1 /* POSIX compatible signal blocking.
2 Copyright (C) 2008-2023 Free Software Foundation, Inc.
3 Written by Eric Blake <ebb9@byu.net>, 2008.
4
5 This file is free software: you can redistribute it and/or modify
6 it under the terms of the GNU Lesser General Public License as
7 published by the Free Software Foundation; either version 2.1 of the
8 License, or (at your option) any later version.
9
10 This file is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public License
16 along with this program. If not, see <https://www.gnu.org/licenses/>. */
17
18 #include <config.h>
19
20 /* Specification. */
21 #include <signal.h>
22
23 #include <errno.h>
24 #include <stdint.h>
25 #include <stdlib.h>
26
27 /* This implementation of sigaction is tailored to native Windows behavior:
28 signal() has SysV semantics (ie. the handler is uninstalled before
29 it is invoked). This is an inherent data race if an asynchronous
30 signal is sent twice in a row before we can reinstall our handler,
31 but there's nothing we can do about it. Meanwhile, sigprocmask()
32 is not present, and while we can use the gnulib replacement to
33 provide critical sections, it too suffers from potential data races
34 in the face of an ill-timed asynchronous signal. And we compound
35 the situation by reading static storage in a signal handler, which
36 POSIX warns is not generically async-signal-safe. Oh well.
37
38 Additionally:
39 - We don't implement SA_NOCLDSTOP or SA_NOCLDWAIT, because SIGCHLD
40 is not defined.
41 - We don't implement SA_ONSTACK, because sigaltstack() is not present.
42 - We ignore SA_RESTART, because blocking native Windows API calls are
43 not interrupted anyway when an asynchronous signal occurs, and the
44 MSVCRT runtime never sets errno to EINTR.
45 - We don't implement SA_SIGINFO because it is impossible to do so
46 portably.
47
48 POSIX states that an application should not mix signal() and
49 sigaction(). We support the use of signal() within the gnulib
50 sigprocmask() substitute, but all other application code linked
51 with this module should stick with only sigaction(). */
52
53 /* Check some of our assumptions. */
54 #if defined SIGCHLD || defined HAVE_SIGALTSTACK || defined HAVE_SIGINTERRUPT
55 # error "Revisit the assumptions made in the sigaction module"
56 #endif
57
58 /* Out-of-range substitutes make a good fallback for uncatchable
59 signals. */
60 #ifndef SIGKILL
61 # define SIGKILL (-1)
62 #endif
63 #ifndef SIGSTOP
64 # define SIGSTOP (-1)
65 #endif
66
67 /* On native Windows, as of 2008, the signal SIGABRT_COMPAT is an alias
68 for the signal SIGABRT. Only one signal handler is stored for both
69 SIGABRT and SIGABRT_COMPAT. SIGABRT_COMPAT is not a signal of its own. */
70 #if defined _WIN32 && ! defined __CYGWIN__
71 # undef SIGABRT_COMPAT
72 # define SIGABRT_COMPAT 6
73 #endif
74
75 /* A signal handler. */
76 typedef void (*handler_t) (int signal);
77
78 /* Set of current actions. If sa_handler for an entry is NULL, then
79 that signal is not currently handled by the sigaction handler. */
80 static struct sigaction volatile action_array[NSIG] /* = 0 */;
81
82 /* Signal handler that is installed for signals. */
83 static void
84 sigaction_handler (int sig)
85 {
86 handler_t handler;
87 sigset_t mask;
88 sigset_t oldmask;
89 int saved_errno = errno;
90 if (sig < 0 || NSIG <= sig || !action_array[sig].sa_handler)
91 {
92 /* Unexpected situation; be careful to avoid recursive abort. */
93 if (sig == SIGABRT)
94 signal (SIGABRT, SIG_DFL);
95 abort ();
96 }
97
98 /* Reinstall the signal handler when required; otherwise update the
99 bookkeeping so that the user's handler may call sigaction and get
100 accurate results. We know the signal isn't currently blocked, or
101 we wouldn't be in its handler, therefore we know that we are not
102 interrupting a sigaction() call. There is a race where any
103 asynchronous instance of the same signal occurring before we
104 reinstall the handler will trigger the default handler; oh
105 well. */
106 handler = action_array[sig].sa_handler;
107 if ((action_array[sig].sa_flags & SA_RESETHAND) == 0)
108 signal (sig, sigaction_handler);
109 else
110 action_array[sig].sa_handler = NULL;
111
112 /* Block appropriate signals. */
113 mask = action_array[sig].sa_mask;
114 if ((action_array[sig].sa_flags & SA_NODEFER) == 0)
115 sigaddset (&mask, sig);
116 sigprocmask (SIG_BLOCK, &mask, &oldmask);
117
118 /* Invoke the user's handler, then restore prior mask. */
119 errno = saved_errno;
120 handler (sig);
121 saved_errno = errno;
122 sigprocmask (SIG_SETMASK, &oldmask, NULL);
123 errno = saved_errno;
124 }
125
126 /* Change and/or query the action that will be taken on delivery of
127 signal SIG. If not NULL, ACT describes the new behavior. If not
128 NULL, OACT is set to the prior behavior. Return 0 on success, or
129 set errno and return -1 on failure. */
130 int
131 sigaction (int sig, const struct sigaction *restrict act,
132 struct sigaction *restrict oact)
133 {
134 sigset_t mask;
135 sigset_t oldmask;
136 int saved_errno;
137
138 if (sig < 0 || NSIG <= sig || sig == SIGKILL || sig == SIGSTOP
139 || (act && act->sa_handler == SIG_ERR))
140 {
141 errno = EINVAL;
142 return -1;
143 }
144
145 #ifdef SIGABRT_COMPAT
146 if (sig == SIGABRT_COMPAT)
147 sig = SIGABRT;
148 #endif
149
150 /* POSIX requires sigaction() to be async-signal-safe. In other
151 words, if an asynchronous signal can occur while we are anywhere
152 inside this function, the user's handler could then call
153 sigaction() recursively and expect consistent results. We meet
154 this rule by using sigprocmask to block all signals before
155 modifying any data structure that could be read from a signal
156 handler; this works since we know that the gnulib sigprocmask
157 replacement does not try to use sigaction() from its handler. */
158 if (!act && !oact)
159 return 0;
160 sigfillset (&mask);
161 sigprocmask (SIG_BLOCK, &mask, &oldmask);
162 if (oact)
163 {
164 if (action_array[sig].sa_handler)
165 *oact = action_array[sig];
166 else
167 {
168 /* Safe to change the handler at will here, since all
169 signals are currently blocked. */
170 oact->sa_handler = signal (sig, SIG_DFL);
171 if (oact->sa_handler == SIG_ERR)
172 goto failure;
173 signal (sig, oact->sa_handler);
174 oact->sa_flags = SA_RESETHAND | SA_NODEFER;
175 sigemptyset (&oact->sa_mask);
176 }
177 }
178
179 if (act)
180 {
181 /* Safe to install the handler before updating action_array,
182 since all signals are currently blocked. */
183 if (act->sa_handler == SIG_DFL || act->sa_handler == SIG_IGN)
184 {
185 if (signal (sig, act->sa_handler) == SIG_ERR)
186 goto failure;
187 action_array[sig].sa_handler = NULL;
188 }
189 else
190 {
191 if (signal (sig, sigaction_handler) == SIG_ERR)
192 goto failure;
193 action_array[sig] = *act;
194 }
195 }
196 sigprocmask (SIG_SETMASK, &oldmask, NULL);
197 return 0;
198
199 failure:
200 saved_errno = errno;
201 sigprocmask (SIG_SETMASK, &oldmask, NULL);
202 errno = saved_errno;
203 return -1;
204 }