sd-login: add new sd_pid_get_cgroup() API

This adds a new sd_pid_get_cgroup() call to sd-login which may be used
to query the control path of a process. This is useful for programs when
making use of delegation units, in order to figure out which subtree has
been delegated.

In light of the unified control group hierarchy this is finally safe to
do, hence let's add a proper API for it, to make it easier to use this.

2 files changed, 45 insertions, 1 deletions

diff --git a/src/libsystemd/sd-login/sd-login.c b/src/libsystemd/sd-login/sd-login.c
index 180bdd09ad..55da26e9d9 100644
--- a/src/libsystemd/sd-login/sd-login.c
+++ b/src/libsystemd/sd-login/sd-login.c
@@ -101,6 +101,32 @@ _public_ int sd_pid_get_owner_uid(pid_t pid, uid_t *uid) {
         return cg_pid_get_owner_uid(pid, uid);
 }
 
+_public_ int sd_pid_get_cgroup(pid_t pid, char **cgroup) {
+        char *c;
+        int r;
+
+        assert_return(pid >= 0, -EINVAL);
+        assert_return(cgroup, -EINVAL);
+
+        r = cg_pid_get_path(SYSTEMD_CGROUP_CONTROLLER, pid, &c);
+        if (r < 0)
+                return r;
+
+        /* The internal APIs return the empty string for the root
+         * cgroup, let's return the "/" in the public APIs instead, as
+         * that's easier and less ambigious for people to grok. */
+        if (isempty(c)) {
+                free(c);
+                c = strdup("/");
+                if (!c)
+                        return -ENOMEM;
+
+        }
+
+        *cgroup = c;
+        return 0;
+}
+
 _public_ int sd_peer_get_session(int fd, char **session) {
         struct ucred ucred = {};
         int r;
@@ -199,6 +225,20 @@ _public_ int sd_peer_get_user_slice(int fd, char **slice) {
         return cg_pid_get_user_slice(ucred.pid, slice);
 }
 
+_public_ int sd_peer_get_cgroup(int fd, char **cgroup) {
+        struct ucred ucred;
+        int r;
+
+        assert_return(fd >= 0, -EBADF);
+        assert_return(cgroup, -EINVAL);
+
+        r = getpeercred(fd, &ucred);
+        if (r < 0)
+                return r;
+
+        return sd_pid_get_cgroup(ucred.pid, cgroup);
+}
+
 static int file_of_uid(uid_t uid, char **p) {
 
         assert_return(uid_is_valid(uid), -EINVAL);
diff --git a/src/libsystemd/sd-login/test-login.c b/src/libsystemd/sd-login/test-login.c
index 70b0345848..f734ce9eee 100644
--- a/src/libsystemd/sd-login/test-login.c
+++ b/src/libsystemd/sd-login/test-login.c
@@ -33,7 +33,7 @@ static void test_login(void) {
         _cleanup_free_ char *pp = NULL, *qq = NULL;
         int r, k;
         uid_t u, u2;
-        char *seat, *type, *class, *display, *remote_user, *remote_host, *display_session;
+        char *seat, *type, *class, *display, *remote_user, *remote_host, *display_session, *cgroup;
         char *session;
         char *state;
         char *session2;
@@ -50,6 +50,10 @@ static void test_login(void) {
         assert_se(sd_pid_get_owner_uid(0, &u2) == 0);
         printf("user = "UID_FMT"\n", u2);
 
+        assert_se(sd_pid_get_cgroup(0, &cgroup) == 0);
+        printf("cgroup = %s\n", cgroup);
+        free(cgroup);
+
         display_session = NULL;
         r = sd_uid_get_display(u2, &display_session);
         assert_se(r >= 0 || r == -ENODATA);