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 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 4

=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 terminates the process.

=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 terminates the process.

=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 terminates the process.

=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 terminates the process.

=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 4

=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 terminates the process.

=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 4

=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 4

=item GNU/Linux

=item FreeBSD

=item NetBSD

=item OpenBSD

=item macOS

=item Solaris

=item AIX

=back

=head1 BUGS

You kiddin'?

=head1 AUTHORS

Emanuele Giaquinta <e.giaquinta@glauco.it>, Marc Alexander Lehmann
<rxvt-unicode@schmorp.de>.

Revision:	1.21
Committed:	Mon May 24 13:52:05 2021 UTC (5 years, 4 months ago) by sf-exg
Branch:	MAIN
Changes since 1.20:	+21 -0 lines
Log Message:	doc: add portability 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	root	1.4	=head1 SECURITY CONSIDERATIONS
64	root	1.3
65	ayin	1.5	I<< B<It is of paramount importance that you at least read the following
66	root	1.4	paragraph!> >>
67
68	root	1.16	If you write a typical terminal-like program that just wants one or more
69	root	1.8	ptys, you should call the C<ptytty::init ()> method (C: C<ptytty_init ()>
70	root	1.7	function) as the very first thing in your program:
71	root	1.4
72			int main (int argc, char *argv[])
73			{
74			// do nothing here
75			ptytty::init ();
76	root	1.7	// in C: ptytty_init ();
77	root	1.4
78			// initialise, parse arguments, etc.
79			}
80
81	sf-exg	1.17	This checks whether the program runs setuid or setgid. If yes then it will
82	root	1.4	fork a helper process and drop privileges.
83
84	root	1.6	Some programs need finer control over if and when this helper process
85			is started, and if and how to drop privileges. For those programs, the
86	root	1.14	methods C<ptytty::use_helper> and C<ptytty::drop_privileges> (and possibly
87			C<ptytty::sanitise_stdfd>) are more useful.
88	root	1.6
89	root	1.7	=head1 C++ INTERFACE: THE ptytty CLASS
90	root	1.6
91			=head2 STATIC METHODS
92
93			=over 4
94
95			=item ptytty::init ()
96
97	root	1.14	The default way to initialise libptytty. Must be called immediately as
98	root	1.6	the first thing in the C<main> function, or earlier e.g. during static
99			construction time. The earlier, the better.
100
101	sf-exg	1.17	This method calls C<sanitise_stdfd> and then checks whether the program runs
102	root	1.14	with setuid/setgid permissions and, if yes, spawns a helper process for
103			pty/tty management. It then drops the privileges completely, so the actual
104			program runs without setuid/setgid privileges.
105	root	1.6
106	sf-exg	1.19	On failure, this method terminates the process.
107
108	root	1.6	=item ptytty::use_helper ()
109
110	root	1.7	Tries to start a helper process that retains privileges even when the
111			calling process does not. This is usually called from C<ptytty::init> when
112			it detects that the program is running setuid or setgid, but can be called
113	sf-exg	1.17	manually if it is inconvenient to drop privileges at startup, or when
114	root	1.7	you are not running setuid/setgid but want to drop privileges (e.g. when
115			running as a root-started daemon).
116
117			This method will try not to start more than one helper process. The same
118	root	1.13	helper process can usually be used both from the process starting it and
119			all its fork'ed (not exec'ed) children.
120	root	1.6
121	sf-exg	1.19	On failure, this method terminates the process.
122
123	root	1.6	=item ptytty::drop_privileges ()
124
125	sf-exg	1.19	Drops privileges completely, i.e. sets real, effective and saved user
126			id to the real user id. Useful to make sure that the process doesn't
127			run with special privileges.
128
129			On failure, this method terminates the process.
130	root	1.6
131	root	1.14	=item ptytty::sanitise_stdfd ()
132
133	sf-exg	1.19	Checks whether file descriptors 0, 1 and 2 (stdin, stdout and stderr)
134			are valid (open) and, if not, connects them to F</dev/tty> or
135			F</dev/null> if possible. This is necessary because libptytty might
136			want to output error messages to those descriptors, which at the time
137			of outputting the error message, might be connected to something
138			unsuitable opened by the unsuspecting program itself (this can be a
139			security issue).
140
141			On failure, this method terminates the process.
142	root	1.14
143	root	1.6	=item bool success = ptytty::send_fd (int socket, int fd)
144
145			Utility method to send a file descriptor over a unix domain
146			socket. Returns true if successful, false otherwise. This method is only
147	sf-exg	1.17	exposed for your convenience and is not required for normal operation.
148	root	1.6
149			=item int fd = ptytty::recv_fd (int socket)
150
151			Utility method to receive a file descriptor over a unix domain
152	sf-exg	1.17	socket. Returns the fd if successful and C<-1> otherwise. This method
153			is only exposed for your convenience and is not required for normal
154	root	1.6	operation.
155	root	1.4
156	root	1.6	=item ptytty *pty = ptytty::create ()
157	root	1.3
158	root	1.6	Creates new ptytty object. Creation does not yet do anything besides
159			allocating the structure.
160
161			A static method is used because the actual ptytty implementation can
162			differ at runtime, so you need a dynamic object creation facility.
163
164			=back
165
166	root	1.7
167	root	1.6	=head2 DYNAMIC/SESSION-RELATED DATA MEMBERS AND METHODS
168	root	1.1
169			=over 4
170
171	root	1.6	=item int pty_fd = pty->pty
172
173			=item int tty_fd = pty->tty
174
175			These members contain the pty and tty file descriptors, respectively. They
176	sf-exg	1.20	initially contain C<-1> until a successful call to C<ptytty::get>.
177	root	1.6
178			=item bool success = pty->get ()
179
180			Tries to find, allocate and initialise a new pty/tty pair. Returns C<true>
181			when successful.
182
183	sf-exg	1.19	If the helper process is running and there is a protocol error, this
184			method terminates the process.
185
186	root	1.6	=item pty->login (int cmd_pid, bool login_shell, const char *hostname)
187
188			Creates an entry in the systems session database(s) (utmp, wtmp, lastlog).
189			C<cmd_pid> must be the pid of the process representing the session
190	sf-exg	1.17	(such as the login shell), C<login_shell> defines whether the session is
191			associated with a login, which influences whether wtmp and lastlog entries
192	root	1.6	are created, and C<hostname> should identify the "hostname" the user logs
193			in from, which often is the value of the C<DISPLAY> variable or tty line
194			in case of local logins.
195
196			Calling this method is optional. A session starts at the time of the login
197			call and extends until the ptytty object is destroyed.
198
199			=item pty->close_tty ()
200
201			Closes the tty. Useful after forking in the parent/pty process.
202
203			=item bool success = pty->make_controlling_tty ()
204
205			Tries to make the pty/tty pair the controlling terminal of the current
206			process. Useful after forking in the child/tty process.
207
208			=item pty->set_utf8_mode (bool on)
209	root	1.1
210	root	1.13	On systems supporting special UTF-8 line disciplines (e.g. Linux), this
211			tries to enable this discipline for the given pty. Can be called at any
212			time to change the mode.
213	root	1.1
214			=back
215
216	root	1.7
217			=head1 C INTERFACE: THE ptytty FAMILY OF FUNCTIONS
218
219			=over 4
220
221			=item ptytty_init ()
222
223			See C<ptytty::init ()>.
224	ayin	1.15
225	root	1.7	=item PTYTTY ptytty_create ()
226
227			Creates a new opaque PTYTTY object and returns it. Do not try to access it
228	root	1.13	in any way except by testing it for truthness (e.g. C<if (pty) ....>). See
229	root	1.7	C<ptytty::create ()>.
230
231			=item int ptytty_pty (PTYTTY ptytty)
232
233			Return the pty file descriptor. See C<< pty->pty >>.
234	ayin	1.15
235	root	1.7	=item int ptytty_tty (PTYTTY ptytty)
236
237			Return the tty file descriptor. See C<< pty->tty >>.
238	ayin	1.15
239	root	1.7	=item void ptytty_delete (PTYTTY ptytty)
240
241			Destroys the PTYTTY object, freeing the pty/tty pair and cleaning up the
242			utmp/wtmp/lastlog databases, if initialised/used. Same as C<delete pty> in
243			C++.
244
245			=item int ptytty_get (PTYTTY ptytty)
246
247			See C<< pty->get >>, returns 0 in case of an error, non-zero otherwise.
248
249			=item void ptytty_login (PTYTTY ptytty, int cmd_pid, bool login_shell, const char *hostname)
250
251			See C<< pty->login >>.
252
253			=item void ptytty_close_tty (PTYTTY ptytty)
254
255			See C<< pty->close_tty >>.
256	ayin	1.15
257	root	1.7	=item int ptytty_make_controlling_tty (PTYTTY ptytty)
258
259			See C<< pty->make_controlling_tty >>.
260	ayin	1.15
261	root	1.7	=item void ptytty_set_utf8_mode (PTYTTY ptytty, int on)
262
263			See C<< pty->set_utf8_mode >>.
264
265			=item void ptytty_drop_privileges ()
266
267			See C<< ptytty::drop_privileges >>.
268	ayin	1.15
269	root	1.7	=item void ptytty_use_helper ()
270
271			See C<< ptytty::use_helper >>.
272
273			=back
274
275	sf-exg	1.21	=head1 PORTABILITY
276
277			To date, libptytty has been tested on the following platforms:
278
279			=over 4
280
281			=item GNU/Linux
282
283			=item FreeBSD
284
285			=item NetBSD
286
287			=item OpenBSD
288
289			=item macOS
290
291			=item Solaris
292
293			=item AIX
294
295			=back
296	root	1.7
297	root	1.1	=head1 BUGS
298
299			You kiddin'?
300
301			=head1 AUTHORS
302
303	root	1.18	Emanuele Giaquinta <e.giaquinta@glauco.it>, Marc Alexander Lehmann
304			<rxvt-unicode@schmorp.de>.
305