signal: new signal handling backend based on signalfd

Linux-specific signal handling backend based on signalfd(2)
system call, and public function event_base_get_signal_method()
to obtain an underlying kernel signal handling mechanism.

Signed-off-by: Dmitry Antipov <dantipov@cloudlinux.com>

1 files changed, 213 insertions, 0 deletions

diff --git a/signalfd.c b/signalfd.c
new file mode 100644
index 00000000..376a04d5
--- /dev/null
+++ b/signalfd.c
@@ -0,0 +1,213 @@
+/*
+ * Signal handling backend based on signalfd(2) system call
+ * Written by Dmitry Antipov <dantipov@cloudlinux.com> 2022
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "event2/event-config.h"
+
+#include <unistd.h>
+#include <sys/signalfd.h>
+
+#include "event2/event.h"
+#include "event-internal.h"
+#include "evmap-internal.h"
+#include "evsignal-internal.h"
+#include "evthread-internal.h"
+
+static int sigfd_add(struct event_base *, evutil_socket_t, short, short, void *);
+static int sigfd_del(struct event_base *, evutil_socket_t, short, short, void *);
+
+static const struct eventop sigfdops = {
+	"signalfd_signal",
+	NULL,
+	sigfd_add,
+	sigfd_del,
+	NULL,
+	NULL,
+	0, 0, 0
+};
+
+static void
+sigfd_cb(evutil_socket_t fd, short what, void *arg)
+{
+	struct signalfd_siginfo fdsi;
+	struct event_base *base = arg;
+	ssize_t ret = read(fd, &fdsi, sizeof(fdsi));
+
+	EVUTIL_ASSERT(ret == sizeof(fdsi));
+	EVUTIL_ASSERT(fdsi.ssi_signo > 0 && fdsi.ssi_signo < NSIG);
+	EVUTIL_ASSERT(base->sig.ev_sigevent[fdsi.ssi_signo] != NULL);
+
+	EVBASE_ACQUIRE_LOCK(base, th_base_lock);
+	evmap_signal_active_(base, fdsi.ssi_signo, 1);
+	EVBASE_RELEASE_LOCK(base, th_base_lock);
+}
+
+static void
+sigfd_free_sigevent(struct event_base *base, int signo)
+{
+	int ret;
+	struct event* sigev = base->sig.ev_sigevent[signo];
+
+	EVUTIL_ASSERT(sigev != NULL);
+	event_del_nolock_(sigev, EVENT_DEL_AUTOBLOCK);
+	ret = close(sigev->ev_fd);
+	EVUTIL_ASSERT(ret == 0);
+	mm_free(sigev);
+	base->sig.ev_sigevent[signo] = NULL;
+}
+
+static int
+sigfd_add(struct event_base *base, int signo, short old, short events, void *p)
+{
+	int sigfd;
+	sigset_t mask;
+	struct event* sigev;
+	struct evsig_info *sig = &base->sig;
+
+	/* EV_SIGNAL event passed from evmap_signal_add_() when setting
+           up and from evmap_signal_reinit_iter_fn() during reinit. */
+	EVUTIL_ASSERT(p != NULL);
+
+	EVUTIL_ASSERT(signo > 0 && signo < NSIG);
+	sigev = base->sig.ev_sigevent[signo];
+
+	if (sigev != NULL) {
+		if (old) {
+			/* We're performing reinit after fork(). This is
+			   required at least for epoll(2)-based backend
+			   because if the process uses fork(2) to create
+			   a child process, then the child will be able
+			   to read(2) signals that are sent to it using the
+			   signalfd(2) file descriptor, but epoll_wait(2)
+			   will not indicate that the signalfd file
+			   descriptor is ready. */
+			sigfd_free_sigevent(base, signo);
+		} else {
+			/* We have an active signal fd
+			   for this signal already. */
+			return 0;
+		}
+	}
+
+	/* Save previous handler just like evsig_set_handler_() does. */
+	if (evsig_ensure_saved_(sig, signo) < 0)
+		return -1;
+
+	sig->sh_old[signo] = mm_malloc(sizeof *sig->sh_old[signo]);
+	if (sig->sh_old[signo] == NULL) {
+		event_warn("malloc() failed");
+		return -1;
+	}
+
+	if (sigaction(signo, NULL, sig->sh_old[signo]) == -1) {
+		event_warn("sigaction() failed");
+		mm_free(sig->sh_old[signo]);
+		sig->sh_old[signo] = NULL;
+		return -1;
+	}
+
+	/* Block the signal from being handled according to its default
+	   disposition so it may be received via the descriptor. */
+	sigemptyset(&mask);
+	sigaddset(&mask, signo);
+	if (sigprocmask(SIG_BLOCK, &mask, NULL)) {
+		event_warn("sigprocmask() failed");
+		return -1;
+	}
+
+	sigfd = signalfd(-1, &mask, SFD_NONBLOCK | SFD_CLOEXEC);
+	if (sigfd < 0) {
+		event_warn("signalfd() failed");
+		goto unblock;
+	}
+
+	/* EV_READ event used to wakeup corresponding EV_SIGNAL ones. */
+	sigev = event_new(base, sigfd, EV_READ | EV_PERSIST, sigfd_cb, base);
+	if (!sigev)
+		goto close_fd;
+
+	/* This was blindly copied from evsig_init_(). */
+	sigev->ev_flags |= EVLIST_INTERNAL;
+	event_priority_set(sigev, 0);
+
+	if (event_add_nolock_(sigev, NULL, 0) < 0)
+		goto free_ev;
+
+	base->sig.ev_sigevent[signo] = sigev;
+	return 0;
+free_ev:
+	mm_free(sigev);
+close_fd:
+	close(sigfd);
+unblock:
+	sigprocmask(SIG_UNBLOCK, &mask, NULL);
+	return -1;
+}
+
+static int
+sigfd_del(struct event_base *base, int signo, short old, short events, void *p)
+{
+	sigset_t mask;
+	struct event *sigev;
+	struct evsig_info *sig = &base->sig;
+
+	EVUTIL_ASSERT(signo > 0 && signo < NSIG);
+	sigev = base->sig.ev_sigevent[signo];
+	EVUTIL_ASSERT(sigev != NULL);
+
+	sigemptyset(&mask);
+	sigaddset(&mask, signo);
+	if (sigprocmask(SIG_UNBLOCK, &mask, NULL)) {
+		event_warn("sigprocmask() failed");
+		return -1;
+	}
+
+	/* Restore previous handler, if any. */
+	if (signo < sig->sh_old_max) {
+		struct sigaction *sa = sig->sh_old[signo];
+		if (sa) {
+			if (sigaction(signo, sa, NULL) == -1) {
+				event_warn("sigaction() failed");
+				return -1;
+			}
+			mm_free(sa);
+			sig->sh_old[signo] = NULL;
+		}
+	}
+
+	sigfd_free_sigevent(base, signo);
+	return 0;
+}
+
+int sigfd_init_(struct event_base *base)
+{
+	EVUTIL_ASSERT(base != NULL);
+	if ((base->flags & EVENT_BASE_FLAG_DISALLOW_SIGNALFD) ||
+	    getenv("EVENT_DISALLOW_SIGNALFD"))
+		return -1;
+	base->evsigsel = &sigfdops;
+	return 0;
+}