Return correct length with MSG_TRUNC for unix sockets.

This change calls a new Truncate method on the EndpointReader in RecvMsg for
both netlink and unix sockets.  This allows readers such as sockets to peek at
the length of data without actually reading it to a buffer.

Fixes #993 #1240

PiperOrigin-RevId: 288800167

1 files changed, 54 insertions, 1 deletions

diff --git a/test/syscalls/linux/socket_stream.cc b/test/syscalls/linux/socket_stream.cc
index 346443f96..6522b2e01 100644
--- a/test/syscalls/linux/socket_stream.cc
+++ b/test/syscalls/linux/socket_stream.cc
@@ -104,7 +104,60 @@ TEST_P(StreamSocketPairTest, RecvmsgMsghdrFlagsNoMsgTrunc) {
   EXPECT_EQ(0, memcmp(received_data, sent_data, sizeof(received_data)));
 
   // Check that msghdr flags were cleared (MSG_TRUNC was not set).
-  EXPECT_EQ(msg.msg_flags, 0);
+  ASSERT_EQ(msg.msg_flags & MSG_TRUNC, 0);
+}
+
+TEST_P(StreamSocketPairTest, RecvmsgTruncZeroLen) {
+  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[0] = {};
+
+  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_TRUNC),
+              SyscallSucceedsWithValue(0));
+
+  // Check that msghdr flags were cleared (MSG_TRUNC was not set).
+  ASSERT_EQ(msg.msg_flags & MSG_TRUNC, 0);
+}
+
+TEST_P(StreamSocketPairTest, RecvmsgTruncPeekZeroLen) {
+  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[0] = {};
+
+  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_TRUNC | MSG_PEEK),
+      SyscallSucceedsWithValue(0));
+
+  // Check that msghdr flags were cleared (MSG_TRUNC was not set).
+  ASSERT_EQ(msg.msg_flags & MSG_TRUNC, 0);
 }
 
 TEST_P(StreamSocketPairTest, MsgTrunc) {