From 084e7706c25bf6cf0d6af4cc07fb1bb47e26b25e Mon Sep 17 00:00:00 2001
From: Yu Watanabe <watanabe.yu+github@gmail.com>
Date: Sun, 13 Feb 2022 20:39:04 +0900
Subject: [PATCH 1/2] cgroup-util: introduce cg_is_threaded()

---
 src/basic/cgroup-util.c | 24 ++++++++++++++++++++++++
 src/basic/cgroup-util.h |  2 ++
 2 files changed, 26 insertions(+)

diff --git a/src/basic/cgroup-util.c b/src/basic/cgroup-util.c
index 289e4fe073..95bf177a6b 100644
--- a/src/basic/cgroup-util.c
+++ b/src/basic/cgroup-util.c
@@ -1688,6 +1688,30 @@ int cg_slice_to_path(const char *unit, char **ret) {
         return 0;
 }
 
+int cg_is_threaded(const char *controller, const char *path) {
+        _cleanup_free_ char *fs = NULL, *contents = NULL;
+        _cleanup_strv_free_ char **v = NULL;
+        int r;
+
+        r = cg_get_path(controller, path, "cgroup.type", &fs);
+        if (r < 0)
+                return r;
+
+        r = read_full_virtual_file(fs, &contents, NULL);
+        if (r == -ENOENT)
+                return false; /* Assume no. */
+        if (r < 0)
+                return r;
+
+        v = strv_split(contents, NULL);
+        if (!v)
+                return -ENOMEM;
+
+        /* If the cgroup is in the threaded mode, it contains "threaded".
+         * If one of the parents or siblings is in the threaded mode, it may contain "invalid". */
+        return strv_contains(v, "threaded") || strv_contains(v, "invalid");
+}
+
 int cg_set_attribute(const char *controller, const char *path, const char *attribute, const char *value) {
         _cleanup_free_ char *p = NULL;
         int r;
diff --git a/src/basic/cgroup-util.h b/src/basic/cgroup-util.h
index 461c01b3c2..4c413a8d17 100644
--- a/src/basic/cgroup-util.h
+++ b/src/basic/cgroup-util.h
@@ -205,6 +205,8 @@ int cg_pid_get_path(const char *controller, pid_t pid, char **path);
 
 int cg_rmdir(const char *controller, const char *path);
 
+int cg_is_threaded(const char *controller, const char *path);
+
 typedef enum  {
         CG_KEY_MODE_GRACEFUL = 1 << 0,
 } CGroupKeyMode;

From 702cf08fceaa7df8476e56e3648b211e58700bc0 Mon Sep 17 00:00:00 2001
From: Yu Watanabe <watanabe.yu+github@gmail.com>
Date: Sun, 13 Feb 2022 20:52:53 +0900
Subject: [PATCH 2/2] core/execute: warn when threaded mode is detected

Prompted by #22486.
---
 src/core/execute.c        | 6 ++++++
 src/shared/cgroup-setup.c | 3 +++
 2 files changed, 9 insertions(+)

diff --git a/src/core/execute.c b/src/core/execute.c
index 380755eb09..72f3534908 100644
--- a/src/core/execute.c
+++ b/src/core/execute.c
@@ -4250,6 +4250,12 @@ static int exec_child(
                 }
 
                 r = cg_attach_everywhere(params->cgroup_supported, p, 0, NULL, NULL);
+                if (r == -EUCLEAN) {
+                        *exit_status = EXIT_CGROUP;
+                        return log_unit_error_errno(unit, r, "Failed to attach process to cgroup %s "
+                                                    "because the cgroup or one of its parents or "
+                                                    "siblings is in the threaded mode: %m", p);
+                }
                 if (r < 0) {
                         *exit_status = EXIT_CGROUP;
                         return log_unit_error_errno(unit, r, "Failed to attach to cgroup %s: %m", p);
diff --git a/src/shared/cgroup-setup.c b/src/shared/cgroup-setup.c
index 8bda66ca36..38a6755d17 100644
--- a/src/shared/cgroup-setup.c
+++ b/src/shared/cgroup-setup.c
@@ -346,6 +346,9 @@ int cg_attach(const char *controller, const char *path, pid_t pid) {
         xsprintf(c, PID_FMT "\n", pid);
 
         r = write_string_file(fs, c, WRITE_STRING_FILE_DISABLE_BUFFER);
+        if (r == -EOPNOTSUPP && cg_is_threaded(controller, path) > 0)
+                /* When the threaded mode is used, we cannot read/write the file. Let's return recognizable error. */
+                return -EUCLEAN;
         if (r < 0)
                 return r;