process-util: introduce pid_is_my_child() helper

No functional changes.

Thanks Renaud Métrich for backporting this to RHEL.
Resolves: #1744972

2 files changed, 4 insertions, 11 deletions

diff --git a/src/core/cgroup.c b/src/core/cgroup.c
index 62ab41a288..b7ed07e65b 100644
--- a/src/core/cgroup.c
+++ b/src/core/cgroup.c
@@ -1876,7 +1876,7 @@ void unit_prune_cgroup(Unit *u) {
 
 int unit_search_main_pid(Unit *u, pid_t *ret) {
         _cleanup_fclose_ FILE *f = NULL;
-        pid_t pid = 0, npid, mypid;
+        pid_t pid = 0, npid;
         int r;
 
         assert(u);
@@ -1889,15 +1889,12 @@ int unit_search_main_pid(Unit *u, pid_t *ret) {
         if (r < 0)
                 return r;
 
-        mypid = getpid_cached();
         while (cg_read_pid(f, &npid) > 0)  {
-                pid_t ppid;
 
                 if (npid == pid)
                         continue;
 
-                /* Ignore processes that aren't our kids */
-                if (get_process_ppid(npid, &ppid) >= 0 && ppid != mypid)
+                if (pid_is_my_child(npid) == 0)
                         continue;
 
                 if (pid != 0)
diff --git a/src/core/service.c b/src/core/service.c
index 24f167572a..614ba05d89 100644
--- a/src/core/service.c
+++ b/src/core/service.c
@@ -139,8 +139,6 @@ static void service_unwatch_pid_file(Service *s) {
 }
 
 static int service_set_main_pid(Service *s, pid_t pid) {
-        pid_t ppid;
-
         assert(s);
 
         if (pid <= 1)
@@ -159,12 +157,10 @@ static int service_set_main_pid(Service *s, pid_t pid) {
 
         s->main_pid = pid;
         s->main_pid_known = true;
+        s->main_pid_alien = pid_is_my_child(pid) == 0;
 
-        if (get_process_ppid(pid, &ppid) >= 0 && ppid != getpid_cached()) {
+        if (s->main_pid_alien)
                 log_unit_warning(UNIT(s), "Supervising process "PID_FMT" which is not our child. We'll most likely not notice when it exits.", pid);
-                s->main_pid_alien = true;
-        } else
-                s->main_pid_alien = false;
 
         return 0;
 }