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upstream: 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@

OpenBSD-Commit-ID: 202c85de0b3bdf1744fe53529a05404c5480d813
This commit is contained in:
dtucker@openbsd.org 2021-06-25 03:38:17 +00:00 committed by Darren Tucker
parent c9f7bba2e6
commit 4bdf7a0479
1 changed files with 28 additions and 80 deletions
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108
serverloop.c
View File

@ -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 <ylo@cs.hut.fi>
* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@ -88,11 +88,6 @@ 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) */
@ -115,59 +110,11 @@ 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*/
@ -211,8 +158,8 @@ 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).
*/
@ -220,16 +167,16 @@ 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);
@ -262,7 +209,6 @@ 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
@ -280,26 +226,26 @@ 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 +
@ -310,8 +256,6 @@ wait_until_can_do_something(struct ssh *ssh,
last_client_time = now;
}
}
notify_done(*readsetp);
}
/*
@ -374,13 +318,8 @@ 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 ||
@ -389,7 +328,6 @@ collect_children(struct ssh *ssh)
session_close_by_pid(ssh, pid, status);
child_terminated = 0;
}
sigprocmask(SIG_SETMASK, &oset, NULL);
}
void
@ -399,9 +337,12 @@ 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);
@ -413,10 +354,7 @@ 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);
@ -434,8 +372,19 @@ 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);
@ -443,7 +392,6 @@ 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)