Clear msghdr flags on successful recvmsg.

.net sets these flags to -1 and then uses their result, especting it to be
zero.

Does not set actual flags (e.g. MSG_TRUNC), but setting to zero is more correct
than what we did before.

PiperOrigin-RevId: 239657951
Change-Id: I89c5f84bc9b94a2cd8ff84e8ecfea09e01142030

2 files changed, 95 insertions, 0 deletions

diff --git a/pkg/sentry/syscalls/linux/sys_socket.go b/pkg/sentry/syscalls/linux/sys_socket.go
index 1513f28e7..564357bac 100644
--- a/pkg/sentry/syscalls/linux/sys_socket.go
+++ b/pkg/sentry/syscalls/linux/sys_socket.go
@@ -57,6 +57,10 @@ const nameLenOffset = 8
 // to the ControlLen field.
 const controlLenOffset = 40
 
+// flagsOffset is the offset form the start of the MessageHeader64 struct
+// to the Flags field.
+const flagsOffset = 48
+
 // messageHeader64Len is the length of a MessageHeader64 struct.
 var messageHeader64Len = uint64(binary.Size(MessageHeader64{}))
 
@@ -743,6 +747,16 @@ func recvSingleMsg(t *kernel.Task, s socket.Socket, msgPtr usermem.Addr, flags i
 			return 0, syserror.ConvertIntr(err.ToError(), kernel.ERESTARTSYS)
 		}
 		cms.Unix.Release()
+
+		if msg.Flags != 0 {
+			// Copy out the flags to the caller.
+			//
+			// TODO: Plumb through actual flags.
+			if _, err := t.CopyOut(msgPtr+flagsOffset, int32(0)); err != nil {
+				return 0, err
+			}
+		}
+
 		return uintptr(n), nil
 	}
 
@@ -787,6 +801,13 @@ func recvSingleMsg(t *kernel.Task, s socket.Socket, msgPtr usermem.Addr, flags i
 		}
 	}
 
+	// Copy out the flags to the caller.
+	//
+	// TODO: Plumb through actual flags.
+	if _, err := t.CopyOut(msgPtr+flagsOffset, int32(0)); err != nil {
+		return 0, err
+	}
+
 	return uintptr(n), nil
 }
 
diff --git a/test/syscalls/linux/socket_generic.cc b/test/syscalls/linux/socket_generic.cc
index 974c0dd7b..c83fb82fe 100644
--- a/test/syscalls/linux/socket_generic.cc
+++ b/test/syscalls/linux/socket_generic.cc
@@ -183,6 +183,80 @@ TEST_P(AllSocketPairTest, SendmsgRecvmsg16KB) {
             memcmp(sent_data.data(), received_data.data(), sent_data.size()));
 }
 
+TEST_P(AllSocketPairTest, RecvmsgMsghdrFlagsNotClearedOnFailure) {
+  auto sockets = ASSERT_NO_ERRNO_AND_VALUE(NewSocketPair());
+
+  char received_data[10] = {};
+
+  struct iovec iov;
+  iov.iov_base = received_data;
+  iov.iov_len = sizeof(received_data);
+  struct msghdr msg = {};
+  msg.msg_flags = -1;
+  msg.msg_iov = &iov;
+  msg.msg_iovlen = 1;
+
+  ASSERT_THAT(RetryEINTR(recvmsg)(sockets->second_fd(), &msg, MSG_DONTWAIT),
+              SyscallFailsWithErrno(EAGAIN));
+
+  // Check that msghdr flags were not changed.
+  EXPECT_EQ(msg.msg_flags, -1);
+}
+
+TEST_P(AllSocketPairTest, RecvmsgMsghdrFlagsCleared) {
+  auto sockets = ASSERT_NO_ERRNO_AND_VALUE(NewSocketPair());
+
+  char sent_data[10];
+  RandomizeBuffer(sent_data, sizeof(sent_data));
+  ASSERT_THAT(
+      RetryEINTR(send)(sockets->first_fd(), sent_data, sizeof(sent_data), 0),
+      SyscallSucceedsWithValue(sizeof(sent_data)));
+
+  char received_data[sizeof(sent_data)] = {};
+
+  struct iovec iov;
+  iov.iov_base = received_data;
+  iov.iov_len = sizeof(received_data);
+  struct msghdr msg = {};
+  msg.msg_flags = -1;
+  msg.msg_iov = &iov;
+  msg.msg_iovlen = 1;
+
+  ASSERT_THAT(RetryEINTR(recvmsg)(sockets->second_fd(), &msg, 0),
+              SyscallSucceedsWithValue(sizeof(sent_data)));
+  EXPECT_EQ(0, memcmp(received_data, sent_data, sizeof(sent_data)));
+
+  // Check that msghdr flags were cleared.
+  EXPECT_EQ(msg.msg_flags, 0);
+}
+
+TEST_P(AllSocketPairTest, RecvmsgPeekMsghdrFlagsCleared) {
+  auto sockets = ASSERT_NO_ERRNO_AND_VALUE(NewSocketPair());
+
+  char sent_data[10];
+  RandomizeBuffer(sent_data, sizeof(sent_data));
+  ASSERT_THAT(
+      RetryEINTR(send)(sockets->first_fd(), sent_data, sizeof(sent_data), 0),
+      SyscallSucceedsWithValue(sizeof(sent_data)));
+
+  char received_data[sizeof(sent_data)] = {};
+
+  struct iovec iov;
+  iov.iov_base = received_data;
+  iov.iov_len = sizeof(received_data);
+  struct msghdr msg = {};
+  msg.msg_flags = -1;
+  msg.msg_iov = &iov;
+  msg.msg_iovlen = 1;
+
+  ASSERT_THAT(RetryEINTR(recvmsg)(sockets->second_fd(), &msg, MSG_PEEK),
+              SyscallSucceedsWithValue(sizeof(sent_data)));
+  EXPECT_EQ(0, memcmp(received_data, sent_data, sizeof(sent_data)));
+
+  // Check that msghdr flags were cleared.
+  EXPECT_EQ(msg.msg_flags, 0);
+}
+
 TEST_P(AllSocketPairTest, RecvmmsgInvalidTimeout) {
   auto sockets = ASSERT_NO_ERRNO_AND_VALUE(NewSocketPair());
   char buf[10];