[ViewVC] Contents of: cvs/libptytty/README

NAME
    libptytty - 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.

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 "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.

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 terminates the process.

    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 terminates the process.

    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.

    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 terminates the process.

    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 terminates the process.

    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".

BUGS
    You kiddin'?

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

Revision:	1.8
Committed:	Thu Feb 25 20:21:49 2016 UTC (8 years, 2 months ago) by root
Branch:	MAIN
CVS Tags:	rxvt-unicode-rel-9_26, rxvt-unicode-rel-9_25, rxvt-unicode-rel-9_22, rel-1_8
Changes since 1.7:	+19 -9 lines
Log Message:	1.8
#	Content
1	NAME
2	libptytty - OS independent and secure pty/tty and utmp/wtmp/lastlog
3	handling
4
5	SYNOPSIS
6	cc ... -lptytty
7
8	#include <libptytty.h>
9
10
11	// C++
12	ptytty *pty = ptytty::create ();
13
14	if (!pty->get ())
15	// error allocating pty
16
17	if (we want utmp)
18	pty->login (process_pid, 0, "remote.host");
19	else if (we want utmp AND wtmp/lastlog)
20	pty->login (process_pid, 1, "remote.host");
21
22	// we are done with it
23	delete pty;
24
25
26	// C
27	PTYTTY pty = ptytty_create ();
28
29	if (!ptytty_get (pty))
30	// error allocating pty
31
32	if (we want utmp)
33	ptytty_login (pty, process_pid, 0, "remote.host");
34	else if (we want utmp AND wtmp/lastlog)
35	ptytty_login (pty, process_pid, 1, "remote.host");
36
37	// we are done with it
38	ptytty_delete (pty);
39
40	See also the eg/ directory, which currently contains the c-sample.c file
41	that spawns a login shell from C using libptytty.
42
43	DESCRIPTION
44	Libptytty is a small library that offers pseudo-tty management in an
45	OS-independent way. It was created out of frustration over the many
46	differences of pty/tty handling in different operating systems for the
47	use inside "rxvt-unicode".
48
49	In addition to offering mere pty/tty management, it also offers session
50	database support (utmp and optional wtmp/lastlog updates for login
51	shells).
52
53	It also supports fork'ing after startup and dropping privileges in the
54	calling process, so in case the calling process gets compromised by the
55	user starting the program there is less to gain, as only the helper
56	process runs with privileges (e.g. setuid/setgid), which reduces the
57	area of attack immensely.
58
59	Libptytty is written in C++, but it also offers a C-only API.
60
61	SECURITY CONSIDERATIONS
62	*It is of paramount importance that you at least read the following
63	paragraph!*
64
65	If you write a typical terminal-like program that just wants one or more
66	ptys, you should call the "ptytty::init ()" method (C: "ptytty_init ()"
67	function) as the very first thing in your program:
68
69	int main (int argc, char *argv[])
70	{
71	// do nothing here
72	ptytty::init ();
73	// in C: ptytty_init ();
74
75	// initialise, parse arguments, etc.
76	}
77
78	This checks whether the program runs setuid or setgid. If yes then it
79	will fork a helper process and drop privileges.
80
81	Some programs need finer control over if and when this helper process is
82	started, and if and how to drop privileges. For those programs, the
83	methods "ptytty::use_helper" and "ptytty::drop_privileges" (and possibly
84	"ptytty::sanitise_stdfd") are more useful.
85
86	C++ INTERFACE: THE ptytty CLASS
87	STATIC METHODS
88	ptytty::init ()
89	The default way to initialise libptytty. Must be called immediately
90	as the first thing in the "main" function, or earlier e.g. during
91	static construction time. The earlier, the better.
92
93	This method calls "sanitise_stdfd" and then checks whether the
94	program runs with setuid/setgid permissions and, if yes, spawns a
95	helper process for pty/tty management. It then drops the privileges
96	completely, so the actual program runs without setuid/setgid
97	privileges.
98
99	On failure, this method terminates the process.
100
101	ptytty::use_helper ()
102	Tries to start a helper process that retains privileges even when
103	the calling process does not. This is usually called from
104	"ptytty::init" when it detects that the program is running setuid or
105	setgid, but can be called manually if it is inconvenient to drop
106	privileges at startup, or when you are not running setuid/setgid but
107	want to drop privileges (e.g. when running as a root-started
108	daemon).
109
110	This method will try not to start more than one helper process. The
111	same helper process can usually be used both from the process
112	starting it and all its fork'ed (not exec'ed) children.
113
114	On failure, this method terminates the process.
115
116	ptytty::drop_privileges ()
117	Drops privileges completely, i.e. sets real, effective and saved
118	user id to the real user id. Useful to make sure that the process
119	doesn't run with special privileges.
120
121	On failure, this method terminates the process.
122
123	ptytty::sanitise_stdfd ()
124	Checks whether file descriptors 0, 1 and 2 (stdin, stdout and
125	stderr) are valid (open) and, if not, connects them to /dev/tty or
126	/dev/null if possible. This is necessary because libptytty might
127	want to output error messages to those descriptors, which at the
128	time of outputting the error message, might be connected to
129	something unsuitable opened by the unsuspecting program itself (this
130	can be a security issue).
131
132	On failure, this method terminates the process.
133
134	bool success = ptytty::send_fd (int socket, int fd)
135	Utility method to send a file descriptor over a unix domain socket.
136	Returns true if successful, false otherwise. This method is only
137	exposed for your convenience and is not required for normal
138	operation.
139
140	int fd = ptytty::recv_fd (int socket)
141	Utility method to receive a file descriptor over a unix domain
142	socket. Returns the fd if successful and -1 otherwise. This method
143	is only exposed for your convenience and is not required for normal
144	operation.
145
146	ptytty *pty = ptytty::create ()
147	Creates new ptytty object. Creation does not yet do anything besides
148	allocating the structure.
149
150	A static method is used because the actual ptytty implementation can
151	differ at runtime, so you need a dynamic object creation facility.
152
153	DYNAMIC/SESSION-RELATED DATA MEMBERS AND METHODS
154	int pty_fd = pty->pty
155	int tty_fd = pty->tty
156	These members contain the pty and tty file descriptors,
157	respectively. They initially contain -1 until a successful call to
158	"ptytty::get".
159
160	bool success = pty->get ()
161	Tries to find, allocate and initialise a new pty/tty pair. Returns
162	"true" when successful.
163
164	If the helper process is running and there is a protocol error, this
165	method terminates the process.
166
167	pty->login (int cmd_pid, bool login_shell, const char *hostname)
168	Creates an entry in the systems session database(s) (utmp, wtmp,
169	lastlog). "cmd_pid" must be the pid of the process representing the
170	session (such as the login shell), "login_shell" defines whether the
171	session is associated with a login, which influences whether wtmp
172	and lastlog entries are created, and "hostname" should identify the
173	"hostname" the user logs in from, which often is the value of the
174	"DISPLAY" variable or tty line in case of local logins.
175
176	Calling this method is optional. A session starts at the time of the
177	login call and extends until the ptytty object is destroyed.
178
179	pty->close_tty ()
180	Closes the tty. Useful after forking in the parent/pty process.
181
182	bool success = pty->make_controlling_tty ()
183	Tries to make the pty/tty pair the controlling terminal of the
184	current process. Useful after forking in the child/tty process.
185
186	pty->set_utf8_mode (bool on)
187	On systems supporting special UTF-8 line disciplines (e.g. Linux),
188	this tries to enable this discipline for the given pty. Can be
189	called at any time to change the mode.
190
191	C INTERFACE: THE ptytty FAMILY OF FUNCTIONS
192	ptytty_init ()
193	See "ptytty::init ()".
194
195	PTYTTY ptytty_create ()
196	Creates a new opaque PTYTTY object and returns it. Do not try to
197	access it in any way except by testing it for truthness (e.g. "if
198	(pty) ...."). See "ptytty::create ()".
199
200	int ptytty_pty (PTYTTY ptytty)
201	Return the pty file descriptor. See "pty->pty".
202
203	int ptytty_tty (PTYTTY ptytty)
204	Return the tty file descriptor. See "pty->tty".
205
206	void ptytty_delete (PTYTTY ptytty)
207	Destroys the PTYTTY object, freeing the pty/tty pair and cleaning up
208	the utmp/wtmp/lastlog databases, if initialised/used. Same as
209	"delete pty" in C++.
210
211	int ptytty_get (PTYTTY ptytty)
212	See "pty->get", returns 0 in case of an error, non-zero otherwise.
213
214	void ptytty_login (PTYTTY ptytty, int cmd_pid, bool login_shell, const
215	char *hostname)
216	See "pty->login".
217
218	void ptytty_close_tty (PTYTTY ptytty)
219	See "pty->close_tty".
220
221	int ptytty_make_controlling_tty (PTYTTY ptytty)
222	See "pty->make_controlling_tty".
223
224	void ptytty_set_utf8_mode (PTYTTY ptytty, int on)
225	See "pty->set_utf8_mode".
226
227	void ptytty_drop_privileges ()
228	See "ptytty::drop_privileges".
229
230	void ptytty_use_helper ()
231	See "ptytty::use_helper".
232
233	BUGS
234	You kiddin'?
235
236	AUTHORS
237	Emanuele Giaquinta <e.giaquinta@glauco.it>, Marc Alexander Lehmann
238	<rxvt-unicode@schmorp.de>.
239