unix: return zero if peer is closed

Previously, recvmsg() on a unix stream socket with its peer closed will
never return, with goroutine call trace like this:

  ...
  2  in gvisor.dev/gvisor/pkg/sentry/kernel.(*Task).block
     at pkg/sentry/kernel/task_block.go:124
  3  in gvisor.dev/gvisor/pkg/sentry/kernel.(*Task).BlockWithDeadline
     at pkg/sentry/kernel/task_block.go:69
  4  in gvisor.dev/gvisor/pkg/sentry/socket/unix.(*SocketOperations).RecvMsg
     at pkg/sentry/socket/unix/unix.go:612
  5  in gvisor.dev/gvisor/pkg/sentry/syscalls/linux.recvFrom
     at pkg/sentry/syscalls/linux/sys_socket.go:885
  6  in gvisor.dev/gvisor/pkg/sentry/syscalls/linux.RecvFrom
     at pkg/sentry/syscalls/linux/sys_socket.go:910
  ...

The issue is caused by that ErrClosedForReceive returned by
unix/transport.queue is turned into nil in
unix.(*EndpointReader).ReadToBlocks():

  err.ToError()

As a result, in unix.(*SocketOperations).RecvMsg():

  n == 0 and err == nil

We shall differentiate it from another case - no data to read where
ErrWouldBlock shall be returned; and return 0 immediately.

Fixes: #734

Reported-by: chenglang.hy <chenglang.hy@antfin.com>
Signed-off-by: Jianfeng Tan <henry.tjf@antfin.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/pkg/sentry/socket/unix/unix.go b/pkg/sentry/socket/unix/unix.go
index 0d0cb68df..8e4f06a22 100644
--- a/pkg/sentry/socket/unix/unix.go
+++ b/pkg/sentry/socket/unix/unix.go
@@ -595,7 +595,8 @@ func (s *SocketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 				total += n
 			}
 
-			if err != nil || !waitAll || isPacket || n >= dst.NumBytes() {
+			streamPeerClosed := s.stype == linux.SOCK_STREAM && n == 0 && err == nil
+			if err != nil || !waitAll || isPacket || n >= dst.NumBytes() || streamPeerClosed {
 				if total > 0 {
 					err = nil
 				}