Add pselect implementation for platforms without.

This is basically the existing notify_pipe kludge from serverloop.c
moved behind a pselect interface.  It works by installing a signal
handler that writes to a pipe that the select is watching, then calls
the original handler.

The select call in serverloop will become pselect soon, at which point the
kludge will be removed from thereand will only exist in the compat layer.
Original code by markus, help from djm.

configure.ac

@@ -1858,6 +1858,7 @@ AC_CHECK_FUNCS([ \
 	pledge \
 	poll \
 	prctl \
+	pselect \
 	pstat \
 	raise \
 	readpassphrase \
@@ -3972,7 +3973,7 @@ AC_CHECK_TYPES([intmax_t, uintmax_t], , , [
 
 TYPE_SOCKLEN_T
 
-AC_CHECK_TYPES([sig_atomic_t], , , [#include <signal.h>])
+AC_CHECK_TYPES([sig_atomic_t, sighandler_t], , , [#include <signal.h>])
 AC_CHECK_TYPES([fsblkcnt_t, fsfilcnt_t], , , [
 #include <sys/types.h>
 #ifdef HAVE_SYS_BITYPES_H

openbsd-compat/Makefile.in

@@ -79,6 +79,7 @@ COMPAT=	arc4random.o \
 	bsd-nextstep.o \
 	bsd-openpty.o \
 	bsd-poll.o \
+	bsd-pselect.o \
 	bsd-setres_id.o \
 	bsd-signal.o \
 	bsd-snprintf.o \

openbsd-compat/bsd-misc.h

@@ -125,6 +125,11 @@ int	isblank(int);
 pid_t getpgid(pid_t);
 #endif
 
+#ifndef HAVE_PSELECT
+int pselect(int, fd_set *, fd_set *, fd_set *, const struct timespec *,
+    const sigset_t *);
+#endif
+
 #ifndef HAVE_ENDGRENT
 # define endgrent() do { } while(0)
 #endif

openbsd-compat/bsd-pselect.c

@@ -0,0 +1,188 @@
+/*
+ * Copyright (c) 2001 Markus Friedl.  All rights reserved.
+ * Copyright (c) 2021 Darren Tucker (dtucker at dtucker net).
+ *
+ * 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.
+ *
+ * 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 "includes.h"
+#ifndef HAVE_PSELECT
+
+#include <sys/types.h>
+#include <sys/time.h>
+#ifdef HAVE_SYS_SELECT_H
+# include <sys/select.h>
+#endif
+
+#include <errno.h>
+#include <fcntl.h>
+#include <signal.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "log.h"
+#include "misc.h"      /* for set_nonblock */
+
+#ifndef HAVE_SIGHANDLER_T
+typedef void (*sighandler_t)(int);
+#endif
+
+static sighandler_t saved_sighandler[_NSIG];
+
+/*
+ * Set up the descriptors.  Because they are close-on-exec, in the case
+ * where sshd's re-exec fails notify_pipe will still point to a descriptor
+ * that was closed by the exec attempt but if that descriptor has been
+ * reopened then we'll attempt to use that.  Ensure that notify_pipe is
+ * outside of the range used by sshd re-exec but within NFDBITS (so we don't
+ * need to expand the fd_sets).
+ */
+#define REEXEC_MIN_FREE_FD (STDERR_FILENO + 4)
+static int
+notify_setup_fd(int *fd)
+{
+       int r;
+
+       if ((r = fcntl(*fd, F_DUPFD, REEXEC_MIN_FREE_FD)) < 0 ||
+           fcntl(r, F_SETFD, FD_CLOEXEC) < 0 || r >= FD_SETSIZE)
+               return -1;
+       (void)close(*fd);
+       return (*fd = r);
+}
+
+/*
+ * we write to this pipe if a SIGCHLD is caught in order to avoid
+ * the race between select() and child_terminated
+ */
+static int notify_pipe[2];
+static void
+notify_setup(void)
+{
+       static int initialized;
+
+       if (initialized)
+               return;
+       if (pipe(notify_pipe) == -1) {
+               error("pipe(notify_pipe) failed %s", strerror(errno));
+       } else if (notify_setup_fd(&notify_pipe[0]) == -1 ||
+           notify_setup_fd(&notify_pipe[1]) == -1) {
+               error("fcntl(notify_pipe, ...) failed %s", strerror(errno));
+               close(notify_pipe[0]);
+               close(notify_pipe[1]);
+       } else {
+               set_nonblock(notify_pipe[0]);
+               set_nonblock(notify_pipe[1]);
+               initialized = 1;
+               return;
+       }
+       notify_pipe[0] = -1;    /* read end */
+       notify_pipe[1] = -1;    /* write end */
+}
+static void
+notify_parent(void)
+{
+       if (notify_pipe[1] != -1)
+               (void)write(notify_pipe[1], "", 1);
+}
+static void
+notify_prepare(fd_set *readset)
+{
+       if (notify_pipe[0] != -1)
+               FD_SET(notify_pipe[0], readset);
+}
+static void
+notify_done(fd_set *readset)
+{
+       char c;
+
+       if (notify_pipe[0] != -1 && FD_ISSET(notify_pipe[0], readset))
+               while (read(notify_pipe[0], &c, 1) != -1)
+                       debug2_f("reading");
+}
+
+/*ARGSUSED*/
+static void
+sig_handler(int sig)
+{
+       int save_errno = errno;
+
+       notify_parent();
+       if (saved_sighandler[sig] != NULL)
+               (*saved_sighandler[sig])(sig);  /* call original handler */
+       errno = save_errno;
+}
+
+/*
+ * A minimal implementation of pselect(2), built on top of select(2).
+ */
+
+int
+pselect(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
+    const struct timespec *timeout, const sigset_t *mask)
+{
+       int ret, sig, saved_errno, unmasked = 0;
+       sigset_t osig;
+       struct sigaction sa, osa;
+       struct timeval tv, *tvp = NULL;
+
+       if (timeout != NULL) {
+               tv.tv_sec = timeout->tv_sec;
+               tv.tv_usec = timeout->tv_nsec / 1000;
+               tvp = &tv;
+       }
+       if (mask == NULL)  /* no signal mask, just call select */
+               return select(nfds, readfds, writefds, exceptfds, tvp);
+
+       /* For each signal we're unmasking, install our handler if needed. */
+       for (sig = 0; sig < _NSIG; sig++) {
+               if (sig == SIGKILL || sig == SIGSTOP || sigismember(mask, sig))
+                       continue;
+               if (sigaction(sig, NULL, &sa) == 0 &&
+                   sa.sa_handler != SIG_IGN && sa.sa_handler != SIG_DFL &&
+                   sa.sa_handler != sig_handler) {
+                       sa.sa_handler = sig_handler;
+                       if (sigaction(sig, &sa, &osa) == 0) {
+                               debug3_f("installing signal handler for %s, "
+                                   "previous %p", strsignal(sig),
+                                    osa.sa_handler);
+                               saved_sighandler[sig] = osa.sa_handler;
+                               unmasked = 1;
+                       }
+               }
+       }
+       if (unmasked) {
+               notify_setup();
+               notify_prepare(readfds);
+               nfds = MAX(nfds, notify_pipe[0]);
+       }
+
+       /* Unmask signals, call select then restore signal mask. */
+       sigprocmask(SIG_SETMASK, mask, &osig);
+       ret = select(nfds, readfds, writefds, exceptfds, tvp);
+       saved_errno = errno;
+       sigprocmask(SIG_SETMASK, &osig, NULL);
+
+       notify_done(readfds);
+       errno = saved_errno;
+       return ret;
+}
+#endif