pidfs: support FS_IOC_GETVERSION

This will allow 32 bit userspace to detect when a given inode number has
been recycled and also to construct a unique 64 bit identifier.

Link: https://lore.kernel.org/r/20241129-work-pidfs-v2-3-61043d66fbce@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/pidfs.c

@@ -271,6 +271,14 @@ static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	struct ns_common *ns_common = NULL;
 	struct pid_namespace *pid_ns;
 
+	if (cmd == FS_IOC_GETVERSION) {
+		if (!arg)
+			return -EINVAL;
+
+		__u32 __user *argp = (__u32 __user *)arg;
+		return put_user(file_inode(file)->i_generation, argp);
+	}
+
 	task = get_pid_task(pid, PIDTYPE_PID);
 	if (!task)
 		return -ESRCH;