Make sure all stdio fds are open. Refactor detach-from-tty code.

Fixes unpleasant behavior in mosh-server when, say, stderr is closed.

Author: cgull

src/frontend/mosh-client.cc

@@ -40,6 +40,7 @@
 #include "crypto.h"
 #include "locale_utils.h"
 #include "fatal_assert.h"
+#include "stdfds.h"
 
 /* These need to be included last because of conflicting defines. */
 /*
@@ -108,6 +109,9 @@ int main( int argc, char *argv[] )
 #endif
 {
   unsigned int verbose = 0;
+  /* Make sure all standard i/o fds are open on something. */
+  open_stdfds();
+
   /* For security, make sure we don't dump core */
   Crypto::disable_dumping_core();
 

src/frontend/mosh-server.cc

@@ -88,6 +88,7 @@
 #include "select.h"
 #include "timestamp.h"
 #include "fatal_assert.h"
+#include "stdfds.h"
 
 #ifndef _PATH_BSHELL
 #define _PATH_BSHELL "/bin/sh"
@@ -170,6 +171,9 @@ static string get_SSH_IP( void )
 
 int main( int argc, char *argv[] )
 {
+  /* Make sure all standard i/o fds are open on something. */
+  open_stdfds();
+
   /* For security, make sure we don't dump core */
   Crypto::disable_dumping_core();
 
@@ -478,35 +482,16 @@ static int run_server( const char *desired_ip, const char *desired_port,
     exit( 0 );
   }
 
-  int master;
-
-  /* close file descriptors */
+  /* Close file descriptors on tty */
   if ( verbose == 0 ) {
-    /* Necessary to properly detach on old versions of sshd (e.g. RHEL/CentOS 5.0). */
-    int nullfd;
-
-    nullfd = open( "/dev/null", O_RDWR );
-    if ( nullfd == -1 ) {
-      perror( "open" );
-      exit( 1 );
-    }
-
-    if ( dup2 ( nullfd, STDIN_FILENO ) < 0 ||
-         dup2 ( nullfd, STDOUT_FILENO ) < 0 ||
-         dup2 ( nullfd, STDERR_FILENO ) < 0 ) {
-      perror( "dup2" );
-      exit( 1 );
-    }
-
-    if ( close( nullfd ) < 0 ) {
-      perror( "close" );
-      exit( 1 );
-    }
+    detach_stdfds();
   }
 
   char utmp_entry[ 64 ] = { 0 };
   snprintf( utmp_entry, 64, "mosh [%ld]", static_cast<long int>( getpid() ) );
 
+  int master;
+
   /* Fork child process */
   pid_t child = forkpty( &master, NULL, NULL, &window_size );
 

src/util/Makefile.am

@@ -2,4 +2,4 @@ AM_CXXFLAGS = $(WARNING_CXXFLAGS) $(PICKY_CXXFLAGS) $(HARDEN_CFLAGS) $(MISC_CXXF
 
 noinst_LIBRARIES = libmoshutil.a
 
-libmoshutil_a_SOURCES = locale_utils.cc locale_utils.h swrite.cc swrite.h dos_assert.h fatal_assert.h select.h select.cc timestamp.h timestamp.cc pty_compat.cc pty_compat.h shared.h
+libmoshutil_a_SOURCES = locale_utils.cc locale_utils.h swrite.cc swrite.h dos_assert.h fatal_assert.h select.h select.cc timestamp.h timestamp.cc pty_compat.cc pty_compat.h shared.h stdfds.cc stdfds.h

src/util/stdfds.cc

@@ -0,0 +1,95 @@
+/*
+    Mosh: the mobile shell
+    Copyright 2012 Keith Winstein
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+    In addition, as a special exception, the copyright holders give
+    permission to link the code of portions of this program with the
+    OpenSSL library under certain conditions as described in each
+    individual source file, and distribute linked combinations including
+    the two.
+
+    You must obey the GNU General Public License in all respects for all
+    of the code used other than OpenSSL. If you modify file(s) with this
+    exception, you may extend this exception to your version of the
+    file(s), but you are not obligated to do so. If you do not wish to do
+    so, delete this exception statement from your version. If you delete
+    this exception statement from all source files in the program, then
+    also delete it here.
+*/
+
+#include "config.h"
+
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#ifdef HAVE_PATHS_H
+#include <paths.h>
+#endif
+
+#include <cstring>
+
+#include "stdfds.h"
+
+#ifndef _PATH_DEVNULL
+#define _PATH_DEVNULL "/dev/null"
+#endif
+
+void open_stdfds()
+{
+  /* Make sure all standard i/o fds are open on something. */
+  for ( int fd = 0; fd <= STDERR_FILENO; fd++) {
+    if ( ::fcntl( fd, F_GETFD ) < 0 ) {
+      if ( ::open( _PATH_DEVNULL, O_RDWR ) != fd ) {
+	/* given the circumstances, even writing an error may fail */
+	const char* stdErr = "cannot open standard file descriptor\n";
+	if ( ::write( STDERR_FILENO, stdErr, strlen(stdErr) ) < static_cast<ssize_t>( strlen(stdErr) ) ) {
+	  ::abort();
+	}
+	::exit(1);
+      }
+    }
+  }
+}
+
+void detach_stdfds()
+{
+  /* Necessary to properly detach on old versions of sshd (e.g. RHEL/CentOS 5.0). */
+  int nullfd;
+
+  nullfd = ::open( _PATH_DEVNULL, O_RDWR );
+  if ( nullfd == -1 ) {
+    ::perror( "open" );
+    ::exit( 1 );
+  }
+
+  if ( ::dup2 ( nullfd, STDIN_FILENO ) < 0 ||
+       ::dup2 ( nullfd, STDOUT_FILENO ) < 0 ||
+       ::dup2 ( nullfd, STDERR_FILENO ) < 0 ) {
+    ::perror( "dup2" );
+    ::exit( 1 );
+  }
+
+  if ( nullfd > STDERR_FILENO && ::close( nullfd ) < 0 ) {
+    /*
+     * This goes to /dev/null, but do it anyway, because it will
+     * show up on system call traces.
+     */
+    ::perror( "close" );
+    ::exit( 1 );
+  }
+}

src/util/stdfds.h

@@ -0,0 +1,39 @@
+/*
+    Mosh: the mobile shell
+    Copyright 2012 Keith Winstein
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+    In addition, as a special exception, the copyright holders give
+    permission to link the code of portions of this program with the
+    OpenSSL library under certain conditions as described in each
+    individual source file, and distribute linked combinations including
+    the two.
+
+    You must obey the GNU General Public License in all respects for all
+    of the code used other than OpenSSL. If you modify file(s) with this
+    exception, you may extend this exception to your version of the
+    file(s), but you are not obligated to do so. If you do not wish to do
+    so, delete this exception statement from your version. If you delete
+    this exception statement from all source files in the program, then
+    also delete it here.
+*/
+
+#ifndef STDFDS_HPP
+#define STDFDS_HPP
+
+void open_stdfds();
+void detach_stdfds();
+
+#endif