Use PIPE_READ_END, PIPE_WRITE_END to clarify use of a socketpair

Both sockets in the socket pair are technically bidirectional, but we
are using them as though they were unidirectional, with the same
convention as pipe(): the first socket in the array is used for reading,
and the second is for writing.

Signed-off-by: Simon McVittie <smcv@collabora.com>

Author: smcv

bubblewrap.c

@@ -3144,11 +3144,11 @@ main (int    argc,
     {
       /* Parent, outside sandbox, privileged (initially) */
 
-      if (intermediate_pids_sockets[0] != -1)
+      if (intermediate_pids_sockets[PIPE_READ_END] != -1)
         {
-          close (intermediate_pids_sockets[1]);
-          pid = read_pid_from_socket (intermediate_pids_sockets[0]);
-          close (intermediate_pids_sockets[0]);
+          close (intermediate_pids_sockets[PIPE_WRITE_END]);
+          pid = read_pid_from_socket (intermediate_pids_sockets[PIPE_READ_END]);
+          close (intermediate_pids_sockets[PIPE_READ_END]);
         }
 
       /* Discover namespace ids before we drop privileges */
@@ -3232,9 +3232,9 @@ main (int    argc,
 
       /* We're back, either in a child or grandchild, so message the actual pid to the monitor */
 
-      close (intermediate_pids_sockets[0]);
-      send_pid_on_socket (intermediate_pids_sockets[1]);
-      close (intermediate_pids_sockets[1]);
+      close (intermediate_pids_sockets[PIPE_READ_END]);
+      send_pid_on_socket (intermediate_pids_sockets[PIPE_WRITE_END]);
+      close (intermediate_pids_sockets[PIPE_WRITE_END]);
     }
 
   /* Child, in sandbox, privileged in the parent or in the user namespace (if --unshare-user).

utils.c

@@ -782,6 +782,12 @@ send_pid_on_socket (int sockfd)
     die_with_error ("Can't send pid");
 }
 
+/*
+ * Create a socket pair such that if one process calls
+ * send_pid_on_socket(sockets[PIPE_WRITE_END]),
+ * another process will be able to call
+ * read_pid_from_socket(sockets[PIPE_READ_END]).
+ */
 void
 create_pid_socketpair (int sockets[2])
 {
@@ -790,7 +796,7 @@ create_pid_socketpair (int sockets[2])
   if (socketpair (AF_UNIX, SOCK_SEQPACKET | SOCK_CLOEXEC, 0, sockets) != 0)
     die_with_error ("Can't create intermediate pids socket");
 
-  if (setsockopt (sockets[0], SOL_SOCKET, SO_PASSCRED, &enable, sizeof (enable)) < 0)
+  if (setsockopt (sockets[PIPE_READ_END], SOL_SOCKET, SO_PASSCRED, &enable, sizeof (enable)) < 0)
     die_with_error ("Can't set SO_PASSCRED");
 }