From c738d2f7f6935f25f4f96171080f774f172f3e5a Mon Sep 17 00:00:00 2001 From: dtucker Date: Fri, 25 Jun 2021 03:38:17 +0000 Subject: [PATCH] Replace SIGCHLD/notify_pipe kludge with pselect. Previously sshd's SIGCHLD handler would wake up select() by writing a byte to notify_pipe. We can remove this by blocking SIGCHLD, checking for child terminations then passing the original signal mask through to pselect. This ensures that the pselect will immediately wake up if a child terminates between wait()ing on them and the pselect. In -portable, for platforms that do not have pselect the kludge is still there but is hidden behind a pselect interface. Based on other changes for bz#2158, ok djm@ --- usr.bin/ssh/serverloop.c | 108 ++++++++++----------------------------- 1 file changed, 28 insertions(+), 80 deletions(-) diff --git a/usr.bin/ssh/serverloop.c b/usr.bin/ssh/serverloop.c index b1e8db5bb13..dd13b792029 100644 --- a/usr.bin/ssh/serverloop.c +++ b/usr.bin/ssh/serverloop.c @@ -1,4 +1,4 @@ -/* $OpenBSD: serverloop.c,v 1.226 2021/04/03 06:18:41 djm Exp $ */ +/* $OpenBSD: serverloop.c,v 1.227 2021/06/25 03:38:17 dtucker Exp $ */ /* * Author: Tatu Ylonen * Copyright (c) 1995 Tatu Ylonen , Espoo, Finland @@ -84,11 +84,6 @@ extern int use_privsep; static int no_more_sessions = 0; /* Disallow further sessions. */ -/* - * This SIGCHLD kludge is used to detect when the child exits. The server - * will exit after that, as soon as forwarded connections have terminated. - */ - static volatile sig_atomic_t child_terminated = 0; /* The child has terminated. */ /* Cleanup on signals (!use_privsep case only) */ @@ -111,59 +106,11 @@ bind_permitted(int port, uid_t uid) return 1; } -/* - * 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) -{ - if (pipe(notify_pipe) == -1) { - error("pipe(notify_pipe) failed %s", strerror(errno)); - } else if ((fcntl(notify_pipe[0], F_SETFD, FD_CLOEXEC) == -1) || - (fcntl(notify_pipe[1], F_SETFD, FD_CLOEXEC) == -1)) { - error("fcntl(notify_pipe, F_SETFD) failed %s", strerror(errno)); - close(notify_pipe[0]); - close(notify_pipe[1]); - } else { - set_nonblock(notify_pipe[0]); - set_nonblock(notify_pipe[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 sigchld_handler(int sig) { - int save_errno = errno; child_terminated = 1; - notify_parent(); - errno = save_errno; } /*ARGSUSED*/ @@ -207,8 +154,8 @@ client_alive_check(struct ssh *ssh) } /* - * Sleep in select() until we can do something. This will initialize the - * select masks. Upon return, the masks will indicate which descriptors + * Sleep in pselect() until we can do something. This will initialize the + * pselect masks. Upon return, the masks will indicate which descriptors * have data or can accept data. Optionally, a maximum time can be specified * for the duration of the wait (0 = infinite). */ @@ -216,16 +163,16 @@ static void wait_until_can_do_something(struct ssh *ssh, int connection_in, int connection_out, fd_set **readsetp, fd_set **writesetp, int *maxfdp, - u_int *nallocp, u_int64_t max_time_ms) + u_int *nallocp, u_int64_t max_time_ms, sigset_t *sigsetp) { - struct timeval tv, *tvp; + struct timespec ts, *tsp; int ret; time_t minwait_secs = 0; int client_alive_scheduled = 0; /* time we last heard from the client OR sent a keepalive */ static time_t last_client_time; - /* Allocate and update select() masks for channel descriptors. */ + /* Allocate and update pselect() masks for channel descriptors. */ channel_prepare_select(ssh, readsetp, writesetp, maxfdp, nallocp, &minwait_secs); @@ -258,7 +205,6 @@ wait_until_can_do_something(struct ssh *ssh, if (channel_not_very_much_buffered_data()) #endif FD_SET(connection_in, *readsetp); - notify_prepare(*readsetp); /* * If we have buffered packet data going to the client, mark that @@ -276,26 +222,26 @@ wait_until_can_do_something(struct ssh *ssh, max_time_ms = 100; if (max_time_ms == 0) - tvp = NULL; + tsp = NULL; else { - tv.tv_sec = max_time_ms / 1000; - tv.tv_usec = 1000 * (max_time_ms % 1000); - tvp = &tv; + ts.tv_sec = max_time_ms / 1000; + ts.tv_nsec = 1000000 * (max_time_ms % 1000); + tsp = &ts; } /* Wait for something to happen, or the timeout to expire. */ - ret = select((*maxfdp)+1, *readsetp, *writesetp, NULL, tvp); + ret = pselect((*maxfdp)+1, *readsetp, *writesetp, NULL, tsp, sigsetp); if (ret == -1) { memset(*readsetp, 0, *nallocp); memset(*writesetp, 0, *nallocp); if (errno != EINTR) - error("select: %.100s", strerror(errno)); + error("pselect: %.100s", strerror(errno)); } else if (client_alive_scheduled) { time_t now = monotime(); /* - * If the select timed out, or returned for some other reason + * If the pselect timed out, or returned for some other reason * but we haven't heard from the client in time, send keepalive. */ if (ret == 0 || (last_client_time != 0 && last_client_time + @@ -306,8 +252,6 @@ wait_until_can_do_something(struct ssh *ssh, last_client_time = now; } } - - notify_done(*readsetp); } /* @@ -369,13 +313,8 @@ static void collect_children(struct ssh *ssh) { pid_t pid; - sigset_t oset, nset; int status; - /* block SIGCHLD while we check for dead children */ - sigemptyset(&nset); - sigaddset(&nset, SIGCHLD); - sigprocmask(SIG_BLOCK, &nset, &oset); if (child_terminated) { debug("Received SIGCHLD."); while ((pid = waitpid(-1, &status, WNOHANG)) > 0 || @@ -384,7 +323,6 @@ collect_children(struct ssh *ssh) session_close_by_pid(ssh, pid, status); child_terminated = 0; } - sigprocmask(SIG_SETMASK, &oset, NULL); } void @@ -394,9 +332,12 @@ server_loop2(struct ssh *ssh, Authctxt *authctxt) int max_fd; u_int nalloc = 0, connection_in, connection_out; u_int64_t rekey_timeout_ms = 0; + sigset_t bsigset, osigset; debug("Entering interactive session for SSH2."); + if (sigemptyset(&bsigset) == -1 || sigaddset(&bsigset, SIGCHLD) == -1) + error_f("bsigset setup: %s", strerror(errno)); ssh_signal(SIGCHLD, sigchld_handler); child_terminated = 0; connection_in = ssh_packet_get_connection_in(ssh); @@ -408,10 +349,7 @@ server_loop2(struct ssh *ssh, Authctxt *authctxt) ssh_signal(SIGQUIT, sigterm_handler); } - notify_setup(); - max_fd = MAXIMUM(connection_in, connection_out); - max_fd = MAXIMUM(max_fd, notify_pipe[0]); server_init_dispatch(ssh); @@ -429,8 +367,19 @@ server_loop2(struct ssh *ssh, Authctxt *authctxt) rekey_timeout_ms = 0; } + /* + * Block SIGCHLD while we check for dead children, then pass + * the old signal mask through to pselect() so that it'll wake + * up immediately if a child exits after we've called waitpid(). + */ + if (sigprocmask(SIG_BLOCK, &bsigset, &osigset) == -1) + error_f("bsigset sigprocmask: %s", strerror(errno)); + collect_children(ssh); wait_until_can_do_something(ssh, connection_in, connection_out, - &readset, &writeset, &max_fd, &nalloc, rekey_timeout_ms); + &readset, &writeset, &max_fd, &nalloc, rekey_timeout_ms, + &osigset); + if (sigprocmask(SIG_UNBLOCK, &bsigset, &osigset) == -1) + error_f("osigset sigprocmask: %s", strerror(errno)); if (received_sigterm) { logit("Exiting on signal %d", (int)received_sigterm); @@ -438,7 +387,6 @@ server_loop2(struct ssh *ssh, Authctxt *authctxt) cleanup_exit(255); } - collect_children(ssh); if (!ssh_packet_is_rekeying(ssh)) channel_after_select(ssh, readset, writeset); if (process_input(ssh, readset, connection_in) < 0) -- 2.20.1