Fix blanker to work when screen is suid root

* Change RunBlanker to call OpenDevice so permissions on slave
  PTY are correctly set.
* Update handling of file descriptors after fork to be similar to
  ForkWindow on at pty (fixes debug and leaked descriptors)
* DEBUG now creates screen.blanker to debug blanker fork
* Allow display of error message when display blocked by blanker
  (because message is probably from blanker failing to start)

Bug: 55512

Signed-off-by: Scott Shambarger <devel@shambarger.net>
Signed-off-by: Amadeusz Sławiński <amade@asmblr.net>

4 files changed, 47 insertions, 16 deletions

diff --git a/src/display.c b/src/display.c
index 531597e..a4d953a 100644
--- a/src/display.c
+++ b/src/display.c
@@ -3959,6 +3959,7 @@ char **cmdv;
   char *m;
   int pid;
   int slave = -1;
+  int ptype = 0;
   char termname[MAXTERMLEN + 6];
 #ifndef TIOCSWINSZ
   char libuf[20], cobuf[20];
@@ -3970,9 +3971,9 @@ char **cmdv;
   termname[sizeof(termname) - 1] = 0;
   KillBlanker();
   D_blankerpid = -1;
-  if ((D_blankerev.fd = OpenPTY(&m)) == -1)
+  if ((D_blankerev.fd = OpenDevice(cmdv, 0, &ptype, &m)) == -1)
     {
-      Msg(0, "OpenPty failed");
+      Msg(0, "OpenDevice failed");
       return;
     }
 #ifdef O_NOCTTY
@@ -3998,12 +3999,8 @@ char **cmdv;
     case 0:
       displays = 0;
       ServerSocket = -1;
-#ifdef DEBUG
-      if (dfp && dfp != stderr)
-	{
-	  fclose(dfp);
-	  dfp = 0;
-	}
+#ifdef SIGPIPE
+      signal(SIGPIPE, SIG_DFL);
 #endif
       if (setgid(real_gid) || setuid(real_uid))
         Panic(errno, "setuid/setgid");
@@ -4011,14 +4008,41 @@ char **cmdv;
       eff_gid = real_gid;
       brktty(D_userfd);
       freetty();
+#ifdef DEBUG
+      if (dfp && dfp != stderr)
+	  fclose(dfp);
+#endif
+      if (slave != -1)
+	{
+	  close(0);
+	  dup(slave);
+	  close(slave);
+	  closeallfiles(D_blankerev.fd);
+	  slave = dup(0);
+	}
+      else
+        closeallfiles(D_blankerev.fd);
+#ifdef DEBUG
+      if (dfp)
+        {
+          char buf[256];
+
+	  sprintf(buf, "%s/screen.blanker", DEBUGDIR);
+	  if ((dfp = fopen(buf, "a")) == 0)
+	    dfp = stderr;
+	  else
+	    (void) chmod(buf, 0666);
+	}
+      debug1("=== RunBlanker: pid %d\n", (int)getpid());
+#endif
       close(0);
       close(1);
       close(2);
-      closeallfiles(slave);
       if (open(m, O_RDWR))
 	Panic(errno, "Cannot open %s", m);
       dup(0);
       dup(0);
+      close(D_blankerev.fd);
       if (slave != -1)
 	close(slave);
       InitPTY(0);
@@ -4032,17 +4056,17 @@ char **cmdv;
       glwz.ws_row = D_height;
       (void)ioctl(0, TIOCSWINSZ, (char *)&glwz);
 #else
+      /* Always turn off nonblocking mode */
+      (void)fcntl(0, F_SETFL, 0);
       sprintf(libuf, "LINES=%d", D_height);
       sprintf(cobuf, "COLUMNS=%d", D_width);
       *np++ = libuf;
       *np++ = cobuf;
 #endif
-#ifdef SIGPIPE
-      signal(SIGPIPE, SIG_DFL);
-#endif
-      display = 0;
+      debug1("calling execvpe %s\n", *cmdv);
       execvpe(*cmdv, cmdv, NewEnv + 3);
-      Panic(errno, "%s", *cmdv);
+      debug1("exec error: %d\n", errno);
+      Panic(errno, "Cannot exec '%s'", *cmdv);
     default:
       break;
     }
diff --git a/src/socket.c b/src/socket.c
index ebf55f9..ba995bb 100644
--- a/src/socket.c
+++ b/src/socket.c
@@ -742,6 +742,7 @@ char *tty, *buf;
   struct msg m;
   bool is_socket;
 
+  debug2("SendErrorMsg: %s %s\n", tty, buf);
   strncpy(m.m.message, buf, sizeof(m.m.message) - 1);
   m.m.message[sizeof(m.m.message) - 1] = 0;
   is_socket = IsSocket(SockPath);
@@ -1237,7 +1238,13 @@ ReceiveMsg()
           FinishAttach(&m);
         break;
       case MSG_ERROR:
+      {
+        int blocked=D_blocked;
+        if(D_blocked == 4) /* allow error messages while in blanker mode */
+          D_blocked=0; /* likely they're from failed blanker */
         Msg(0, "%s", m.m.message);
+        D_blocked=blocked;
+      }
         break;
       case MSG_HANGUP:
         if (!wi) /* ignore hangups from inside */
diff --git a/src/window.c b/src/window.c
index 77b7e63..58411e2 100644
--- a/src/window.c
+++ b/src/window.c
@@ -100,7 +100,6 @@ static void pseu_writeev_fn __P((struct event *, char *));
 static void win_silenceev_fn __P((struct event *, char *));
 static void win_destroyev_fn __P((struct event *, char *));
 
-static int  OpenDevice __P((char **, int, int *, char **));
 static int  ForkWindow __P((struct win *, char **, char *));
 #ifdef ZMODEM
 static void zmodem_found __P((struct win *, int, char *, int));
@@ -1089,7 +1088,7 @@ struct win *wp;
   free((char *)wp);
 }
 
-static int
+int
 OpenDevice(args, lflag, typep, namep)
 char **args;
 int lflag;
diff --git a/src/window.h b/src/window.h
index 99f44c0..fd016a3 100644
--- a/src/window.h
+++ b/src/window.h
@@ -351,6 +351,7 @@ struct win
 #define Layer2Window(l) ((struct win *)(l)->l_bottom->l_data)
 
 int WindowChangeNumber __P((int, int));
+int OpenDevice __P((char **, int, int *, char **));
 
 #endif /* SCREEN_WINDOW_H */