Fix RecvMsgWithPid on broken socket pair

This prevents RecvMsgWithPid from DCHECK failing under Debug builds
when reading EOF from a broken socket pair.  Noticed while testing
crrev.com/269543014.

Unfortunately, Linux allows sending zero length messages that the
recvmsg() API does not distinguish from reading EOF.  To be
conservative, if we didn't receive any message or control bytes, then
assume we received an EOF and tolerate not knowing the sender's PID.

BUG=357670

Review URL: https://codereview.chromium.org/276593008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270967 0039d316-1c4b-4281-b951-d872f2087c98

base/posix/unix_domain_socket_linux.cc

@@ -152,7 +152,13 @@ ssize_t UnixDomainSocket::RecvMsgWithFlags(int fd,
   }
 
   if (out_pid) {
-    DCHECK(pid != -1);
+    // |pid| will legitimately be -1 if we read EOF, so only DCHECK if we
+    // actually received a message.  Unfortunately, Linux allows sending zero
+    // length messages, which are indistinguishable from EOF, so this check
+    // has false negatives.
+    if (r > 0 || msg.msg_controllen > 0)
+      DCHECK_GE(pid, 0);
+
     *out_pid = pid;
   }
 

base/posix/unix_domain_socket_linux_unittest.cc

@@ -134,6 +134,28 @@ TEST(UnixDomainSocketTest, RecvPidWithMaxDescriptors) {
   ASSERT_EQ(getpid(), sender_pid);
 }
 
+// Check that RecvMsgWithPid doesn't DCHECK fail when reading EOF from a
+// disconnected socket.
+TEST(UnixDomianSocketTest, RecvPidDisconnectedSocket) {
+  int fds[2];
+  ASSERT_EQ(0, socketpair(AF_UNIX, SOCK_SEQPACKET, 0, fds));
+  base::ScopedFD recv_sock(fds[0]);
+  base::ScopedFD send_sock(fds[1]);
+
+  ASSERT_TRUE(UnixDomainSocket::EnableReceiveProcessId(recv_sock.get()));
+
+  send_sock.reset();
+
+  char ch;
+  base::ProcessId sender_pid;
+  ScopedVector<base::ScopedFD> recv_fds;
+  const ssize_t nread = UnixDomainSocket::RecvMsgWithPid(
+      recv_sock.get(), &ch, sizeof(ch), &recv_fds, &sender_pid);
+  ASSERT_EQ(0, nread);
+  ASSERT_EQ(-1, sender_pid);
+  ASSERT_EQ(0U, recv_fds.size());
+}
+
 }  // namespace
 
 }  // namespace base