/* POSIX compatible signal blocking. Copyright (C) 2008-2017 Free Software Foundation, Inc. Written by Eric Blake , 2008. This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see . */ #include /* Specification. */ #include #include #include #include /* This implementation of sigaction is tailored to native Windows behavior: signal() has SysV semantics (ie. the handler is uninstalled before it is invoked). This is an inherent data race if an asynchronous signal is sent twice in a row before we can reinstall our handler, but there's nothing we can do about it. Meanwhile, sigprocmask() is not present, and while we can use the gnulib replacement to provide critical sections, it too suffers from potential data races in the face of an ill-timed asynchronous signal. And we compound the situation by reading static storage in a signal handler, which POSIX warns is not generically async-signal-safe. Oh well. Additionally: - We don't implement SA_NOCLDSTOP or SA_NOCLDWAIT, because SIGCHLD is not defined. - We don't implement SA_ONSTACK, because sigaltstack() is not present. - We ignore SA_RESTART, because blocking native Windows API calls are not interrupted anyway when an asynchronous signal occurs, and the MSVCRT runtime never sets errno to EINTR. - We don't implement SA_SIGINFO because it is impossible to do so portably. POSIX states that an application should not mix signal() and sigaction(). We support the use of signal() within the gnulib sigprocmask() substitute, but all other application code linked with this module should stick with only sigaction(). */ /* Check some of our assumptions. */ #if defined SIGCHLD || defined HAVE_SIGALTSTACK || defined HAVE_SIGINTERRUPT # error "Revisit the assumptions made in the sigaction module" #endif /* Out-of-range substitutes make a good fallback for uncatchable signals. */ #ifndef SIGKILL # define SIGKILL (-1) #endif #ifndef SIGSTOP # define SIGSTOP (-1) #endif /* On native Windows, as of 2008, the signal SIGABRT_COMPAT is an alias for the signal SIGABRT. Only one signal handler is stored for both SIGABRT and SIGABRT_COMPAT. SIGABRT_COMPAT is not a signal of its own. */ #if (defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__ # undef SIGABRT_COMPAT # define SIGABRT_COMPAT 6 #endif /* A signal handler. */ typedef void (*handler_t) (int signal); /* Set of current actions. If sa_handler for an entry is NULL, then that signal is not currently handled by the sigaction handler. */ static struct sigaction volatile action_array[NSIG] /* = 0 */; /* Signal handler that is installed for signals. */ static void sigaction_handler (int sig) { handler_t handler; sigset_t mask; sigset_t oldmask; int saved_errno = errno; if (sig < 0 || NSIG <= sig || !action_array[sig].sa_handler) { /* Unexpected situation; be careful to avoid recursive abort. */ if (sig == SIGABRT) signal (SIGABRT, SIG_DFL); abort (); } /* Reinstall the signal handler when required; otherwise update the bookkeeping so that the user's handler may call sigaction and get accurate results. We know the signal isn't currently blocked, or we wouldn't be in its handler, therefore we know that we are not interrupting a sigaction() call. There is a race where any asynchronous instance of the same signal occurring before we reinstall the handler will trigger the default handler; oh well. */ handler = action_array[sig].sa_handler; if ((action_array[sig].sa_flags & SA_RESETHAND) == 0) signal (sig, sigaction_handler); else action_array[sig].sa_handler = NULL; /* Block appropriate signals. */ mask = action_array[sig].sa_mask; if ((action_array[sig].sa_flags & SA_NODEFER) == 0) sigaddset (&mask, sig); sigprocmask (SIG_BLOCK, &mask, &oldmask); /* Invoke the user's handler, then restore prior mask. */ errno = saved_errno; handler (sig); saved_errno = errno; sigprocmask (SIG_SETMASK, &oldmask, NULL); errno = saved_errno; } /* Change and/or query the action that will be taken on delivery of signal SIG. If not NULL, ACT describes the new behavior. If not NULL, OACT is set to the prior behavior. Return 0 on success, or set errno and return -1 on failure. */ int sigaction (int sig, const struct sigaction *restrict act, struct sigaction *restrict oact) { sigset_t mask; sigset_t oldmask; int saved_errno; if (sig < 0 || NSIG <= sig || sig == SIGKILL || sig == SIGSTOP || (act && act->sa_handler == SIG_ERR)) { errno = EINVAL; return -1; } #ifdef SIGABRT_COMPAT if (sig == SIGABRT_COMPAT) sig = SIGABRT; #endif /* POSIX requires sigaction() to be async-signal-safe. In other words, if an asynchronous signal can occur while we are anywhere inside this function, the user's handler could then call sigaction() recursively and expect consistent results. We meet this rule by using sigprocmask to block all signals before modifying any data structure that could be read from a signal handler; this works since we know that the gnulib sigprocmask replacement does not try to use sigaction() from its handler. */ if (!act && !oact) return 0; sigfillset (&mask); sigprocmask (SIG_BLOCK, &mask, &oldmask); if (oact) { if (action_array[sig].sa_handler) *oact = action_array[sig]; else { /* Safe to change the handler at will here, since all signals are currently blocked. */ oact->sa_handler = signal (sig, SIG_DFL); if (oact->sa_handler == SIG_ERR) goto failure; signal (sig, oact->sa_handler); oact->sa_flags = SA_RESETHAND | SA_NODEFER; sigemptyset (&oact->sa_mask); } } if (act) { /* Safe to install the handler before updating action_array, since all signals are currently blocked. */ if (act->sa_handler == SIG_DFL || act->sa_handler == SIG_IGN) { if (signal (sig, act->sa_handler) == SIG_ERR) goto failure; action_array[sig].sa_handler = NULL; } else { if (signal (sig, sigaction_handler) == SIG_ERR) goto failure; action_array[sig] = *act; } } sigprocmask (SIG_SETMASK, &oldmask, NULL); return 0; failure: saved_errno = errno; sigprocmask (SIG_SETMASK, &oldmask, NULL); errno = saved_errno; return -1; }