libptytty - Man Page

OS independent and secure pty/tty and utmp/wtmp/lastlog handling

Synopsis

   cc ... -lptytty

   #include <libptytty.h>


   // C++
   ptytty *pty = ptytty::create ();

   if (!pty->get ())
     // error allocating pty

   if (we want utmp)
     pty->login (process_pid, 0, "remote.host");
   else if (we want utmp AND wtmp/lastlog)
     pty->login (process_pid, 1, "remote.host");

   // we are done with it
   delete pty;


   // C
   PTYTTY pty = ptytty_create ();

   if (!ptytty_get (pty))
     // error allocating pty

   if (we want utmp)
     ptytty_login (pty, process_pid, 0, "remote.host");
   else if (we want utmp AND wtmp/lastlog)
     ptytty_login (pty, process_pid, 1, "remote.host");

   // we are done with it
   ptytty_delete (pty);

See also the eg/ directory, which currently contains the c-sample.c file that spawns a login shell from C using libptytty.

Libptytty is a small library that offers pseudo-tty management in an OS-independent way. It was created out of frustration over the many differences of pty/tty handling in different operating systems for the use inside rxvt-unicode.

In addition to offering mere pty/tty management, it also offers session database support (utmp and optional wtmp/lastlog updates for login shells).

It also supports fork'ing after startup and dropping privileges in the calling process, so in case the calling process gets compromised by the user starting the program there is less to gain, as only the helper process runs with privileges (e.g. setuid/setgid), which reduces the area of attack immensely.

Libptytty is written in C++, but it also offers a C-only API.

Installation

libptytty uses CMake as build system. To build libptytty, install CMake and run the following commands from either the libptytty source directory or a separate build directory:

   cmake -DCMAKE_INSTALL_PREFIX=<prefix> -DBUILD_SHARED_LIBS=ON <path/to/libptytty>
   cmake --build .
   cmake --install .

Security Considerations

It is of paramount importance that you at least read the following paragraph!

If you write a typical terminal-like program that just wants one or more ptys, you should call the ptytty::init () method (C: ptytty_init () function) as the very first thing in your program:

   int main (int argc, char *argv[])
   {
      // do nothing here
      ptytty::init ();
      // in C: ptytty_init ();

      // initialise, parse arguments, etc.
   }

This checks whether the program runs setuid or setgid. If yes then it will fork a helper process and drop privileges.

Some programs need finer control over if and when this helper process is started, and if and how to drop privileges. For those programs, the methods ptytty::use_helper and ptytty::drop_privileges (and possibly ptytty::sanitise_stdfd) are more useful.

C++ INTERFACE: THE ptytty CLASS

Static Methods

ptytty::init ()

The default way to initialise libptytty. Must be called immediately as the first thing in the main function, or earlier e.g. during static construction time. The earlier, the better.

This method calls sanitise_stdfd and then checks whether the program runs with setuid/setgid permissions and, if yes, spawns a helper process for pty/tty management. It then drops the privileges completely, so the actual program runs without setuid/setgid privileges.

On failure, this method throws a ptytty_error exception.

ptytty::use_helper ()

Tries to start a helper process that retains privileges even when the calling process does not. This is usually called from ptytty::init when it detects that the program is running setuid or setgid, but can be called manually if it is inconvenient to drop privileges at startup, or when you are not running setuid/setgid but want to drop privileges (e.g. when running as a root-started daemon).

This method will try not to start more than one helper process. The same helper process can usually be used both from the process starting it and all its fork'ed (not exec'ed) children.

On failure, this method throws a ptytty_error exception.

ptytty::drop_privileges ()

Drops privileges completely, i.e. sets real, effective and saved user id to the real user id. Useful to make sure that the process doesn't run with special privileges.

On failure, this method throws a ptytty_error exception.

ptytty::sanitise_stdfd ()

Checks whether file descriptors 0, 1 and 2 (stdin, stdout and stderr) are valid (open) and, if not, connects them to /dev/tty or /dev/null if possible. This is necessary because libptytty might want to output error messages to those descriptors, which at the time of outputting the error message, might be connected to something unsuitable opened by the unsuspecting program itself (this can be a security issue).

On failure, this method throws a ptytty_error exception.

bool success = ptytty::send_fd (int socket, int fd)

Utility method to send a file descriptor over a unix domain socket. Returns true if successful, false otherwise. This method is only exposed for your convenience and is not required for normal operation.

int fd = ptytty::recv_fd (int socket)

Utility method to receive a file descriptor over a unix domain socket. Returns the fd if successful and -1 otherwise. This method is only exposed for your convenience and is not required for normal operation.

ptytty *pty = ptytty::create ()

Creates new ptytty object. Creation does not yet do anything besides allocating the structure.

A static method is used because the actual ptytty implementation can differ at runtime, so you need a dynamic object creation facility.

Dynamic/Session-Related Data Members and Methods

int pty_fd = pty->pty

int tty_fd = pty->tty

These members contain the pty and tty file descriptors, respectively. They initially contain -1 until a successful call to ptytty::get.

bool success = pty->get ()

Tries to find, allocate and initialise a new pty/tty pair. Returns true when successful.

If the helper process is running and there is a protocol error, this method throws a ptytty_error exception.

pty->login (int cmd_pid, bool login_shell, const char *hostname)

Creates an entry in the systems session database(s) (utmp, wtmp, lastlog). cmd_pid must be the pid of the process representing the session (such as the login shell), login_shell defines whether the session is associated with a login, which influences whether wtmp and lastlog entries are created, and hostname should identify the “hostname” the user logs in from, which often is the value of the DISPLAY variable or tty line in case of local logins.

Calling this method is optional. A session starts at the time of the login call and extends until the ptytty object is destroyed.

pty->close_tty ()

Closes the tty. Useful after forking in the parent/pty process.

bool success = pty->make_controlling_tty ()

Tries to make the pty/tty pair the controlling terminal of the current process. Useful after forking in the child/tty process.

pty->set_utf8_mode (bool on)

On systems supporting special UTF-8 line disciplines (e.g. Linux), this tries to enable this discipline for the given pty. Can be called at any time to change the mode.

C INTERFACE: THE ptytty FAMILY OF FUNCTIONS

ptytty_init (): See ptytty::init ().
PTYTTY ptytty_create (): Creates a new opaque PTYTTY object and returns it. Do not try to access it in any way except by testing it for truthness (e.g. if (pty) ....). See ptytty::create ().
int ptytty_pty (PTYTTY ptytty): Return the pty file descriptor. See pty->pty.
int ptytty_tty (PTYTTY ptytty): Return the tty file descriptor. See pty->tty.
void ptytty_delete (PTYTTY ptytty): Destroys the PTYTTY object, freeing the pty/tty pair and cleaning up the utmp/wtmp/lastlog databases, if initialised/used. Same as delete pty in C++.
int ptytty_get (PTYTTY ptytty): See pty->get, returns 0 in case of an error, non-zero otherwise.
void ptytty_login (PTYTTY ptytty, int cmd_pid, bool login_shell, const char *hostname): See pty->login.
void ptytty_close_tty (PTYTTY ptytty): See pty->close_tty.
int ptytty_make_controlling_tty (PTYTTY ptytty): See pty->make_controlling_tty.
void ptytty_set_utf8_mode (PTYTTY ptytty, int on): See pty->set_utf8_mode.
void ptytty_drop_privileges (): See ptytty::drop_privileges.
void ptytty_use_helper (): See ptytty::use_helper.

Portability

To date, libptytty has been tested on the following platforms:

GNU/Linux

FreeBSD

NetBSD

OpenBSD

macOS

Solaris

AIX

libptytty - Man Page

Synopsis

Description

Installation

Security Considerations

C++ INTERFACE: THE ptytty CLASS

Static Methods

Dynamic/Session-Related Data Members and Methods

C INTERFACE: THE ptytty FAMILY OF FUNCTIONS

Portability

Bugs

Authors

Info