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Systemd/src/shared/ptyfwd.c
Lennart Poettering ae3dde8012 machinectl: fix race when opening new shells with "machinectl shell" 
Previously, we'd allocate the TTY, spawn a service on it, but
immediately start processing the TTY and forwarding it to whatever the
commnd was started on. This is however problematic, as the TTY might get
actually opened only much later by the service. We'll hence first get
EIOs on the master as the other side is still closed, and hence
considered it hung up and terminated the session.

With this change we add a flag to the pty forwarding logic:
PTY_FORWARD_IGNORE_INITIAL_VHANGUP. If set, we'll ignore all hangups
(i.e. EIOs) on the master PTY until the first byte is successfully read.
From that point on we consider a hangup/EIO a regular connection termination. This
way, we handle the race: when we get EIO initially we'll ignore it,
until the connection is properly set up, at which time we start
honouring it.
2015-10-07 20:10:48 +02:00

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/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
/***
This file is part of systemd.
Copyright 2010-2013 Lennart Poettering
systemd 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.
systemd 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 systemd; If not, see <http://www.gnu.org/licenses/>.
***/
#include <sys/epoll.h>
#include <sys/ioctl.h>
#include <limits.h>
#include <termios.h>
#include "util.h"
#include "ptyfwd.h"
struct PTYForward {
sd_event *event;
int master;
PTYForwardFlags flags;
sd_event_source *stdin_event_source;
sd_event_source *stdout_event_source;
sd_event_source *master_event_source;
sd_event_source *sigwinch_event_source;
struct termios saved_stdin_attr;
struct termios saved_stdout_attr;
bool saved_stdin:1;
bool saved_stdout:1;
bool stdin_readable:1;
bool stdin_hangup:1;
bool stdout_writable:1;
bool stdout_hangup:1;
bool master_readable:1;
bool master_writable:1;
bool master_hangup:1;
bool read_from_master:1;
bool last_char_set:1;
char last_char;
char in_buffer[LINE_MAX], out_buffer[LINE_MAX];
size_t in_buffer_full, out_buffer_full;
usec_t escape_timestamp;
unsigned escape_counter;
};
#define ESCAPE_USEC (1*USEC_PER_SEC)
static bool look_for_escape(PTYForward *f, const char *buffer, size_t n) {
const char *p;
assert(f);
assert(buffer);
assert(n > 0);
for (p = buffer; p < buffer + n; p++) {
/* Check for ^] */
if (*p == 0x1D) {
usec_t nw = now(CLOCK_MONOTONIC);
if (f->escape_counter == 0 || nw > f->escape_timestamp + ESCAPE_USEC) {
f->escape_timestamp = nw;
f->escape_counter = 1;
} else {
(f->escape_counter)++;
if (f->escape_counter >= 3)
return true;
}
} else {
f->escape_timestamp = 0;
f->escape_counter = 0;
}
}
return false;
}
static bool ignore_vhangup(PTYForward *f) {
assert(f);
if (f->flags & PTY_FORWARD_IGNORE_VHANGUP)
return true;
if ((f->flags & PTY_FORWARD_IGNORE_INITIAL_VHANGUP) && !f->read_from_master)
return true;
return false;
}
static int shovel(PTYForward *f) {
ssize_t k;
assert(f);
while ((f->stdin_readable && f->in_buffer_full <= 0) ||
(f->master_writable && f->in_buffer_full > 0) ||
(f->master_readable && f->out_buffer_full <= 0) ||
(f->stdout_writable && f->out_buffer_full > 0)) {
if (f->stdin_readable && f->in_buffer_full < LINE_MAX) {
k = read(STDIN_FILENO, f->in_buffer + f->in_buffer_full, LINE_MAX - f->in_buffer_full);
if (k < 0) {
if (errno == EAGAIN)
f->stdin_readable = false;
else if (errno == EIO || errno == EPIPE || errno == ECONNRESET) {
f->stdin_readable = false;
f->stdin_hangup = true;
f->stdin_event_source = sd_event_source_unref(f->stdin_event_source);
} else {
log_error_errno(errno, "read(): %m");
return sd_event_exit(f->event, EXIT_FAILURE);
}
} else if (k == 0) {
/* EOF on stdin */
f->stdin_readable = false;
f->stdin_hangup = true;
f->stdin_event_source = sd_event_source_unref(f->stdin_event_source);
} else {
/* Check if ^] has been
* pressed three times within
* one second. If we get this
* we quite immediately. */
if (look_for_escape(f, f->in_buffer + f->in_buffer_full, k))
return sd_event_exit(f->event, EXIT_FAILURE);
f->in_buffer_full += (size_t) k;
}
}
if (f->master_writable && f->in_buffer_full > 0) {
k = write(f->master, f->in_buffer, f->in_buffer_full);
if (k < 0) {
if (errno == EAGAIN || errno == EIO)
f->master_writable = false;
else if (errno == EPIPE || errno == ECONNRESET) {
f->master_writable = f->master_readable = false;
f->master_hangup = true;
f->master_event_source = sd_event_source_unref(f->master_event_source);
} else {
log_error_errno(errno, "write(): %m");
return sd_event_exit(f->event, EXIT_FAILURE);
}
} else {
assert(f->in_buffer_full >= (size_t) k);
memmove(f->in_buffer, f->in_buffer + k, f->in_buffer_full - k);
f->in_buffer_full -= k;
}
}
if (f->master_readable && f->out_buffer_full < LINE_MAX) {
k = read(f->master, f->out_buffer + f->out_buffer_full, LINE_MAX - f->out_buffer_full);
if (k < 0) {
/* Note that EIO on the master device
* might be caused by vhangup() or
* temporary closing of everything on
* the other side, we treat it like
* EAGAIN here and try again, unless
* ignore_vhangup is off. */
if (errno == EAGAIN || (errno == EIO && ignore_vhangup(f)))
f->master_readable = false;
else if (errno == EPIPE || errno == ECONNRESET || errno == EIO) {
f->master_readable = f->master_writable = false;
f->master_hangup = true;
f->master_event_source = sd_event_source_unref(f->master_event_source);
} else {
log_error_errno(errno, "read(): %m");
return sd_event_exit(f->event, EXIT_FAILURE);
}
} else {
f->read_from_master = true;
f->out_buffer_full += (size_t) k;
}
}
if (f->stdout_writable && f->out_buffer_full > 0) {
k = write(STDOUT_FILENO, f->out_buffer, f->out_buffer_full);
if (k < 0) {
if (errno == EAGAIN)
f->stdout_writable = false;
else if (errno == EIO || errno == EPIPE || errno == ECONNRESET) {
f->stdout_writable = false;
f->stdout_hangup = true;
f->stdout_event_source = sd_event_source_unref(f->stdout_event_source);
} else {
log_error_errno(errno, "write(): %m");
return sd_event_exit(f->event, EXIT_FAILURE);
}
} else {
if (k > 0) {
f->last_char = f->out_buffer[k-1];
f->last_char_set = true;
}
assert(f->out_buffer_full >= (size_t) k);
memmove(f->out_buffer, f->out_buffer + k, f->out_buffer_full - k);
f->out_buffer_full -= k;
}
}
}
if (f->stdin_hangup || f->stdout_hangup || f->master_hangup) {
/* Exit the loop if any side hung up and if there's
* nothing more to write or nothing we could write. */
if ((f->out_buffer_full <= 0 || f->stdout_hangup) &&
(f->in_buffer_full <= 0 || f->master_hangup))
return sd_event_exit(f->event, EXIT_SUCCESS);
}
return 0;
}
static int on_master_event(sd_event_source *e, int fd, uint32_t revents, void *userdata) {
PTYForward *f = userdata;
assert(f);
assert(e);
assert(e == f->master_event_source);
assert(fd >= 0);
assert(fd == f->master);
if (revents & (EPOLLIN|EPOLLHUP))
f->master_readable = true;
if (revents & (EPOLLOUT|EPOLLHUP))
f->master_writable = true;
return shovel(f);
}
static int on_stdin_event(sd_event_source *e, int fd, uint32_t revents, void *userdata) {
PTYForward *f = userdata;
assert(f);
assert(e);
assert(e == f->stdin_event_source);
assert(fd >= 0);
assert(fd == STDIN_FILENO);
if (revents & (EPOLLIN|EPOLLHUP))
f->stdin_readable = true;
return shovel(f);
}
static int on_stdout_event(sd_event_source *e, int fd, uint32_t revents, void *userdata) {
PTYForward *f = userdata;
assert(f);
assert(e);
assert(e == f->stdout_event_source);
assert(fd >= 0);
assert(fd == STDOUT_FILENO);
if (revents & (EPOLLOUT|EPOLLHUP))
f->stdout_writable = true;
return shovel(f);
}
static int on_sigwinch_event(sd_event_source *e, const struct signalfd_siginfo *si, void *userdata) {
PTYForward *f = userdata;
struct winsize ws;
assert(f);
assert(e);
assert(e == f->sigwinch_event_source);
/* The window size changed, let's forward that. */
if (ioctl(STDOUT_FILENO, TIOCGWINSZ, &ws) >= 0)
(void) ioctl(f->master, TIOCSWINSZ, &ws);
return 0;
}
int pty_forward_new(
sd_event *event,
int master,
PTYForwardFlags flags,
PTYForward **ret) {
_cleanup_(pty_forward_freep) PTYForward *f = NULL;
struct winsize ws;
int r;
f = new0(PTYForward, 1);
if (!f)
return -ENOMEM;
f->flags = flags;
if (event)
f->event = sd_event_ref(event);
else {
r = sd_event_default(&f->event);
if (r < 0)
return r;
}
if (!(flags & PTY_FORWARD_READ_ONLY)) {
r = fd_nonblock(STDIN_FILENO, true);
if (r < 0)
return r;
r = fd_nonblock(STDOUT_FILENO, true);
if (r < 0)
return r;
}
r = fd_nonblock(master, true);
if (r < 0)
return r;
f->master = master;
if (ioctl(STDOUT_FILENO, TIOCGWINSZ, &ws) >= 0)
(void) ioctl(master, TIOCSWINSZ, &ws);
if (!(flags & PTY_FORWARD_READ_ONLY)) {
if (tcgetattr(STDIN_FILENO, &f->saved_stdin_attr) >= 0) {
struct termios raw_stdin_attr;
f->saved_stdin = true;
raw_stdin_attr = f->saved_stdin_attr;
cfmakeraw(&raw_stdin_attr);
raw_stdin_attr.c_oflag = f->saved_stdin_attr.c_oflag;
tcsetattr(STDIN_FILENO, TCSANOW, &raw_stdin_attr);
}
if (tcgetattr(STDOUT_FILENO, &f->saved_stdout_attr) >= 0) {
struct termios raw_stdout_attr;
f->saved_stdout = true;
raw_stdout_attr = f->saved_stdout_attr;
cfmakeraw(&raw_stdout_attr);
raw_stdout_attr.c_iflag = f->saved_stdout_attr.c_iflag;
raw_stdout_attr.c_lflag = f->saved_stdout_attr.c_lflag;
tcsetattr(STDOUT_FILENO, TCSANOW, &raw_stdout_attr);
}
r = sd_event_add_io(f->event, &f->stdin_event_source, STDIN_FILENO, EPOLLIN|EPOLLET, on_stdin_event, f);
if (r < 0 && r != -EPERM)
return r;
}
r = sd_event_add_io(f->event, &f->stdout_event_source, STDOUT_FILENO, EPOLLOUT|EPOLLET, on_stdout_event, f);
if (r == -EPERM)
/* stdout without epoll support. Likely redirected to regular file. */
f->stdout_writable = true;
else if (r < 0)
return r;
r = sd_event_add_io(f->event, &f->master_event_source, master, EPOLLIN|EPOLLOUT|EPOLLET, on_master_event, f);
if (r < 0)
return r;
r = sd_event_add_signal(f->event, &f->sigwinch_event_source, SIGWINCH, on_sigwinch_event, f);
if (r < 0)
return r;
*ret = f;
f = NULL;
return 0;
}
PTYForward *pty_forward_free(PTYForward *f) {
if (f) {
sd_event_source_unref(f->stdin_event_source);
sd_event_source_unref(f->stdout_event_source);
sd_event_source_unref(f->master_event_source);
sd_event_unref(f->event);
if (f->saved_stdout)
tcsetattr(STDOUT_FILENO, TCSANOW, &f->saved_stdout_attr);
if (f->saved_stdin)
tcsetattr(STDIN_FILENO, TCSANOW, &f->saved_stdin_attr);
free(f);
}
/* STDIN/STDOUT should not be nonblocking normally, so let's
* unconditionally reset it */
fd_nonblock(STDIN_FILENO, false);
fd_nonblock(STDOUT_FILENO, false);
return NULL;
}
int pty_forward_get_last_char(PTYForward *f, char *ch) {
assert(f);
assert(ch);
if (!f->last_char_set)
return -ENXIO;
*ch = f->last_char;
return 0;
}
int pty_forward_set_ignore_vhangup(PTYForward *f, bool b) {
int r;
assert(f);
if (!!(f->flags & PTY_FORWARD_IGNORE_VHANGUP) == b)
return 0;
if (b)
f->flags |= PTY_FORWARD_IGNORE_VHANGUP;
else
f->flags &= ~PTY_FORWARD_IGNORE_VHANGUP;
if (!ignore_vhangup(f)) {
/* We shall now react to vhangup()s? Let's check
* immediately if we might be in one */
f->master_readable = true;
r = shovel(f);
if (r < 0)
return r;
}
return 0;
}
int pty_forward_get_ignore_vhangup(PTYForward *f) {
assert(f);
return !!(f->flags & PTY_FORWARD_IGNORE_VHANGUP);
}
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