pidfd fixes for v5.2-rc1

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Merge tag 'pidfd-fixes-v5.2-rc1' of gitolite.kernel.org:pub/scm/linux/kernel/git/brauner/linux

Pull pidfd fixes from Christian Brauner:
 "This fixes two bugs:

   - The first one reported by Linus whereby the pidfd-metadata binary
     was not placed in a .gitignore file.

   - The second one is rather urgent and fixes a locking issue found by
     syzkaller.

     What happened is that during process creation we need to check
     whether the cgroup we are in allows us to fork. To perform this
     check the cgroup needs to guard itself against threadgroup changes
     and takes a lock.

     Prior to CLONE_PIDFD the cleanup target "bad_fork_free_pid" would
     also need to release said lock. That's not true anymore since
     CLONE_PIDFD so this is fixed here.

     Syzkaller has tested the patch and was not able to reproduce the
     issue"

* tag 'pidfd-fixes-v5.2-rc1' of gitolite.kernel.org:pub/scm/linux/kernel/git/brauner/linux:
  fork: do not release lock that wasn't taken
  samples: add .gitignore for pidfd-metadata

kernel/fork.c

@@ -2102,7 +2102,7 @@ static __latent_entropy struct task_struct *copy_process(
 	 */
 	retval = cgroup_can_fork(p);
 	if (retval)
-		goto bad_fork_put_pidfd;
+		goto bad_fork_cgroup_threadgroup_change_end;
 
 	/*
 	 * From this point on we must avoid any synchronous user-space
@@ -2217,11 +2217,12 @@ bad_fork_cancel_cgroup:
 	spin_unlock(&current->sighand->siglock);
 	write_unlock_irq(&tasklist_lock);
 	cgroup_cancel_fork(p);
+bad_fork_cgroup_threadgroup_change_end:
+	cgroup_threadgroup_change_end(current);
 bad_fork_put_pidfd:
 	if (clone_flags & CLONE_PIDFD)
 		ksys_close(pidfd);
 bad_fork_free_pid:
-	cgroup_threadgroup_change_end(current);
 	if (pid != &init_struct_pid)
 		free_pid(pid);
 bad_fork_cleanup_thread:

samples/pidfd/.gitignore

@@ -0,0 +1 @@
+pidfd-metadata