libptytty/doc/libptytty.3.pod

=head1 NAME

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

=head1 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 F<eg/> directory, which currently contains the F<c-sample.c>
file that spawns a login shell from C using libptytty.

=head1 DESCRIPTION

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 C<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.

=head1 INSTALLATION

libptytty uses C<CMake> as build system. To build libptytty, install
C<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 .

=head1 SECURITY CONSIDERATIONS

I<< B<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 C<ptytty::init ()> method (C: 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 C<ptytty::use_helper> and C<ptytty::drop_privileges> (and possibly
C<ptytty::sanitise_stdfd>) are more useful.

=head1 C++ INTERFACE: THE ptytty CLASS

=head2 STATIC METHODS

=over

=item ptytty::init ()

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

This method calls C<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 C<ptytty_error> exception.

=item ptytty::use_helper ()

Tries to start a helper process that retains privileges even when the
calling process does not. This is usually called from C<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 C<ptytty_error> exception.

=item 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 C<ptytty_error> exception.

=item ptytty::sanitise_stdfd ()

Checks whether file descriptors 0, 1 and 2 (stdin, stdout and stderr)
are valid (open) and, if not, connects them to F</dev/tty> or
F</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 C<ptytty_error> exception.

=item 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.

=item 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 C<-1> otherwise. This method
is only exposed for your convenience and is not required for normal
operation.

=item 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.

=back


=head2 DYNAMIC/SESSION-RELATED DATA MEMBERS AND METHODS

=over

=item int pty_fd = pty->pty

=item int tty_fd = pty->tty

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

=item bool success = pty->get ()

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

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

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

Creates an entry in the systems session database(s) (utmp, wtmp, lastlog).
C<cmd_pid> must be the pid of the process representing the session
(such as the login shell), C<login_shell> defines whether the session is
associated with a login, which influences whether wtmp and lastlog entries
are created, and C<hostname> should identify the "hostname" the user logs
in from, which often is the value of the C<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.

=item pty->close_tty ()

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

=item 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.

=item 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.

=back


=head1 C INTERFACE: THE ptytty FAMILY OF FUNCTIONS

=over

=item ptytty_init ()

See C<ptytty::init ()>.

=item 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. C<if (pty) ....>). See
C<ptytty::create ()>.

=item int ptytty_pty (PTYTTY ptytty)

Return the pty file descriptor. See C<< pty->pty >>.

=item int ptytty_tty (PTYTTY ptytty)

Return the tty file descriptor. See C<< pty->tty >>.

=item 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 C<delete pty> in
C++.

=item int ptytty_get (PTYTTY ptytty)

See C<< pty->get >>, returns 0 in case of an error, non-zero otherwise.

=item void ptytty_login (PTYTTY ptytty, int cmd_pid, bool login_shell, const char *hostname)

See C<< pty->login >>.

=item void ptytty_close_tty (PTYTTY ptytty)

See C<< pty->close_tty >>.

=item int ptytty_make_controlling_tty (PTYTTY ptytty)

See C<< pty->make_controlling_tty >>.

=item void ptytty_set_utf8_mode (PTYTTY ptytty, int on)

See C<< pty->set_utf8_mode >>.

=item void ptytty_drop_privileges ()

See C<< ptytty::drop_privileges >>.

=item void ptytty_use_helper ()

See C<< ptytty::use_helper >>.

=back

=head1 PORTABILITY

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

=over

=item GNU/Linux

=item FreeBSD

=item NetBSD

=item OpenBSD

=item macOS

=item Solaris

=item AIX

=back

=head1 BUGS

You kiddin'?

=head1 AUTHORS

Emanuele Giaquinta <emanuele.giaquinta@gmail.com>, Marc Alexander Lehmann
<rxvt-unicode@schmorp.de>.

Revision:	1.25
Committed:	Sat Jul 24 12:41:35 2021 UTC (5 years, 2 months ago) by sf-exg
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.24:	+10 -0 lines
Log Message:	doc: add installation section
#	User	Rev	Content
1	root	1.1	=head1 NAME
2
3	root	1.2	libptytty - OS independent and secure pty/tty and utmp/wtmp/lastlog handling
4	root	1.1
5			=head1 SYNOPSIS
6
7	root	1.11	cc ... -lptytty
8	root	1.9
9			#include <libptytty.h>
10
11
12			// C++
13			ptytty *pty = ptytty::create ();
14
15			if (!pty->get ())
16			// error allocating pty
17
18			if (we want utmp)
19			pty->login (process_pid, 0, "remote.host");
20			else if (we want utmp AND wtmp/lastlog)
21			pty->login (process_pid, 1, "remote.host");
22
23			// we are done with it
24			delete pty;
25
26
27			// C
28			PTYTTY pty = ptytty_create ();
29
30			if (!ptytty_get (pty))
31			// error allocating pty
32
33			if (we want utmp)
34			ptytty_login (pty, process_pid, 0, "remote.host");
35			else if (we want utmp AND wtmp/lastlog)
36			ptytty_login (pty, process_pid, 1, "remote.host");
37
38			// we are done with it
39			ptytty_delete (pty);
40
41	root	1.12	See also the F<eg/> directory, which currently contains the F<c-sample.c>
42	sf-exg	1.17	file that spawns a login shell from C using libptytty.
43	root	1.1
44			=head1 DESCRIPTION
45
46	root	1.9	Libptytty is a small library that offers pseudo-tty management in an
47			OS-independent way. It was created out of frustration over the many
48			differences of pty/tty handling in different operating systems for the use
49			inside C<rxvt-unicode>.
50
51			In addition to offering mere pty/tty management, it also offers session
52			database support (utmp and optional wtmp/lastlog updates for login
53			shells).
54
55			It also supports fork'ing after startup and dropping privileges in the
56			calling process, so in case the calling process gets compromised by the
57			user starting the program there is less to gain, as only the helper
58			process runs with privileges (e.g. setuid/setgid), which reduces the area
59			of attack immensely.
60
61			Libptytty is written in C++, but it also offers a C-only API.
62	root	1.6
63	sf-exg	1.25	=head1 INSTALLATION
64
65			libptytty uses C<CMake> as build system. To build libptytty, install
66			C<CMake> and run the following commands from either the libptytty
67			source directory or a separate build directory:
68
69			cmake -DCMAKE_INSTALL_PREFIX=<prefix> -DBUILD_SHARED_LIBS=ON <path/to/libptytty>
70			cmake --build .
71			cmake --install .
72
73	root	1.4	=head1 SECURITY CONSIDERATIONS
74	root	1.3
75	ayin	1.5	I<< B<It is of paramount importance that you at least read the following
76	root	1.4	paragraph!> >>
77
78	root	1.16	If you write a typical terminal-like program that just wants one or more
79	root	1.8	ptys, you should call the C<ptytty::init ()> method (C: C<ptytty_init ()>
80	root	1.7	function) as the very first thing in your program:
81	root	1.4
82			int main (int argc, char *argv[])
83			{
84			// do nothing here
85			ptytty::init ();
86	root	1.7	// in C: ptytty_init ();
87	root	1.4
88			// initialise, parse arguments, etc.
89			}
90
91	sf-exg	1.17	This checks whether the program runs setuid or setgid. If yes then it will
92	root	1.4	fork a helper process and drop privileges.
93
94	root	1.6	Some programs need finer control over if and when this helper process
95			is started, and if and how to drop privileges. For those programs, the
96	root	1.14	methods C<ptytty::use_helper> and C<ptytty::drop_privileges> (and possibly
97			C<ptytty::sanitise_stdfd>) are more useful.
98	root	1.6
99	root	1.7	=head1 C++ INTERFACE: THE ptytty CLASS
100	root	1.6
101			=head2 STATIC METHODS
102
103	sf-exg	1.24	=over
104	root	1.6
105			=item ptytty::init ()
106
107	root	1.14	The default way to initialise libptytty. Must be called immediately as
108	root	1.6	the first thing in the C<main> function, or earlier e.g. during static
109			construction time. The earlier, the better.
110
111	sf-exg	1.17	This method calls C<sanitise_stdfd> and then checks whether the program runs
112	root	1.14	with setuid/setgid permissions and, if yes, spawns a helper process for
113			pty/tty management. It then drops the privileges completely, so the actual
114			program runs without setuid/setgid privileges.
115	root	1.6
116	sf-exg	1.22	On failure, this method throws a C<ptytty_error> exception.
117	sf-exg	1.19
118	root	1.6	=item ptytty::use_helper ()
119
120	root	1.7	Tries to start a helper process that retains privileges even when the
121			calling process does not. This is usually called from C<ptytty::init> when
122			it detects that the program is running setuid or setgid, but can be called
123	sf-exg	1.17	manually if it is inconvenient to drop privileges at startup, or when
124	root	1.7	you are not running setuid/setgid but want to drop privileges (e.g. when
125			running as a root-started daemon).
126
127			This method will try not to start more than one helper process. The same
128	root	1.13	helper process can usually be used both from the process starting it and
129			all its fork'ed (not exec'ed) children.
130	root	1.6
131	sf-exg	1.22	On failure, this method throws a C<ptytty_error> exception.
132	sf-exg	1.19
133	root	1.6	=item ptytty::drop_privileges ()
134
135	sf-exg	1.19	Drops privileges completely, i.e. sets real, effective and saved user
136			id to the real user id. Useful to make sure that the process doesn't
137			run with special privileges.
138
139	sf-exg	1.22	On failure, this method throws a C<ptytty_error> exception.
140	root	1.6
141	root	1.14	=item ptytty::sanitise_stdfd ()
142
143	sf-exg	1.19	Checks whether file descriptors 0, 1 and 2 (stdin, stdout and stderr)
144			are valid (open) and, if not, connects them to F</dev/tty> or
145			F</dev/null> if possible. This is necessary because libptytty might
146			want to output error messages to those descriptors, which at the time
147			of outputting the error message, might be connected to something
148			unsuitable opened by the unsuspecting program itself (this can be a
149			security issue).
150
151	sf-exg	1.22	On failure, this method throws a C<ptytty_error> exception.
152	root	1.14
153	root	1.6	=item bool success = ptytty::send_fd (int socket, int fd)
154
155			Utility method to send a file descriptor over a unix domain
156			socket. Returns true if successful, false otherwise. This method is only
157	sf-exg	1.17	exposed for your convenience and is not required for normal operation.
158	root	1.6
159			=item int fd = ptytty::recv_fd (int socket)
160
161			Utility method to receive a file descriptor over a unix domain
162	sf-exg	1.17	socket. Returns the fd if successful and C<-1> otherwise. This method
163			is only exposed for your convenience and is not required for normal
164	root	1.6	operation.
165	root	1.4
166	root	1.6	=item ptytty *pty = ptytty::create ()
167	root	1.3
168	root	1.6	Creates new ptytty object. Creation does not yet do anything besides
169			allocating the structure.
170
171			A static method is used because the actual ptytty implementation can
172			differ at runtime, so you need a dynamic object creation facility.
173
174			=back
175
176	root	1.7
177	root	1.6	=head2 DYNAMIC/SESSION-RELATED DATA MEMBERS AND METHODS
178	root	1.1
179	sf-exg	1.24	=over
180	root	1.1
181	root	1.6	=item int pty_fd = pty->pty
182
183			=item int tty_fd = pty->tty
184
185			These members contain the pty and tty file descriptors, respectively. They
186	sf-exg	1.20	initially contain C<-1> until a successful call to C<ptytty::get>.
187	root	1.6
188			=item bool success = pty->get ()
189
190			Tries to find, allocate and initialise a new pty/tty pair. Returns C<true>
191			when successful.
192
193	sf-exg	1.19	If the helper process is running and there is a protocol error, this
194	sf-exg	1.22	method throws a C<ptytty_error> exception.
195	sf-exg	1.19
196	root	1.6	=item pty->login (int cmd_pid, bool login_shell, const char *hostname)
197
198			Creates an entry in the systems session database(s) (utmp, wtmp, lastlog).
199			C<cmd_pid> must be the pid of the process representing the session
200	sf-exg	1.17	(such as the login shell), C<login_shell> defines whether the session is
201			associated with a login, which influences whether wtmp and lastlog entries
202	root	1.6	are created, and C<hostname> should identify the "hostname" the user logs
203			in from, which often is the value of the C<DISPLAY> variable or tty line
204			in case of local logins.
205
206			Calling this method is optional. A session starts at the time of the login
207			call and extends until the ptytty object is destroyed.
208
209			=item pty->close_tty ()
210
211			Closes the tty. Useful after forking in the parent/pty process.
212
213			=item bool success = pty->make_controlling_tty ()
214
215			Tries to make the pty/tty pair the controlling terminal of the current
216			process. Useful after forking in the child/tty process.
217
218			=item pty->set_utf8_mode (bool on)
219	root	1.1
220	root	1.13	On systems supporting special UTF-8 line disciplines (e.g. Linux), this
221			tries to enable this discipline for the given pty. Can be called at any
222			time to change the mode.
223	root	1.1
224			=back
225
226	root	1.7
227			=head1 C INTERFACE: THE ptytty FAMILY OF FUNCTIONS
228
229	sf-exg	1.24	=over
230	root	1.7
231			=item ptytty_init ()
232
233			See C<ptytty::init ()>.
234	ayin	1.15
235	root	1.7	=item PTYTTY ptytty_create ()
236
237			Creates a new opaque PTYTTY object and returns it. Do not try to access it
238	root	1.13	in any way except by testing it for truthness (e.g. C<if (pty) ....>). See
239	root	1.7	C<ptytty::create ()>.
240
241			=item int ptytty_pty (PTYTTY ptytty)
242
243			Return the pty file descriptor. See C<< pty->pty >>.
244	ayin	1.15
245	root	1.7	=item int ptytty_tty (PTYTTY ptytty)
246
247			Return the tty file descriptor. See C<< pty->tty >>.
248	ayin	1.15
249	root	1.7	=item void ptytty_delete (PTYTTY ptytty)
250
251			Destroys the PTYTTY object, freeing the pty/tty pair and cleaning up the
252			utmp/wtmp/lastlog databases, if initialised/used. Same as C<delete pty> in
253			C++.
254
255			=item int ptytty_get (PTYTTY ptytty)
256
257			See C<< pty->get >>, returns 0 in case of an error, non-zero otherwise.
258
259			=item void ptytty_login (PTYTTY ptytty, int cmd_pid, bool login_shell, const char *hostname)
260
261			See C<< pty->login >>.
262
263			=item void ptytty_close_tty (PTYTTY ptytty)
264
265			See C<< pty->close_tty >>.
266	ayin	1.15
267	root	1.7	=item int ptytty_make_controlling_tty (PTYTTY ptytty)
268
269			See C<< pty->make_controlling_tty >>.
270	ayin	1.15
271	root	1.7	=item void ptytty_set_utf8_mode (PTYTTY ptytty, int on)
272
273			See C<< pty->set_utf8_mode >>.
274
275			=item void ptytty_drop_privileges ()
276
277			See C<< ptytty::drop_privileges >>.
278	ayin	1.15
279	root	1.7	=item void ptytty_use_helper ()
280
281			See C<< ptytty::use_helper >>.
282
283			=back
284
285	sf-exg	1.21	=head1 PORTABILITY
286
287			To date, libptytty has been tested on the following platforms:
288
289	sf-exg	1.24	=over
290	sf-exg	1.21
291			=item GNU/Linux
292
293			=item FreeBSD
294
295			=item NetBSD
296
297			=item OpenBSD
298
299			=item macOS
300
301			=item Solaris
302
303			=item AIX
304
305			=back
306	root	1.7
307	root	1.1	=head1 BUGS
308
309			You kiddin'?
310
311			=head1 AUTHORS
312
313	sf-exg	1.23	Emanuele Giaquinta <emanuele.giaquinta@gmail.com>, Marc Alexander Lehmann
314	root	1.18	<rxvt-unicode@schmorp.de>.
315