Fix flaky udp tests by polling before reading.

On native Linux, calling recv/read right after send/write sometimes returns
EWOULDBLOCK, if the data has not made it to the receiving socket (even though
the endpoints are on the same host). Poll before reading to avoid this.

Making this change also uncovered a hostinet bug (gvisor.dev/issue/2726),
which is noted in this CL.

PiperOrigin-RevId: 312320587

2 files changed, 48 insertions, 2 deletions

diff --git a/pkg/sentry/socket/hostinet/socket.go b/pkg/sentry/socket/hostinet/socket.go
index b49433326..c11e82c10 100644
--- a/pkg/sentry/socket/hostinet/socket.go
+++ b/pkg/sentry/socket/hostinet/socket.go
@@ -555,7 +555,7 @@ func (s *socketOpsCommon) SendMsg(t *kernel.Task, src usermem.IOSequence, to []b
 		if uint64(src.NumBytes()) != srcs.NumBytes() {
 			return 0, nil
 		}
-		if srcs.IsEmpty() {
+		if srcs.IsEmpty() && len(controlBuf) == 0 {
 			return 0, nil
 		}
 
diff --git a/test/syscalls/linux/udp_socket_test_cases.cc b/test/syscalls/linux/udp_socket_test_cases.cc
index 740c7986d..4a3ae5f3f 100644
--- a/test/syscalls/linux/udp_socket_test_cases.cc
+++ b/test/syscalls/linux/udp_socket_test_cases.cc
@@ -17,6 +17,7 @@
 #include <arpa/inet.h>
 #include <fcntl.h>
 #include <netinet/in.h>
+#include <poll.h>
 #include <sys/ioctl.h>
 #include <sys/socket.h>
 #include <sys/types.h>
@@ -673,6 +674,11 @@ TEST_P(UdpSocketTest, ZerolengthWriteAllowed) {
   char buf[3];
   // Send zero length packet from s_ to t_.
   ASSERT_THAT(write(s_, buf, 0), SyscallSucceedsWithValue(0));
+
+  struct pollfd pfd = {t_, POLLIN, 0};
+  ASSERT_THAT(RetryEINTR(poll)(&pfd, 1, /*timeout=*/1000),
+              SyscallSucceedsWithValue(1));
+
   // Receive the packet.
   char received[3];
   EXPECT_THAT(read(t_, received, sizeof(received)),
@@ -698,6 +704,11 @@ TEST_P(UdpSocketTest, ZerolengthWriteAllowedNonBlockRead) {
   char buf[3];
   // Send zero length packet from s_ to t_.
   ASSERT_THAT(write(s_, buf, 0), SyscallSucceedsWithValue(0));
+
+  struct pollfd pfd = {t_, POLLIN, 0};
+  ASSERT_THAT(RetryEINTR(poll)(&pfd, 1, /*timeout=*/1000),
+              SyscallSucceedsWithValue(1));
+
   // Receive the packet.
   char received[3];
   EXPECT_THAT(read(t_, received, sizeof(received)),
@@ -859,6 +870,10 @@ TEST_P(UdpSocketTest, ReadShutdownNonblockPendingData) {
 
   EXPECT_THAT(shutdown(s_, SHUT_RD), SyscallSucceeds());
 
+  struct pollfd pfd = {s_, POLLIN, 0};
+  ASSERT_THAT(RetryEINTR(poll)(&pfd, 1, /*timeout=*/1000),
+              SyscallSucceedsWithValue(1));
+
   // We should get the data even though read has been shutdown.
   EXPECT_THAT(recv(s_, received, 2, 0), SyscallSucceedsWithValue(2));
 
@@ -1112,6 +1127,10 @@ TEST_P(UdpSocketTest, FIONREADWriteShutdown) {
   ASSERT_THAT(send(s_, str, sizeof(str), 0),
               SyscallSucceedsWithValue(sizeof(str)));
 
+  struct pollfd pfd = {s_, POLLIN, 0};
+  ASSERT_THAT(RetryEINTR(poll)(&pfd, 1, /*timeout=*/1000),
+              SyscallSucceedsWithValue(1));
+
   n = -1;
   EXPECT_THAT(ioctl(s_, FIONREAD, &n), SyscallSucceedsWithValue(0));
   EXPECT_EQ(n, sizeof(str));
@@ -1123,7 +1142,7 @@ TEST_P(UdpSocketTest, FIONREADWriteShutdown) {
   EXPECT_EQ(n, sizeof(str));
 }
 
-TEST_P(UdpSocketTest, Fionread) {
+TEST_P(UdpSocketTest, FIONREAD) {
   // Bind s_ to loopback:TestPort.
   ASSERT_THAT(bind(s_, addr_[0], addrlen_), SyscallSucceeds());
 
@@ -1138,10 +1157,14 @@ TEST_P(UdpSocketTest, Fionread) {
   char buf[3 * psize];
   RandomizeBuffer(buf, sizeof(buf));
 
+  struct pollfd pfd = {s_, POLLIN, 0};
   for (int i = 0; i < 3; ++i) {
     ASSERT_THAT(sendto(t_, buf + i * psize, psize, 0, addr_[0], addrlen_),
                 SyscallSucceedsWithValue(psize));
 
+    ASSERT_THAT(RetryEINTR(poll)(&pfd, 1, /*timeout=*/1000),
+                SyscallSucceedsWithValue(1));
+
     // Check that regardless of how many packets are in the queue, the size
     // reported is that of a single packet.
     n = -1;
@@ -1165,10 +1188,18 @@ TEST_P(UdpSocketTest, FIONREADZeroLengthPacket) {
   char buf[3 * psize];
   RandomizeBuffer(buf, sizeof(buf));
 
+  struct pollfd pfd = {s_, POLLIN, 0};
   for (int i = 0; i < 3; ++i) {
     ASSERT_THAT(sendto(t_, buf + i * psize, 0, 0, addr_[0], addrlen_),
                 SyscallSucceedsWithValue(0));
 
+    // TODO(gvisor.dev/issue/2726): sending a zero-length message to a hostinet
+    // socket does not cause a poll event to be triggered.
+    if (!IsRunningWithHostinet()) {
+      ASSERT_THAT(RetryEINTR(poll)(&pfd, 1, /*timeout=*/1000),
+                  SyscallSucceedsWithValue(1));
+    }
+
     // Check that regardless of how many packets are in the queue, the size
     // reported is that of a single packet.
     n = -1;
@@ -1235,6 +1266,10 @@ TEST_P(UdpSocketTest, SoTimestamp) {
   // Send zero length packet from t_ to s_.
   ASSERT_THAT(RetryEINTR(write)(t_, buf, 0), SyscallSucceedsWithValue(0));
 
+  struct pollfd pfd = {s_, POLLIN, 0};
+  ASSERT_THAT(RetryEINTR(poll)(&pfd, 1, /*timeout=*/1000),
+              SyscallSucceedsWithValue(1));
+
   char cmsgbuf[CMSG_SPACE(sizeof(struct timeval))];
   msghdr msg;
   memset(&msg, 0, sizeof(msg));
@@ -1278,6 +1313,10 @@ TEST_P(UdpSocketTest, TimestampIoctl) {
   ASSERT_THAT(RetryEINTR(write)(t_, buf, sizeof(buf)),
               SyscallSucceedsWithValue(sizeof(buf)));
 
+  struct pollfd pfd = {s_, POLLIN, 0};
+  ASSERT_THAT(RetryEINTR(poll)(&pfd, 1, /*timeout=*/1000),
+              SyscallSucceedsWithValue(1));
+
   // There should be no control messages.
   char recv_buf[sizeof(buf)];
   ASSERT_NO_FATAL_FAILURE(RecvNoCmsg(s_, recv_buf, sizeof(recv_buf)));
@@ -1315,6 +1354,10 @@ TEST_P(UdpSocketTest, TimestampIoctlPersistence) {
               SyscallSucceedsWithValue(sizeof(buf)));
   ASSERT_THAT(RetryEINTR(write)(t_, buf, 0), SyscallSucceedsWithValue(0));
 
+  struct pollfd pfd = {s_, POLLIN, 0};
+  ASSERT_THAT(RetryEINTR(poll)(&pfd, 1, /*timeout=*/1000),
+              SyscallSucceedsWithValue(1));
+
   // There should be no control messages.
   char recv_buf[sizeof(buf)];
   ASSERT_NO_FATAL_FAILURE(RecvNoCmsg(s_, recv_buf, sizeof(recv_buf)));
@@ -1330,6 +1373,9 @@ TEST_P(UdpSocketTest, TimestampIoctlPersistence) {
               SyscallSucceeds());
   ASSERT_THAT(RetryEINTR(write)(t_, buf, 0), SyscallSucceedsWithValue(0));
 
+  ASSERT_THAT(RetryEINTR(poll)(&pfd, 1, /*timeout=*/1000),
+              SyscallSucceedsWithValue(1));
+
   // There should be a message for SO_TIMESTAMP.
   char cmsgbuf[CMSG_SPACE(sizeof(struct timeval))];
   msghdr msg = {};