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Diffstat (limited to 'lib/sigaction.c')
| -rw-r--r-- | lib/sigaction.c | 204 |
1 files changed, 204 insertions, 0 deletions
diff --git a/lib/sigaction.c b/lib/sigaction.c new file mode 100644 index 0000000..953a6ca --- /dev/null +++ b/lib/sigaction.c @@ -0,0 +1,204 @@ +/* POSIX compatible signal blocking. + Copyright (C) 2008-2022 Free Software Foundation, Inc. + Written by Eric Blake <ebb9@byu.net>, 2008. + + This file is free software: you can redistribute it and/or modify + it under the terms of the GNU Lesser General Public License as + published by the Free Software Foundation; either version 2.1 of the + License, or (at your option) any later version. + + This file 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 Lesser General Public License for more details. + + You should have received a copy of the GNU Lesser General Public License + along with this program. If not, see <https://www.gnu.org/licenses/>. */ + +#include <config.h> + +/* Specification. */ +#include <signal.h> + +#include <errno.h> +#include <stdint.h> +#include <stdlib.h> + +/* 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 __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; +} |