pidfs: add pidfs_root_path() helper

Allow to return the root of the global pidfs filesystem.

Link: https://lore.kernel.org/20250624-work-pidfs-fhandle-v2-4-d02a04858fe3@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/internal.h

@@ -353,3 +353,4 @@ int anon_inode_getattr(struct mnt_idmap *idmap, const struct path *path,
 		       unsigned int query_flags);
 int anon_inode_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 		       struct iattr *attr);
+void pidfs_get_root(struct path *path);

fs/pidfs.c

@@ -31,6 +31,14 @@
 static struct kmem_cache *pidfs_attr_cachep __ro_after_init;
 static struct kmem_cache *pidfs_xattr_cachep __ro_after_init;
 
+static struct path pidfs_root_path = {};
+
+void pidfs_get_root(struct path *path)
+{
+	*path = pidfs_root_path;
+	path_get(path);
+}
+
 /*
  * Stashes information that userspace needs to access even after the
  * process has been reaped.
@@ -1068,4 +1076,7 @@ void __init pidfs_init(void)
 	pidfs_mnt = kern_mount(&pidfs_type);
 	if (IS_ERR(pidfs_mnt))
 		panic("Failed to mount pidfs pseudo filesystem");
+
+	pidfs_root_path.mnt = pidfs_mnt;
+	pidfs_root_path.dentry = pidfs_mnt->mnt_root;
 }