When forking the child process, wait until the child has exec'd before continuing in the parent.

Also includes a few other changes for safety, e.g. making the child exit via QuitGS rather than exit() or _exit().

The vfork_and_run() routine is based on code from my port of hush.

Makefile.common

@@ -20,6 +20,7 @@ TELNETD_SRCS = \
   libtelnet/getent.c \
   libtelnet/posix_openpt.c \
   libtelnet/vasprintf.c \
+  libtelnet/vfork.and.run.c \
   telnetd/global.c \
   telnetd/slc.c \
   telnetd/state.c \

libtelnet/vfork.and.run.c

@@ -0,0 +1,128 @@
+/*
+ * Copyright (c) 2014, 2016 Stephen Heumann
+ * 
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
+#ifdef __GNO__
+#include <gno/gno.h>
+#endif
+
+/* Like vfork, but calls fn(arg) in the child instead of returning.
+ * This is designed to match the semantics of GNO's fork2 call.
+ */
+#ifndef __GNO__
+
+pid_t vfork_and_run(void (*fn)(void*) /*NORETURN*/, void *arg) {
+	/* Use fork rather than vfork because vfork is problematic on OS X. */
+	pid_t pid = fork();
+
+	if (pid == 0) {
+		fn(arg);
+	}
+
+	return pid;
+}
+
+#else
+
+# include <signal.h>
+# include <gno/kvm.h>
+# include <orca.h>
+
+/* Turn off all ORCA/C stack repair code to avoid corruption. */
+# ifdef __ORCAC__
+#  pragma optimize 72
+# endif
+
+# pragma databank 1
+void fork_thunk(void (*fn)(void*) /*NORETURN*/, void *arg, long sigmask) {
+	sigsetmask(sigmask);
+	fn(arg);
+}
+# pragma databank 0
+
+const char *forked_child_name = "telnetd pty slave proc";
+#define CHILD_STACKSIZE 1024
+
+pid_t vfork_and_run(void (*fn)(void*) /*NORETURN*/, void *arg) {
+	/* GNO's fork2 call will return immediately and allow the parent and 
+	 * child processes to execute concurrently using the same memory
+	 * space.  To prevent them stomping on each other, we want to get
+	 * behavior like a traditional vfork() implementation, where the
+	 * parent blocks until the child terminates or execs.
+	 *
+	 * Our approach is to check the process tables to make sure the
+	 * child has actually finished or exec'd.  If not, we loop and try again.
+	 * We can't just rely on the fact that the child signaled us, because
+	 * it may still be running in libc's implementation of exec*.
+	 */
+	
+	long oldmask;
+	pid_t pid;
+	kvmt *kvm_context;
+	struct pentry *proc_entry;
+	int done = 0;
+	
+	/* Isolate child process's environment from parent */
+	if (environPush() != 0)
+		return  -1;
+	
+	/* Block all signals for now */
+	oldmask = sigblock(-1);
+	
+	pid = fork2(fork_thunk, CHILD_STACKSIZE, 0, forked_child_name, 
+				(sizeof(fn) + sizeof(arg) + sizeof(oldmask) + 1) / 2, 
+				fn, arg, oldmask);
+	if (pid < 0) 
+		goto ret;
+	
+	while (!done) {
+		/* Wait for ~100 ms.  If procsend worked, the child could send a 
+		 * message with it to end the waiting earlier, but this isn't 
+		 * possible in GNO 2.0.6 because procsend is broken.  This isn't
+		 * too big an issue, since 100ms isn't very long to wait anyhow. */
+		procrecvtim(1);
+		
+		/* Check if the child is really dead or forked by inspecting
+		 * the kernel's process entry for it. */
+		kvm_context = kvm_open();
+		if (kvm_context == NULL)
+			break;
+		proc_entry = kvmgetproc(kvm_context, pid);
+		if (proc_entry == NULL 
+			|| (proc_entry->args != NULL 
+				&& strcmp(forked_child_name, proc_entry->args + 8) != 0))
+			done = 1;
+		kvm_close(kvm_context);
+	}
+	
+ret:
+	sigsetmask(oldmask);
+	environPop();
+	return pid;
+}
+
+#endif

libtelnet/vfork.and.run.h

@@ -0,0 +1 @@
+pid_t vfork_and_run(void (*fn)(void*) /*NORETURN*/, void *arg);

telnetd/ext.h

@@ -83,6 +83,8 @@ extern int	SYNCHing;		/* we are in TELNET SYNCH mode */
 /* Buffer for miscellaneous uses in various functions */
 extern char     buf[BUFSIZ > 1024 ? BUFSIZ : 1024];
 
+extern int      parent_pid;            /* pid of parent (server) process */
+
 extern void
 	_termstat(void),
 	add_slc(char, char, cc_t),
@@ -166,6 +168,7 @@ extern int
 	tty_linemode(void);
 
 extern void
+	safe_exit(int),
 	tty_rspeed(int),
 	tty_setecho(int),
 	tty_setedit(int),
@@ -194,7 +197,7 @@ extern char	*nclearto;
  */
 
 extern struct {
-    int
+    long
 	system,			/* what the current time is */
 	echotoggle,		/* last time user entered echo character */
 	modenegotiated,		/* last time operating mode negotiated */

telnetd/sys_term.c

@@ -45,8 +45,13 @@ static const char sccsid[] = "@(#)sys_term.c	8.4+1 (Berkeley) 5/30/95";
 #endif
 #include <stdlib.h>
 
+#ifdef __GNO__
+#include <gsos.h>
+#endif
+
 #include "telnetd.h"
 #include "pathnames.h"
+#include "libtelnet/vfork.and.run.h"
 
 #ifdef	AUTHENTICATION
 #include "libtelnet/auth.h"
@@ -922,22 +927,20 @@ cleanopen(char *li)
 	return(t);
 }
 
-#ifdef __ORCAC__
-# pragma databank 1
-#endif
+struct slaveargs {
+	char *host;
+	int autologin;
+	char *autoname;
+};
+
 static void 
-slaveproc(char *host, int autologin, char *autoname)
+slaveproc(void *slaveargs)
 {
-#ifdef __GNO__
-    environPush();
-#endif
+	struct slaveargs *args = slaveargs;
 	getptyslave();
-	start_login(host, autologin, autoname);
+	start_login(args->host, args->autologin, args->autoname);
 	/*NOTREACHED*/
 }
-#ifdef __ORCAC__
-# pragma databank 0
-#endif
 
 /*
  * startslave(host)
@@ -951,6 +954,7 @@ void
 startslave(char *host, int autologin, char *autoname)
 {
 	int i;
+	struct slaveargs slaveargs = {host, autologin, autoname};
 
 #ifdef	AUTHENTICATION
 	if (!autoname || !autoname[0])
@@ -962,25 +966,9 @@ startslave(char *host, int autologin, char *autoname)
 	}
 #endif
 
-#ifndef __GNO__
-	i = fork();
-#else
-	i = fork2(slaveproc, 1024, 0, "telnetd pty slave proc", 
-    		   (sizeof(host) + sizeof(autologin) + sizeof(autoname))/2, 
-    		   host, autologin, autoname);
-#endif
+	i = vfork_and_run(slaveproc, &slaveargs);
 	if (i < 0)
 		fatalperror(net, "fork");
-	if (i) {
-#ifdef __GNO__
-		do {
-			errno = 0;
-			(void)wait(NULL);
-		} while (errno == EINTR);
-#endif
-	} else {
-		slaveproc(host, autologin, autoname);
-	}
 }
 
 void
@@ -1276,6 +1264,47 @@ scrub_env(void)
 	environ = new_environ;
 }
 
+
+#ifdef __GNO__
+static QuitRecGS quitRec = {0, NULL, 0};
+#endif
+
+/*
+ * safe_exit()
+ *
+ * Exit in a way that's safe for either the forked child or
+ * the parent.
+ */
+void
+safe_exit(int status)
+{
+	if (getpid() == parent_pid) {
+		exit(status);
+	} else {
+#ifndef __GNO__
+		_exit(status);
+#else
+		/* _exit (contrary to its documentation) performs clean-up 
+		 * that's inappropriate for a forked child process (this 
+		 * usually results in corruption of the memory allocator 
+		 * state, and maybe other problems), so we define our own 
+		 * function without this problem.  We call QuitGS in assembly
+		 * so we can push the return value on the stack.
+		 */
+		while (1) {
+			asm {
+				lda status
+				pha
+				jsl 0xE100A8
+				dcw 0x2029	/* QuitGS */
+				dcl quitRec;
+				pla
+			}
+		}
+#endif
+	}
+}
+
 /*
  * cleanup()
  *
@@ -1288,5 +1317,5 @@ cleanup(int sig __unused)
 {
 
 	(void) shutdown(net, SHUT_RDWR);
-	exit(1);
+	safe_exit(1);
 }

telnetd/telnetd.c

@@ -156,13 +156,14 @@ main(int argc, char *argv[])
 		_reportStack();
 
 	/* Make sure our environment is isolated from the parent process's */
-	if (environPush() == 0)
+	if (environPush() == 0 && environInit() == 0)
 		atexit(environPop);
 	else
 		exit(1);
-	environInit();
 #endif
 
+	parent_pid = getpid();
+
 	pfrontp = pbackp = ptyobuf;
 	netip = netibuf;
 	nfrontp = nbackp = netobuf;

telnetd/utility.c

@@ -339,7 +339,7 @@ fatal(int f, const char *msg)
 #endif	/* ENCRYPTION */
 	(void) write(f, buf, (int)strlen(buf));
 	sleep(1);	/*XXX*/
-	exit(1);
+	safe_exit(1);
 }
 
 void