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dbus-wait-for.c
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dbus-wait-for.c
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/*
* A wrapper for implementation of D-Bus readiness notification.
*
* The wrapper utilizes double-forking; it first forks, detaches child from
* the terminal, forks again, and exits the sub-parent (daemonizes).
*
* However, the first parent does not just exit; it first reaps the original
* child and execs the given command (with the given arguments). The daemon
* child then waits until the given bus name is taken, and once that happens,
* writes on the readiness descriptor.
*
* The readiness descriptor can be specified either as a number or as an
* environment variable having a number as its value.
*
* Before writing on the descriptor, we need to make sure the name really
* belongs to us. The default approach is to get the PID of the process
* that owns it and check it against the original parent PID; that works
* as long as the tracked process has not forked again and set up D-Bus
* inside. Alternatively, one may use cgroups to check instead, which
* works robustly, but only assuming the service being tracked has the
* cgroup for itself (does not share it) - we can guarantee this under
* specific service hierarchies, but not generically. Only cgroups v2
* are supported.
*
* SPDX-License-Identifier: BSD-2-Clause
*
* Copyright (c) 2023 q66 <q66@chimera-linux.org>
*
* 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 REGENTS AND CONTRIBUTORS ``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 REGENTS OR CONTRIBUTORS 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.
*/
#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif
/* if not disabled, assume enabled */
#ifndef HAVE_CGROUPS
#ifdef __linux__
#define HAVE_CGROUPS 1
#else
#define HAVE_CGROUPS 0
#endif
#endif
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <limits.h>
#include <time.h>
#include <errno.h>
#include <err.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <dbus/dbus.h>
#if HAVE_CGROUPS
#include <sys/stat.h>
#include <sys/vfs.h>
#include <mntent.h>
#endif
#define TIMEOUT_SECS 60
#define DBUS_IFACE "org.freedesktop.DBus"
#define DBUS_PATH "/org/freedesktop/DBus"
#define DBUS_SIGNAL "NameOwnerChanged"
#define DBUS_METHOD_GET_PID "GetConnectionUnixProcessID"
struct BusData {
char const *name;
int fd;
uint32_t parent_pid;
uint32_t pid_serial;
};
#if HAVE_CGROUPS
/* the fs magic is checked once we have a permanent descriptor, to avoid
* any potential race with something after the mounts check (though unlikely)
*/
#ifndef CGROUP2_SUPER_MAGIC
#define CGROUP2_SUPER_MAGIC 0x63677270
#endif
static int cgr_fd = -1;
#endif
static void usage(FILE *f) {
extern char const *__progname;
fprintf(f, "Usage: %s [OPTION]... COMMAND [CMD_OPTION]...\n"
"\n"
"Exec COMMAND, wait (with TIMEOUT) for NAME to appear on bus,\n"
"write readiness in FD or in file descriptor referenced by ENV.\n"
"\n"
"If requested, the program can use system-specific interfaces\n"
"(e.g. cgroups) for tracking forks and ensuring race-free operation.\n"
"\n"
"Besides a system and session bus, the program can also connect to a\n"
"specific address if given; this is purely optional.\n"
"\n"
" -h Print this message and exit.\n"
" -a ADDRESS The address to connect to.\n"
" -e ENV The environment variable with the file descriptor.\n"
" -f FD The file descriptor to write to.\n"
" -n NAME The bus name to wait for.\n"
" -P Try to use cgroups etc. when available.\n"
" -s Use the system bus (session bus is default).\n"
" -t TIMEOUT How long to wait in seconds (default: %d, 0 to disable).\n",
__progname, TIMEOUT_SECS
);
}
static int clockdiff(struct timespec *tp, int *timeout) {
struct timespec tp2;
int msdiff;
if (*timeout < 0) {
return 1;
}
if (!tp->tv_sec && !tp->tv_nsec) {
clock_gettime(CLOCK_MONOTONIC, tp);
return 1;
}
clock_gettime(CLOCK_MONOTONIC, &tp2);
/* deal with seconds */
msdiff = tp2.tv_sec - tp->tv_sec;
msdiff *= 1000;
/* account for ns */
msdiff += (tp2.tv_nsec - tp->tv_nsec) / 1000000;
/* set original time */
*tp = tp2;
if (msdiff >= *timeout) {
return 0;
}
*timeout -= msdiff;
return 1;
}
#if HAVE_CGROUPS
static int cgr_fsopen(void) {
FILE *mnts = setmntent("/proc/self/mounts", "rb");
struct mntent *me;
struct statfs sfs;
char const *mpath = NULL;
int fd;
if (!mnts) {
/* does not exist, so skip cgroups checks */
if (errno == ENOENT) {
return 0;
}
return -1;
}
while ((me = getmntent(mnts))) {
if (!strcmp(me->mnt_type, "cgroup2")) {
mpath = me->mnt_dir;
break;
}
}
if (!mpath) {
endmntent(mnts);
return 0;
}
/* now open the filesystem */
fd = open(mpath, O_DIRECTORY | O_PATH | O_CLOEXEC);
if ((fd < 0) || fstatfs(fd, &sfs)) {
endmntent(mnts);
close(fd);
return -1;
}
if (sfs.f_type != CGROUP2_SUPER_MAGIC) {
endmntent(mnts);
close(fd);
errno = ENOTSUP;
return -1;
}
endmntent(mnts);
return fd;
}
static int cgr_find(int fsfd, uint32_t pid) {
FILE *pf;
char buf[64];
char *line = NULL;
ssize_t ret;
size_t len = 0;
int cfd = 0;
snprintf(buf, sizeof(buf), "/proc/%u/cgroup", pid);
pf = fopen(buf, "rb");
if (!pf) {
if (errno == ENOENT) {
return 0;
}
return -1;
}
/* locate the v2 cgroup */
while ((ret = getline(&line, &len, pf)) >= 0) {
char *nl;
if (strncmp(line, "0::/", 4)) {
/* v1 cgroup */
continue;
}
nl = strchr(line, '\n');
if (nl) {
*nl = '\0';
}
cfd = openat(fsfd, line + 4, O_DIRECTORY | O_PATH | O_CLOEXEC);
if (cfd < 0) {
return -1;
}
break;
}
free(line);
return cfd;
}
static FILE *cgr_procs_fopen(int fd) {
FILE *f;
int pfd = openat(fd, "cgroup.procs", O_RDONLY | O_NOFOLLOW);
if (pfd < 0) {
return NULL;
}
f = fdopen(pfd, "rb");
if (!f) {
int serr = errno;
close(pfd);
errno = serr;
return NULL;
}
return f;
}
/* cgroups with processes must be leaf, so no recursion
*
* returns 0 if pid is not contained (regardless of number of procs),
* 1 if only pid is contained, and 2 if pid + other(s) is/are contained
*/
static int cgr_check(FILE *f, uint32_t pid) {
char buf[32], buf2[32];
char *ln;
size_t plen;
int oth = 0;
if (!f) {
return 0;
}
snprintf(buf2, sizeof(buf2), "%u", pid);
plen = strlen(buf2);
while ((ln = fgets(buf, sizeof(buf), f))) {
if (strncmp(ln, buf2, plen)) {
oth = 1;
continue;
}
if (!ln[plen] || (ln[plen] == '\n')) {
fclose(f);
return oth + 1;
}
oth = 1;
}
fclose(f);
return 0;
}
#endif
static DBusHandlerResult handle_pid_cb(DBusMessage *msg, struct BusData *bd) {
uint32_t pid;
/* make sure it matches the method we called, otherwise ignore */
if (dbus_message_get_reply_serial(msg) != bd->pid_serial) {
return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}
/* reset just in case, there should be only one reply matching the serial */
bd->pid_serial = 0;
if (!dbus_message_get_args(
msg,
NULL,
DBUS_TYPE_UINT32, &pid,
DBUS_TYPE_INVALID
)) {
return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}
#if HAVE_CGROUPS
/* prefer a cgroup check if we can do it; it's less prone to pid reuse
* (in case of a spurious pid replacement, the new pid would have to
* belong to the same cgroup, which reduces potential harm)
*/
if (cgr_fd >= 0) {
/* check the pid belongs to the same cgroup as the parent */
FILE *f = cgr_procs_fopen(cgr_fd);
if (!f) {
return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}
if (cgr_check(f, pid)) {
fclose(f);
goto handle;
}
fclose(f);
} else
#endif
if (pid == bd->parent_pid) {
goto handle;
}
/* different process claimed it */
return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
handle:
/* ours */
write(bd->fd, "READY=1\n", sizeof("READY=1"));
/* we'll quit */
close(bd->fd);
bd->fd = -1;
return DBUS_HANDLER_RESULT_HANDLED;
}
static DBusHandlerResult filter_cb(
DBusConnection *conn, DBusMessage *msg, void *data
) {
char const *name;
char const *old_owner;
char const *new_owner;
struct BusData *bd = data;
DBusMessage *nmsg;
/* do not handle if we've already done that */
if (bd->fd < 0) {
return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}
/* the method is checked inside */
if (dbus_message_get_type(msg) == DBUS_MESSAGE_TYPE_METHOD_RETURN) {
return handle_pid_cb(msg, bd);
}
/* ignore messages that don't concern us */
if (!dbus_message_is_signal(msg, DBUS_IFACE, DBUS_SIGNAL)) {
return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}
/* if that fails, ignore */
if (!dbus_message_get_args(
msg, NULL,
DBUS_TYPE_STRING, &name,
DBUS_TYPE_STRING, &old_owner,
DBUS_TYPE_STRING, &new_owner,
DBUS_TYPE_INVALID
)) {
return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}
/* if not the one we're tracking, ignore */
if (strcmp(name, bd->name)) {
return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}
/* reset the reply id in case of a subsequent loss */
bd->pid_serial = 0;
/* in case of loss, ignore */
if (!strcmp(new_owner, "")) {
return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}
/* getting the pid of the new owner involves a method call; the logic
* here ensures that if something else gains the name in the meantime,
* any result received from this will not be used (as the reply serial
* is reset earlier)
*/
nmsg = dbus_message_new_method_call(
DBUS_IFACE,
DBUS_PATH,
DBUS_IFACE,
DBUS_METHOD_GET_PID
);
/* could not perform the method call */
if (!nmsg) {
return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}
/* the method takes a string name and returns a uint32_t */
if (!dbus_message_append_args(
nmsg,
DBUS_TYPE_STRING, &new_owner,
DBUS_TYPE_INVALID
)) {
dbus_message_unref(nmsg);
return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}
/* do the call asynchronously, receive reply later in the mainloop */
if (!dbus_connection_send(conn, nmsg, NULL)) {
/* could not send */
dbus_message_unref(nmsg);
return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
}
/* update reply serial; that means we are currently handling the bus
* name for a specific pid (if we lose it before the response could be
* received, it will get reset)
*/
bd->pid_serial = dbus_message_get_serial(nmsg);
dbus_message_unref(nmsg);
/* we've handled it */
return DBUS_HANDLER_RESULT_HANDLED;
}
static int get_fd(char const *str) {
char *end = NULL;
unsigned long fd;
if (!str || !*str) {
return -1;
}
fd = strtoul(str, &end, 10);
if (fd && end && !*end && (fd <= INT_MAX)) {
int tfd = (int)fd;
if (fcntl(tfd, F_GETFD) >= 0) {
if (fcntl(fd, F_SETFD, FD_CLOEXEC) < 0) {
close(tfd);
return -1;
}
return tfd;
}
}
return -1;
}
int main(int argc, char **argv) {
DBusBusType bt = DBUS_BUS_SESSION;
DBusError derr;
DBusConnection *conn;
char const *addr = NULL;
struct BusData bd;
unsigned long timeout_s = TIMEOUT_SECS;
int c, timeout;
#if HAVE_CGROUPS
int do_cgr_check = 0;
#endif
pid_t p;
bd.fd = -1;
bd.name = NULL;
bd.pid_serial = 0;
bd.parent_pid = getpid();
while ((c = getopt(argc, argv, "a:e:f:hn:Pst:")) > 0) {
switch (c) {
case 'h':
usage(stdout);
return 0;
case 'a':
addr = optarg;
break;
case 'e':
bd.fd = get_fd(getenv(optarg));
goto err_fd;
case 'f':
bd.fd = get_fd(optarg);
err_fd:
if (bd.fd <= 0) {
errx(1, "invalid file descriptor given");
}
break;
case 'n':
bd.name = optarg;
if (!*bd.name) {
errx(1, "invalid bus name given");
}
break;
case 'P':
#if HAVE_CGROUPS
do_cgr_check = 1;
#endif
break;
case 's':
bt = DBUS_BUS_SYSTEM;
break;
case 't': {
char *end = NULL;
timeout_s = strtoul(optarg, &end, 10);
if (!end || *end || timeout_s > (INT_MAX / 1000)) {
errx(1, "invalid timeout given");
}
break;
}
default:
fprintf(stderr, "\n");
usage(stderr);
return 1;
}
}
/* convert to milliseconds */
timeout = timeout_s * 1000;
if (!timeout) {
/* for dbus */
timeout = -1;
}
if (bd.fd < 0) {
warnx("no file descriptor given\n");
usage(stderr);
return 1;
}
if (!bd.name) {
warnx("no bus name given\n");
usage(stderr);
return 1;
}
/* establish as much as we can early on to reduce error handling
* after fork (and increase robustness of the while thing)
*/
dbus_error_init(&derr);
if (addr) {
conn = dbus_connection_open(addr, &derr);
} else {
conn = dbus_bus_get(bt, &derr);
}
if (!conn) {
errx(1, "connection error (%s)", derr.message);
}
dbus_connection_set_exit_on_disconnect(conn, FALSE);
dbus_bus_add_match(
conn,
"type='signal',"
"sender='" DBUS_IFACE "',"
"interface='" DBUS_IFACE "',"
"member='" DBUS_SIGNAL "'",
&derr
);
if (dbus_error_is_set(&derr)) {
errx(1, "failed to register match rule (%s)", derr.message);
}
if (!dbus_connection_add_filter(conn, filter_cb, &bd, NULL)) {
errx(1, "failed to register dbus filter");
}
#if HAVE_CGROUPS
/* when tracking cgroups, check what the parent belongs to first */
if (do_cgr_check) {
int cfd = -1;
/* now open the filesystem */
cgr_fd = cgr_fsopen();
if (!cgr_fd) {
goto no_cgr;
}
if (cgr_fd < 0) {
err(1, "could not open cgroup2 filesystem");
}
/* ensure the process belongs to a single cgroup */
cfd = cgr_find(cgr_fd, bd.parent_pid);
if (!cfd) {
close(cgr_fd);
cgr_fd = -1;
goto no_cgr;
}
if (cfd < 0) {
err(1, "could not obtain cgroup for %u", bd.parent_pid);
}
close(cgr_fd);
cgr_fd = cfd;
}
no_cgr:
#endif
p = fork();
if (p < 0) {
err(1, "fork failed");
}
if (p == 0) {
struct timespec tp = {0};
/* child */
setsid();
p = fork();
if (p < 0) {
err(1, "fork failed");
}
if (p) {
/* the "parent" just exits */
return 0;
}
/* if something resets the fd, it's a signal to exit */
while ((bd.fd >= 0) && clockdiff(&tp, &timeout)) {
/* dispatch the dbus connection once */
if (!dbus_connection_read_write_dispatch(conn, timeout)) {
break;
}
}
dbus_connection_unref(conn);
return 0;
}
/* unref the connection */
dbus_connection_unref(conn);
/* original parent; reap forked child first */
while (waitpid(p, NULL, 0) < 0) {
if (errno != EINTR) {
break;
}
}
/* and exec into the intended process */
execvp(argv[optind], &argv[optind]);
return 1;
}