process-util: explicitly handle processes lacking parents in get_process_ppid()

Let's make sure we signal out-of-band via an error message if a process
doesn't have a parent process whose PID we could return. Otherwise we'll
too likely hide errors, as we return an invalid PID 0, which in other
contexts has special meaning (i.e. usually "myself").

Replaces: #20153

This is based on work by @dtardon, but goes a different route, by
ensuring we propagate a proper error in this case.

This modernizes the function in question a bit in other ways, i.e.
renames stuff and makes the return parameter optional.

3 files changed, 72 insertions, 26 deletions

diff --git a/src/basic/process-util.c b/src/basic/process-util.c
index 20b2441468..14259ea8df 100644
--- a/src/basic/process-util.c
+++ b/src/basic/process-util.c
@@ -643,20 +643,23 @@ int get_process_environ(pid_t pid, char **env) {
         return 0;
 }
 
-int get_process_ppid(pid_t pid, pid_t *_ppid) {
-        int r;
+int get_process_ppid(pid_t pid, pid_t *ret) {
         _cleanup_free_ char *line = NULL;
         long unsigned ppid;
         const char *p;
+        int r;
 
         assert(pid >= 0);
-        assert(_ppid);
 
         if (pid == 0 || pid == getpid_cached()) {
-                *_ppid = getppid();
+                if (ret)
+                        *ret = getppid();
                 return 0;
         }
 
+        if (pid == 1) /* PID 1 has no parent, shortcut this case */
+                return -EADDRNOTAVAIL;
+
         p = procfs_file_alloca(pid, "stat");
         r = read_one_line_file(p, &line);
         if (r == -ENOENT)
@@ -664,9 +667,8 @@ int get_process_ppid(pid_t pid, pid_t *_ppid) {
         if (r < 0)
                 return r;
 
-        /* Let's skip the pid and comm fields. The latter is enclosed
-         * in () but does not escape any () in its value, so let's
-         * skip over it manually */
+        /* Let's skip the pid and comm fields. The latter is enclosed in () but does not escape any () in its
+         * value, so let's skip over it manually */
 
         p = strrchr(line, ')');
         if (!p)
@@ -680,10 +682,17 @@ int get_process_ppid(pid_t pid, pid_t *_ppid) {
                    &ppid) != 1)
                 return -EIO;
 
-        if ((long unsigned) (pid_t) ppid != ppid)
+        /* If ppid is zero the process has no parent. Which might be the case for PID 1 but also for
+         * processes originating in other namespaces that are inserted into a pidns. Return a recognizable
+         * error in this case. */
+        if (ppid == 0)
+                return -EADDRNOTAVAIL;
+
+        if ((pid_t) ppid < 0 || (long unsigned) (pid_t) ppid != ppid)
                 return -ERANGE;
 
-        *_ppid = (pid_t) ppid;
+        if (ret)
+                *ret = (pid_t) ppid;
 
         return 0;
 }
diff --git a/src/coredump/coredump.c b/src/coredump/coredump.c
index c6c232c7e7..444b9ec374 100644
--- a/src/coredump/coredump.c
+++ b/src/coredump/coredump.c
@@ -668,8 +668,7 @@ static int get_process_ns(pid_t pid, const char *namespace, ino_t *ns) {
         return 0;
 }
 
-static int get_mount_namespace_leader(pid_t pid, pid_t *container_pid) {
-        pid_t cpid = pid, ppid = 0;
+static int get_mount_namespace_leader(pid_t pid, pid_t *ret) {
         ino_t proc_mntns;
         int r;
 
@@ -679,8 +678,12 @@ static int get_mount_namespace_leader(pid_t pid, pid_t *container_pid) {
 
         for (;;) {
                 ino_t parent_mntns;
+                pid_t ppid;
 
-                r = get_process_ppid(cpid, &ppid);
+                r = get_process_ppid(pid, &ppid);
+                if (r == -EADDRNOTAVAIL) /* Reached the top (i.e. typically PID 1, but could also be a process
+                                          * whose parent is not in our pidns) */
+                        return -ENOENT;
                 if (r < 0)
                         return r;
 
@@ -688,17 +691,13 @@ static int get_mount_namespace_leader(pid_t pid, pid_t *container_pid) {
                 if (r < 0)
                         return r;
 
-                if (proc_mntns != parent_mntns)
-                        break;
-
-                if (ppid == 1)
-                        return -ENOENT;
+                if (proc_mntns != parent_mntns) {
+                        *ret = ppid;
+                        return 0;
+                }
 
-                cpid = ppid;
+                pid = ppid;
         }
-
-        *container_pid = ppid;
-        return 0;
 }
 
 /* Returns 1 if the parent was found.
diff --git a/src/test/test-process-util.c b/src/test/test-process-util.c
index 576a9a8753..8c76392ae9 100644
--- a/src/test/test-process-util.c
+++ b/src/test/test-process-util.c
@@ -14,9 +14,9 @@
 
 #include "alloc-util.h"
 #include "architecture.h"
-#include "errno-util.h"
-#include "errno-list.h"
 #include "dirent-util.h"
+#include "errno-list.h"
+#include "errno-util.h"
 #include "fd-util.h"
 #include "log.h"
 #include "macro.h"
@@ -24,6 +24,7 @@
 #include "missing_syscall.h"
 #include "parse-util.h"
 #include "process-util.h"
+#include "procfs-util.h"
 #include "rlimit-util.h"
 #include "signal-util.h"
 #include "stdio-util.h"
@@ -65,9 +66,12 @@ static void test_get_process_comm(pid_t pid) {
         assert_se(get_process_cmdline(pid, 1, 0, &d) >= 0);
         log_info("PID"PID_FMT" cmdline truncated to 1: '%s'", pid, d);
 
-        assert_se(get_process_ppid(pid, &e) >= 0);
-        log_info("PID"PID_FMT" PPID: "PID_FMT, pid, e);
-        assert_se(pid == 1 ? e == 0 : e > 0);
+        r = get_process_ppid(pid, &e);
+        assert_se(pid == 1 ? r == -EADDRNOTAVAIL : r >= 0);
+        if (r >= 0) {
+                log_info("PID"PID_FMT" PPID: "PID_FMT, pid, e);
+                assert_se(e > 0);
+        }
 
         assert_se(is_kernel_thread(pid) == 0 || pid != 1);
 
@@ -837,6 +841,39 @@ static void test_setpriority_closest(void) {
         }
 }
 
+static void test_get_process_ppid(void) {
+        uint64_t limit;
+        int r;
+
+        log_info("/* %s */", __func__);
+
+        assert_se(get_process_ppid(1, NULL) == -EADDRNOTAVAIL);
+
+        /* the process with the PID above the global limit definitely doesn't exist. Verify that */
+        assert_se(procfs_tasks_get_limit(&limit) >= 0);
+        assert_se(limit >= INT_MAX || get_process_ppid(limit+1, NULL) == -ESRCH);
+
+        for (pid_t pid = 0;;) {
+                _cleanup_free_ char *c1 = NULL, *c2 = NULL;
+                pid_t ppid;
+
+                r = get_process_ppid(pid, &ppid);
+                if (r == -EADDRNOTAVAIL) {
+                        log_info("No further parent PID");
+                        break;
+                }
+
+                assert_se(r >= 0);
+
+                assert_se(get_process_cmdline(pid, SIZE_MAX, PROCESS_CMDLINE_COMM_FALLBACK, &c1) >= 0);
+                assert_se(get_process_cmdline(ppid, SIZE_MAX, PROCESS_CMDLINE_COMM_FALLBACK, &c2) >= 0);
+
+                log_info("Parent of " PID_FMT " (%s) is " PID_FMT " (%s).", pid, c1, ppid, c2);
+
+                pid = ppid;
+        }
+}
+
 int main(int argc, char *argv[]) {
         log_show_color(true);
         test_setup_logging(LOG_INFO);
@@ -866,6 +903,7 @@ int main(int argc, char *argv[]) {
         test_pid_to_ptr();
         test_ioprio_class_from_to_string();
         test_setpriority_closest();
+        test_get_process_ppid();
 
         return 0;
 }