[ViewVC] Annotation 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
#	User	Rev	Content
1	root	1.1	NAME
2	root	1.2	libptytty - OS independent and secure pty/tty and utmp/wtmp/lastlog
3	root	1.1	handling
4
5			SYNOPSIS
6	root	1.4	cc ... -lptytty
7
8			#include <libptytty.h>
9
10	sf-exg	1.7
11	root	1.4	// 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	sf-exg	1.7
26	root	1.4	// 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	root	1.1
40	root	1.5	See also the eg/ directory, which currently contains the c-sample.c file
41	sf-exg	1.7	that spawns a login shell from C using libptytty.
42	root	1.5
43	root	1.1	DESCRIPTION
44	root	1.4	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	root	1.3
61			SECURITY CONSIDERATIONS
62			*It is of paramount importance that you at least read the following
63			paragraph!*
64
65	sf-exg	1.7	If you write a typical terminal-like program that just wants one or more
66	root	1.3	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	ayin	1.6	This checks whether the program runs setuid or setgid. If yes then it
79	root	1.3	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	sf-exg	1.7	methods "ptytty::use_helper" and "ptytty::drop_privileges" (and possibly
84			"ptytty::sanitise_stdfd") are more useful.
85	root	1.3
86			C++ INTERFACE: THE ptytty CLASS
87			STATIC METHODS
88			ptytty::init ()
89	sf-exg	1.7	The default way to initialise libptytty. Must be called immediately
90	root	1.3	as the first thing in the "main" function, or earlier e.g. during
91			static construction time. The earlier, the better.
92
93	sf-exg	1.7	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	root	1.3
99	root	1.8	On failure, this method terminates the process.
100
101	root	1.3	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	sf-exg	1.7	setgid, but can be called manually if it is inconvenient to drop
106	root	1.3	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	root	1.5	same helper process can usually be used both from the process
112			starting it and all its fork'ed (not exec'ed) children.
113	root	1.3
114	root	1.8	On failure, this method terminates the process.
115
116	root	1.3	ptytty::drop_privileges ()
117			Drops privileges completely, i.e. sets real, effective and saved
118	root	1.8	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	root	1.3
123	sf-exg	1.7	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	root	1.8	/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	sf-exg	1.7
134	root	1.3	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	sf-exg	1.7	exposed for your convenience and is not required for normal
138	root	1.3	operation.
139
140			int fd = ptytty::recv_fd (int socket)
141			Utility method to receive a file descriptor over a unix domain
142	sf-exg	1.7	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	root	1.3	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	root	1.8	respectively. They initially contain -1 until a successful call to
158	root	1.3	"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	root	1.8	If the helper process is running and there is a protocol error, this
165			method terminates the process.
166
167	root	1.3	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	ayin	1.6	session (such as the login shell), "login_shell" defines whether the
171	sf-exg	1.7	session is associated with a login, which influences whether wtmp
172			and lastlog entries are created, and "hostname" should identify the
173	root	1.3	"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	root	1.5	this tries to enable this discipline for the given pty. Can be
189			called at any time to change the mode.
190	root	1.3
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	root	1.5	access it in any way except by testing it for truthness (e.g. "if
198	root	1.3	(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	root	1.1
233			BUGS
234			You kiddin'?
235
236			AUTHORS
237			Emanuele Giaquinta <e.giaquinta@glauco.it>, Marc Alexander Lehmann
238			<rxvt-unicode@schmorp.de>.
239