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| 1 | +/* POSIX compatible signal blocking. |
| 2 | + Copyright (C) 2008-2011 Free Software Foundation, Inc. |
| 3 | + Written by Eric Blake <[email protected]>, 2008. |
| 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 | +#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 Woe32 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 Win32 calls are not interrupted |
| 43 | + anyway when an asynchronous signal occurs, and the MSVCRT runtime |
| 44 | + 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 __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 | +} |
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