cgroup: Show the absolute path to cgroup.controllers when a controlle…

…r is not available

This patch will provide the absolute path to the cgroup.controllers file
in the error message if the specified controller is not available.
Hopefully this will help with troubleshooting efforts.

Signed-off-by: Aaron Tomlin <[email protected]>

src/libcrun/cgroup-resources.c

@@ -177,8 +177,18 @@ check_cgroup_v2_controller_available_wrapper (int ret, int cgroup_dirfd, const c
         }
       if (! found)
         {
+          cleanup_free char *absolute_path = NULL;
+          libcrun_error_t tmp_err = NULL;
+
           crun_error_release (err);
-          return crun_make_error (err, 0, "the requested cgroup controller `%s` is not available", key);
+          ret = get_realpath_to_file (cgroup_dirfd, "cgroup.controllers", &absolute_path, &tmp_err);
+          if (LIKELY (ret >= 0))
+            ret = crun_make_error (err, 0, "controller `%s` is not available under %s", key, absolute_path);
+          else
+            {
+              crun_error_release (&tmp_err);
+              ret = crun_make_error (err, 0, "the requested cgroup controller `%s` is not available", key);
+            }
         }
     }
   return ret;

src/libcrun/utils.c

@@ -1056,6 +1056,31 @@ read_all_file (const char *path, char **out, size_t *len, libcrun_error_t *err)
   return read_all_file_at (AT_FDCWD, path, out, len, err);
 }
 
+int
+get_realpath_to_file (int dirfd, const char *path_name, char **absolute_path, libcrun_error_t *err)
+{
+  cleanup_close int targetfd = -1;
+
+  targetfd = TEMP_FAILURE_RETRY (openat (dirfd, path_name, O_RDONLY | O_CLOEXEC));
+  if (UNLIKELY (targetfd < 0))
+    return crun_make_error (err, errno, "error opening file `%s`", path_name);
+  else
+    {
+      ssize_t len;
+      proc_fd_path_t target_fd_path;
+
+      get_proc_self_fd_path (target_fd_path, targetfd);
+      len = safe_readlinkat (AT_FDCWD, target_fd_path, absolute_path, 0, err);
+      if (UNLIKELY (len < 0))
+        {
+          crun_error_release (err);
+          return crun_make_error (err, errno, "error unable to provide absolute path to file `%s`", path_name);
+        }
+    }
+
+  return 0;
+}
+
 int
 open_unix_domain_client_socket (const char *path, int dgram, libcrun_error_t *err)
 {

src/libcrun/utils.h

@@ -305,6 +305,8 @@ read_all_fd (int fd, const char *description, char **out, size_t *len, libcrun_e
   return read_all_fd_with_size_hint (fd, description, out, len, 0, err);
 }
 
+int get_realpath_to_file (int dirfd, const char *path_name, char **absolute_path, libcrun_error_t *err);
+
 int read_all_file (const char *path, char **out, size_t *len, libcrun_error_t *err);
 
 int read_all_file_at (int dirfd, const char *path, char **out, size_t *len, libcrun_error_t *err);