sock, timeout=1, count=1)
+elapsed = time.monotonic() - t0
+assert not sock.recv_called, "blocking recv must not be used with timeout"
+assert elapsed < 3, "sniff should stop on timeout, got %.1fs" % elapsed
+assert len(pkts) == 0
+sock.close()
+
 = GH issue 3306
 ~ netaccess needs_root