Fix bug in RecvMMsgDispatcher.

Fixed a bug introduced in the following commit:
https://github.com/google/gvisor/commit/979d6e7d77b17e94defc29515180cc75d3560383

The commit introduced a bug which causes the recvmmsg dispatcher to never exit
as BlockingPoll is now called with two fds and poll will not return an error
anymore if one of the FD is closed. We need to explicitly check the events for
each FD to determine if the sentry FD is closed.

ReadV dispatcher does not have the same issue as Readv does not rely on sk_err
field of the underlying socket to determine if the socket is in an error
state. Recvmmsg OTOH seems to get confused and always returns EAGAIN if poll()
is called which queries the sk_err field and clears it.

PiperOrigin-RevId: 396676135

pkg/tcpip/link/rawfile/blockingpoll_amd64.s

@@ -27,7 +27,7 @@ TEXT ·BlockingPoll(SB),NOSPLIT,$0-40
 	MOVQ	$0x0, R10  // sigmask parameter which isn't used here
 	MOVQ	$0x10f, AX // SYS_PPOLL
 	SYSCALL
-	CMPQ	AX, $0xfffffffffffff001
+	CMPQ	AX, $0xfffffffffffff002
 	JLS	ok
 	MOVQ	$-1, n+24(FP)
 	NEGQ	AX

pkg/tcpip/link/rawfile/blockingpoll_arm64.s

@@ -27,7 +27,7 @@ TEXT ·BlockingPoll(SB),NOSPLIT,$0-40
 	MOVD	$0x0, R3  // sigmask parameter which isn't used here
 	MOVD	$0x49, R8 // SYS_PPOLL
 	SVC
-	CMP	$0xfffffffffffff001, R0
+	CMP	$0xfffffffffffff002, R0
 	BLS	ok
 	MOVD	$-1, R1
 	MOVD	R1, n+24(FP)

pkg/tcpip/link/rawfile/rawfile_unsafe.go

@@ -181,7 +181,9 @@ func BlockingReadvUntilStopped(efd int, fd int, iovecs []unix.Iovec) (int, tcpip
 		if e == 0 {
 			return int(n), nil
 		}
-
+		if e != 0 && e != unix.EWOULDBLOCK {
+			return 0, TranslateErrno(e)
+		}
 		stopped, e := BlockingPollUntilStopped(efd, fd, unix.POLLIN)
 		if stopped {
 			return -1, nil
@@ -204,6 +206,10 @@ func BlockingRecvMMsgUntilStopped(efd int, fd int, msgHdrs []MMsgHdr) (int, tcpi
 			return int(n), nil
 		}
 
+		if e != 0 && e != unix.EWOULDBLOCK {
+			return 0, TranslateErrno(e)
+		}
+
 		stopped, e := BlockingPollUntilStopped(efd, fd, unix.POLLIN)
 		if stopped {
 			return -1, nil
@@ -228,5 +234,13 @@ func BlockingPollUntilStopped(efd int, fd int, events int16) (bool, unix.Errno)
 		},
 	}
 	_, errno := BlockingPoll(&pevents[0], len(pevents), nil)
+	if errno != 0 {
+		return pevents[0].Revents&unix.POLLIN != 0, errno
+	}
+
+	if pevents[1].Revents&unix.POLLHUP != 0 || pevents[1].Revents&unix.POLLERR != 0 {
+		errno = unix.ECONNRESET
+	}
+
 	return pevents[0].Revents&unix.POLLIN != 0, errno
 }