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core: add GetUnitByPIDFD method and use it in systemctl

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A pid can be recycled, but a pidfd is pinned. Add a new method that is safer
as it takes a pidfd as input.
Return not only the D-Bus object path, but also the unit id and the last
recorded invocation id, as they are both useful (especially the id, as
converting from a path object to a unit id from a script requires another
round-trip via D-Bus).

Note that the manager still tracks processes by pid, so theorethically this
is not fully error-proof, but on the other hand the method response is
synchronous and the manager is single-threaded, so once a call is being
processed the unit database will not change anyway. Once the manager
switches to use pidfds everywhere, this can be further hardened.
This commit is contained in:
Luca Boccassi 2023-01-16 23:46:01 +00:00 committed by Lennart Poettering
parent 9ca3bfb6c4
commit e0e7bc8223
5 changed files with 151 additions and 13 deletions
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12
man/org.freedesktop.systemd1.xml
View File

@ -65,6 +65,10 @@ node /org/freedesktop/systemd1 {
out o unit);
GetUnitByControlGroup(in s cgroup,
out o unit);
GetUnitByPIDFD(in h pidfd,
out o unit,
out s unit_id,
out ay invocation_id);
LoadUnit(in s name,
out o unit);
StartUnit(in s name,
@ -796,6 +800,8 @@ node /org/freedesktop/systemd1 {
<variablelist class="dbus-method" generated="True" extra-ref="GetUnitByControlGroup()"/>
<variablelist class="dbus-method" generated="True" extra-ref="GetUnitByPIDFD()"/>
<variablelist class="dbus-method" generated="True" extra-ref="LoadUnit()"/>
<variablelist class="dbus-method" generated="True" extra-ref="StartUnit()"/>
@ -1219,7 +1225,11 @@ node /org/freedesktop/systemd1 {
will fail.</para>
<para><function>GetUnitByPID()</function> may be used to get the unit object path of the unit a process
ID belongs to. It takes a UNIX PID and returns the object path. The PID must refer to an existing system process.</para>
ID belongs to. It takes a UNIX PID and returns the object path. The PID must refer to an existing system process.
<function>GetUnitByPIDFD()</function> may be used to query with a Linux PIDFD (see:
<citerefentry><refentrytitle>pidfd_open</refentrytitle><manvolnum>2</manvolnum></citerefentry>) instead
of a PID, which is safer as UNIX PIDs can be recycled. The latter method returns the unit id and the
invocation id together with the unit object path.</para>
<para><function>LoadUnit()</function> is similar to <function>GetUnit()</function> but will load the
unit from disk if possible.</para>

62
src/core/dbus-manager.c
View File

@ -653,6 +653,63 @@ static int method_get_unit_by_control_group(sd_bus_message *message, void *userd
return reply_unit_path(u, message, error);
}
static int method_get_unit_by_pidfd(sd_bus_message *message, void *userdata, sd_bus_error *error) {
_cleanup_(sd_bus_message_unrefp) sd_bus_message *reply = NULL;
Manager *m = ASSERT_PTR(userdata);
_cleanup_free_ char *path = NULL;
int r, pidfd;
pid_t pid_early, pid_late;
Unit *u;
assert(message);
r = sd_bus_message_read(message, "h", &pidfd);
if (r < 0)
return r;
r = pidfd_get_pid(pidfd, &pid_early);
if (r < 0)
return sd_bus_error_set_errnof(error, r, "Failed to get PID from PIDFD: %m");
u = manager_get_unit_by_pid(m, pid_early);
if (!u)
return sd_bus_error_setf(error, BUS_ERROR_NO_UNIT_FOR_PID, "PID "PID_FMT" does not belong to any loaded unit.", pid_early);
r = mac_selinux_unit_access_check(u, message, "status", error);
if (r < 0)
return r;
path = unit_dbus_path(u);
if (!path)
return log_oom();
r = sd_bus_message_new_method_return(message, &reply);
if (r < 0)
return r;
r = sd_bus_message_append(reply, "os", path, u->id);
if (r < 0)
return r;
r = sd_bus_message_append_array(reply, 'y', u->invocation_id.bytes, sizeof(u->invocation_id.bytes));
if (r < 0)
return r;
/* Double-check that the process is still alive and that the PID did not change before returning the
* answer. */
r = pidfd_get_pid(pidfd, &pid_late);
if (r < 0)
return sd_bus_error_set_errnof(error, r, "Failed to get PID from PIDFD: %m");
if (pid_early != pid_late)
return sd_bus_error_setf(error,
BUS_ERROR_NO_SUCH_PROCESS,
"The PIDFD's PID "PID_FMT" changed to "PID_FMT" during the lookup operation.",
pid_early,
pid_late);
return sd_bus_send(NULL, reply, NULL);
}
static int method_load_unit(sd_bus_message *message, void *userdata, sd_bus_error *error) {
Manager *m = ASSERT_PTR(userdata);
const char *name;
@ -2911,6 +2968,11 @@ const sd_bus_vtable bus_manager_vtable[] = {
SD_BUS_RESULT("o", unit),
method_get_unit_by_control_group,
SD_BUS_VTABLE_UNPRIVILEGED),
SD_BUS_METHOD_WITH_ARGS("GetUnitByPIDFD",
SD_BUS_ARGS("h", pidfd),
SD_BUS_RESULT("o", unit, "s", unit_id, "ay", invocation_id),
method_get_unit_by_pidfd,
SD_BUS_VTABLE_UNPRIVILEGED),
SD_BUS_METHOD_WITH_ARGS("LoadUnit",
SD_BUS_ARGS("s", name),
SD_BUS_RESULT("o", unit),

4
src/core/org.freedesktop.systemd1.conf
View File

@ -64,6 +64,10 @@
send_interface="org.freedesktop.systemd1.Manager"
send_member="GetUnitByControlGroup"/>
<allow send_destination="org.freedesktop.systemd1"
send_interface="org.freedesktop.systemd1.Manager"
send_member="GetUnitByPIDFD"/>
<allow send_destination="org.freedesktop.systemd1"
send_interface="org.freedesktop.systemd1.Manager"
send_member="LoadUnit"/>

85
src/systemctl/systemctl-show.c
View File

@ -13,6 +13,7 @@
#include "errno-util.h"
#include "exec-util.h"
#include "exit-status.h"
#include "fd-util.h"
#include "format-util.h"
#include "hexdecoct.h"
#include "hostname-util.h"
@ -2100,29 +2101,93 @@ static int show_one(
return 0;
}
static int get_unit_dbus_path_by_pid(
static int get_unit_dbus_path_by_pid_fallback(
sd_bus *bus,
uint32_t pid,
char **unit) {
char **ret_path,
char **ret_unit) {
_cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
_cleanup_(sd_bus_message_unrefp) sd_bus_message *reply = NULL;
char *u;
_cleanup_free_ char *path = NULL, *unit = NULL;
char *p;
int r;
assert(bus);
assert(ret_path);
assert(ret_unit);
r = bus_call_method(bus, bus_systemd_mgr, "GetUnitByPID", &error, &reply, "u", pid);
if (r < 0)
return log_error_errno(r, "Failed to get unit for PID %"PRIu32": %s", pid, bus_error_message(&error, r));
r = sd_bus_message_read(reply, "o", &u);
r = sd_bus_message_read(reply, "o", &p);
if (r < 0)
return bus_log_parse_error(r);
u = strdup(u);
if (!u)
path = strdup(p);
if (!path)
return log_oom();
*unit = u;
r = unit_name_from_dbus_path(path, &unit);
if (r < 0)
return log_oom();
*ret_unit = TAKE_PTR(unit);
*ret_path = TAKE_PTR(path);
return 0;
}
static int get_unit_dbus_path_by_pid(
sd_bus *bus,
uint32_t pid,
char **ret_path,
char **ret_unit) {
_cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
_cleanup_(sd_bus_message_unrefp) sd_bus_message *reply = NULL;
_cleanup_free_ char *path = NULL, *unit = NULL;
_cleanup_close_ int pidfd = -EBADFD;
char *p, *u;
int r;
assert(bus);
assert(ret_path);
assert(ret_unit);
/* First, try to send a PIDFD across the wire, so that we can pin the process and there's no race
* condition possible while we wait for the D-Bus reply. If we either don't have PIDFD support in
* the kernel or the new D-Bus method is not available, then fallback to the older method that
* sends the numeric PID. */
pidfd = pidfd_open(pid, 0);
if (pidfd < 0 && ERRNO_IS_NOT_SUPPORTED(errno))
return get_unit_dbus_path_by_pid_fallback(bus, pid, ret_path, ret_unit);
if (pidfd < 0)
return log_error_errno(errno, "Failed to open PID %"PRIu32": %m", pid);
r = bus_call_method(bus, bus_systemd_mgr, "GetUnitByPIDFD", &error, &reply, "h", pidfd);
if (r < 0 && sd_bus_error_has_name(&error, SD_BUS_ERROR_UNKNOWN_METHOD))
return get_unit_dbus_path_by_pid_fallback(bus, pid, ret_path, ret_unit);
if (r < 0)
return log_error_errno(r, "Failed to get unit for PID %"PRIu32": %s", pid, bus_error_message(&error, r));
r = sd_bus_message_read(reply, "os", &p, &u);
if (r < 0)
return bus_log_parse_error(r);
path = strdup(p);
if (!path)
return log_oom();
unit = strdup(u);
if (!unit)
return log_oom();
*ret_unit = TAKE_PTR(unit);
*ret_path = TAKE_PTR(path);
return 0;
}
@ -2297,15 +2362,11 @@ int verb_show(int argc, char *argv[], void *userdata) {
} else {
/* Interpret as PID */
r = get_unit_dbus_path_by_pid(bus, id, &path);
r = get_unit_dbus_path_by_pid(bus, id, &path, &unit);
if (r < 0) {
ret = r;
continue;
}
r = unit_name_from_dbus_path(path, &unit);
if (r < 0)
return log_oom();
}
r = show_one(bus, path, unit, show_mode, &new_line, &ellipsized);

1
test/units/testsuite-26.sh
View File

@ -245,6 +245,7 @@ systemctl status "systemd-*.timer"
systemctl status "systemd-journald*.socket"
systemctl status "sys-devices-*-ttyS0.device"
systemctl status -- -.mount
systemctl status 1
# --marked
systemctl restart "$UNIT_NAME"