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1 /* Emergency actions in case of a fatal signal.
2    Copyright (C) 2003-2004, 2006-2012 Free Software Foundation, Inc.
3    Written by Bruno Haible <bruno@clisp.org>, 2003.
4 
5    This program is free software: you can redistribute it and/or modify
6    it under the terms of the GNU General Public License as published by
7    the Free Software Foundation; either version 3 of the License, or
8    (at your option) any later version.
9 
10    This program 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 General Public License for more details.
14 
15    You should have received a copy of the GNU General Public License
16    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
17 
18 
19 #include <config.h>
20 
21 /* Specification.  */
22 #include "fatal-signal.h"
23 
24 #include <stdbool.h>
25 #include <stdlib.h>
26 #include <signal.h>
27 #include <unistd.h>
28 
29 #include "sig-handler.h"
30 #include "xalloc.h"
31 
32 #define SIZEOF(a) (sizeof(a) / sizeof(a[0]))
33 
34 /* ========================================================================= */
35 
36 
37 /* The list of fatal signals.
38    These are those signals whose default action is to terminate the process
39    without a core dump, except
40      SIGKILL - because it cannot be caught,
41      SIGALRM SIGUSR1 SIGUSR2 SIGPOLL SIGIO SIGLOST - because applications
42        often use them for their own purpose,
43      SIGPROF SIGVTALRM - because they are used for profiling,
44      SIGSTKFLT - because it is more similar to SIGFPE, SIGSEGV, SIGBUS,
45      SIGSYS - because it is more similar to SIGABRT, SIGSEGV,
46      SIGPWR - because it of too special use,
47      SIGRTMIN...SIGRTMAX - because they are reserved for application use.
48    plus
49      SIGXCPU, SIGXFSZ - because they are quite similar to SIGTERM.  */
50 
51 static int fatal_signals[] =
52   {
53     /* ISO C 99 signals.  */
54 #ifdef SIGINT
55     SIGINT,
56 #endif
57 #ifdef SIGTERM
58     SIGTERM,
59 #endif
60     /* POSIX:2001 signals.  */
61 #ifdef SIGHUP
62     SIGHUP,
63 #endif
64 #ifdef SIGPIPE
65     SIGPIPE,
66 #endif
67     /* BSD signals.  */
68 #ifdef SIGXCPU
69     SIGXCPU,
70 #endif
71 #ifdef SIGXFSZ
72     SIGXFSZ,
73 #endif
74     /* Native Windows signals.  */
75 #ifdef SIGBREAK
76     SIGBREAK,
77 #endif
78     0
79   };
80 
81 #define num_fatal_signals (SIZEOF (fatal_signals) - 1)
82 
83 /* Eliminate signals whose signal handler is SIG_IGN.  */
84 
85 static void
init_fatal_signals(void)86 init_fatal_signals (void)
87 {
88   static bool fatal_signals_initialized = false;
89   if (!fatal_signals_initialized)
90     {
91       size_t i;
92 
93       for (i = 0; i < num_fatal_signals; i++)
94         {
95           struct sigaction action;
96 
97           if (sigaction (fatal_signals[i], NULL, &action) >= 0
98               && get_handler (&action) == SIG_IGN)
99             fatal_signals[i] = -1;
100         }
101 
102       fatal_signals_initialized = true;
103     }
104 }
105 
106 
107 /* ========================================================================= */
108 
109 
110 typedef void (*action_t) (void);
111 
112 /* Type of an entry in the actions array.
113    The 'action' field is accessed from within the fatal_signal_handler(),
114    therefore we mark it as 'volatile'.  */
115 typedef struct
116 {
117   volatile action_t action;
118 }
119 actions_entry_t;
120 
121 /* The registered cleanup actions.  */
122 static actions_entry_t static_actions[32];
123 static actions_entry_t * volatile actions = static_actions;
124 static sig_atomic_t volatile actions_count = 0;
125 static size_t actions_allocated = SIZEOF (static_actions);
126 
127 
128 /* The saved signal handlers.
129    Size 32 would not be sufficient: On HP-UX, SIGXCPU = 33, SIGXFSZ = 34.  */
130 static struct sigaction saved_sigactions[64];
131 
132 
133 /* Uninstall the handlers.  */
134 static void
uninstall_handlers(void)135 uninstall_handlers (void)
136 {
137   size_t i;
138 
139   for (i = 0; i < num_fatal_signals; i++)
140     if (fatal_signals[i] >= 0)
141       {
142         int sig = fatal_signals[i];
143         if (saved_sigactions[sig].sa_handler == SIG_IGN)
144           saved_sigactions[sig].sa_handler = SIG_DFL;
145         sigaction (sig, &saved_sigactions[sig], NULL);
146       }
147 }
148 
149 
150 /* The signal handler.  It gets called asynchronously.  */
151 static void
fatal_signal_handler(int sig)152 fatal_signal_handler (int sig)
153 {
154   for (;;)
155     {
156       /* Get the last registered cleanup action, in a reentrant way.  */
157       action_t action;
158       size_t n = actions_count;
159       if (n == 0)
160         break;
161       n--;
162       actions_count = n;
163       action = actions[n].action;
164       /* Execute the action.  */
165       action ();
166     }
167 
168   /* Now execute the signal's default action.
169      If the signal being delivered was blocked, the re-raised signal would be
170      delivered when this handler returns.  But the way we install this handler,
171      no signal is blocked, and the re-raised signal is delivered already
172      during raise().  */
173   uninstall_handlers ();
174   raise (sig);
175 }
176 
177 
178 /* Install the handlers.  */
179 static void
install_handlers(void)180 install_handlers (void)
181 {
182   size_t i;
183   struct sigaction action;
184 
185   action.sa_handler = &fatal_signal_handler;
186   /* If we get a fatal signal while executing fatal_signal_handler, enter
187      fatal_signal_handler recursively, since it is reentrant.  Hence no
188      SA_RESETHAND.  */
189   action.sa_flags = SA_NODEFER;
190   sigemptyset (&action.sa_mask);
191   for (i = 0; i < num_fatal_signals; i++)
192     if (fatal_signals[i] >= 0)
193       {
194         int sig = fatal_signals[i];
195 
196         if (!(sig < sizeof (saved_sigactions) / sizeof (saved_sigactions[0])))
197           abort ();
198         sigaction (sig, &action, &saved_sigactions[sig]);
199       }
200 }
201 
202 
203 /* Register a cleanup function to be executed when a catchable fatal signal
204    occurs.  */
205 void
at_fatal_signal(action_t action)206 at_fatal_signal (action_t action)
207 {
208   static bool cleanup_initialized = false;
209   if (!cleanup_initialized)
210     {
211       init_fatal_signals ();
212       install_handlers ();
213       cleanup_initialized = true;
214     }
215 
216   if (actions_count == actions_allocated)
217     {
218       /* Extend the actions array.  Note that we cannot use xrealloc(),
219          because then the cleanup() function could access an already
220          deallocated array.  */
221       actions_entry_t *old_actions = actions;
222       size_t old_actions_allocated = actions_allocated;
223       size_t new_actions_allocated = 2 * actions_allocated;
224       actions_entry_t *new_actions =
225         XNMALLOC (new_actions_allocated, actions_entry_t);
226       size_t k;
227 
228       /* Don't use memcpy() here, because memcpy takes non-volatile arguments
229          and is therefore not guaranteed to complete all memory stores before
230          the next statement.  */
231       for (k = 0; k < old_actions_allocated; k++)
232         new_actions[k] = old_actions[k];
233       actions = new_actions;
234       actions_allocated = new_actions_allocated;
235       /* Now we can free the old actions array.  */
236       if (old_actions != static_actions)
237         free (old_actions);
238     }
239   /* The two uses of 'volatile' in the types above (and ISO C 99 section
240      5.1.2.3.(5)) ensure that we increment the actions_count only after
241      the new action has been written to the memory location
242      actions[actions_count].  */
243   actions[actions_count].action = action;
244   actions_count++;
245 }
246 
247 
248 /* ========================================================================= */
249 
250 
251 static sigset_t fatal_signal_set;
252 
253 static void
init_fatal_signal_set(void)254 init_fatal_signal_set (void)
255 {
256   static bool fatal_signal_set_initialized = false;
257   if (!fatal_signal_set_initialized)
258     {
259       size_t i;
260 
261       init_fatal_signals ();
262 
263       sigemptyset (&fatal_signal_set);
264       for (i = 0; i < num_fatal_signals; i++)
265         if (fatal_signals[i] >= 0)
266           sigaddset (&fatal_signal_set, fatal_signals[i]);
267 
268       fatal_signal_set_initialized = true;
269     }
270 }
271 
272 /* Temporarily delay the catchable fatal signals.  */
273 void
block_fatal_signals(void)274 block_fatal_signals (void)
275 {
276   init_fatal_signal_set ();
277   sigprocmask (SIG_BLOCK, &fatal_signal_set, NULL);
278 }
279 
280 /* Stop delaying the catchable fatal signals.  */
281 void
unblock_fatal_signals(void)282 unblock_fatal_signals (void)
283 {
284   init_fatal_signal_set ();
285   sigprocmask (SIG_UNBLOCK, &fatal_signal_set, NULL);
286 }
287