1 /*
2  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
3  * Copyright (c) 2021 Darren Tucker (dtucker at dtucker net).
4  *
5  * Redistribution and use in source and binary forms, with or without
6  * modification, are permitted provided that the following conditions
7  * are met:
8  * 1. Redistributions of source code must retain the above copyright
9  *    notice, this list of conditions and the following disclaimer.
10  * 2. Redistributions in binary form must reproduce the above copyright
11  *    notice, this list of conditions and the following disclaimer in the
12  *    documentation and/or other materials provided with the distribution.
13  *
14  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
15  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
16  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
17  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
18  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
19  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
20  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
21  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
22  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
23  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
24  */
25 
26 #include "includes.h"
27 #ifndef HAVE_PSELECT
28 
29 #include <sys/types.h>
30 #include <sys/time.h>
31 #ifdef HAVE_SYS_SELECT_H
32 # include <sys/select.h>
33 #endif
34 
35 #include <errno.h>
36 #include <fcntl.h>
37 #include <signal.h>
38 #include <stdlib.h>
39 #include <string.h>
40 #include <unistd.h>
41 
42 #include "log.h"
43 #include "misc.h"      /* for set_nonblock */
44 
45 #ifndef HAVE_SIGHANDLER_T
46 typedef void (*sighandler_t)(int);
47 #endif
48 
49 static sighandler_t saved_sighandler[_NSIG];
50 
51 /*
52  * Set up the descriptors.  Because they are close-on-exec, in the case
53  * where sshd's re-exec fails notify_pipe will still point to a descriptor
54  * that was closed by the exec attempt but if that descriptor has been
55  * reopened then we'll attempt to use that.  Ensure that notify_pipe is
56  * outside of the range used by sshd re-exec but within NFDBITS (so we don't
57  * need to expand the fd_sets).
58  */
59 #define REEXEC_MIN_FREE_FD (STDERR_FILENO + 4)
60 static int
61 pselect_notify_setup_fd(int *fd)
62 {
63 	int r;
64 
65 	if ((r = fcntl(*fd, F_DUPFD, REEXEC_MIN_FREE_FD)) < 0 ||
66 	    fcntl(r, F_SETFD, FD_CLOEXEC) < 0 || r >= FD_SETSIZE)
67 		return -1;
68 	(void)close(*fd);
69 	return (*fd = r);
70 }
71 
72 /*
73  * we write to this pipe if a SIGCHLD is caught in order to avoid
74  * the race between select() and child_terminated
75  */
76 static pid_t notify_pid;
77 static int notify_pipe[2];
78 static void
79 pselect_notify_setup(void)
80 {
81 	static int initialized;
82 
83 	if (initialized && notify_pid == getpid())
84 		return;
85 	if (notify_pid == 0)
86 		debug3_f("initializing");
87 	else {
88 		debug3_f("pid changed, reinitializing");
89 		if (notify_pipe[0] != -1)
90 			close(notify_pipe[0]);
91 		if (notify_pipe[1] != -1)
92 			close(notify_pipe[1]);
93 	}
94 	if (pipe(notify_pipe) == -1) {
95 		error("pipe(notify_pipe) failed %s", strerror(errno));
96 	} else if (pselect_notify_setup_fd(&notify_pipe[0]) == -1 ||
97 	    pselect_notify_setup_fd(&notify_pipe[1]) == -1) {
98 		error("fcntl(notify_pipe, ...) failed %s", strerror(errno));
99 		close(notify_pipe[0]);
100 		close(notify_pipe[1]);
101 	} else {
102 		set_nonblock(notify_pipe[0]);
103 		set_nonblock(notify_pipe[1]);
104 		notify_pid = getpid();
105 		debug3_f("pid %d saved %d pipe0 %d pipe1 %d", getpid(),
106 		    notify_pid, notify_pipe[0], notify_pipe[1]);
107 		initialized = 1;
108 		return;
109 	}
110 	notify_pipe[0] = -1;    /* read end */
111 	notify_pipe[1] = -1;    /* write end */
112 }
113 static void
114 pselect_notify_parent(void)
115 {
116 	if (notify_pipe[1] != -1)
117 		(void)write(notify_pipe[1], "", 1);
118 }
119 static void
120 pselect_notify_prepare(fd_set *readset)
121 {
122 	if (notify_pipe[0] != -1)
123 		FD_SET(notify_pipe[0], readset);
124 }
125 static void
126 pselect_notify_done(fd_set *readset)
127 {
128 	char c;
129 
130 	if (notify_pipe[0] != -1 && FD_ISSET(notify_pipe[0], readset)) {
131 		while (read(notify_pipe[0], &c, 1) != -1)
132 			debug2_f("reading");
133 		FD_CLR(notify_pipe[0], readset);
134 	}
135 }
136 
137 /*ARGSUSED*/
138 static void
139 pselect_sig_handler(int sig)
140 {
141 	int save_errno = errno;
142 
143 	pselect_notify_parent();
144 	if (saved_sighandler[sig] != NULL)
145 		(*saved_sighandler[sig])(sig);  /* call original handler */
146 	errno = save_errno;
147 }
148 
149 /*
150  * A minimal implementation of pselect(2), built on top of select(2).
151  */
152 
153 int
154 pselect(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
155     const struct timespec *timeout, const sigset_t *mask)
156 {
157 	int ret, sig, saved_errno, unmasked = 0;
158 	sigset_t osig;
159 	struct sigaction sa, osa;
160 	struct timeval tv, *tvp = NULL;
161 
162 	if (timeout != NULL) {
163 		tv.tv_sec = timeout->tv_sec;
164 		tv.tv_usec = timeout->tv_nsec / 1000;
165 		tvp = &tv;
166 	}
167 	if (mask == NULL)  /* no signal mask, just call select */
168 		return select(nfds, readfds, writefds, exceptfds, tvp);
169 
170 	/* For each signal we're unmasking, install our handler if needed. */
171 	for (sig = 0; sig < _NSIG; sig++) {
172 		if (sig == SIGKILL || sig == SIGSTOP || sigismember(mask, sig))
173 			continue;
174 		if (sigaction(sig, NULL, &sa) == 0 &&
175 		    sa.sa_handler != SIG_IGN && sa.sa_handler != SIG_DFL) {
176 			unmasked = 1;
177 			if (sa.sa_handler == pselect_sig_handler)
178 				continue;
179 			sa.sa_handler = pselect_sig_handler;
180 			if (sigaction(sig, &sa, &osa) == 0) {
181 				debug3_f("installing signal handler for %s, "
182 				    "previous %p", strsignal(sig),
183 				     osa.sa_handler);
184 				saved_sighandler[sig] = osa.sa_handler;
185 			}
186 		}
187 	}
188 	if (unmasked) {
189 		pselect_notify_setup();
190 		pselect_notify_prepare(readfds);
191 		nfds = MAX(nfds, notify_pipe[0] + 1);
192 	}
193 
194 	/* Unmask signals, call select then restore signal mask. */
195 	sigprocmask(SIG_SETMASK, mask, &osig);
196 	ret = select(nfds, readfds, writefds, exceptfds, tvp);
197 	saved_errno = errno;
198 	sigprocmask(SIG_SETMASK, &osig, NULL);
199 
200 	if (unmasked)
201 		pselect_notify_done(readfds);
202 	errno = saved_errno;
203 	return ret;
204 }
205 #endif
206