pid: make ENOMEM return value more obvious

The alloc_pid() codepath used to be simpler. With the introducation of the
ability to choose specific pids in 49cb2fc42ce4 ("fork: extend clone3() to
support setting a PID") it got more complex. It hasn't been super obvious
that ENOMEM is returned when the pid namespace init process/child subreaper
of the pid namespace has died. As can be seen from multiple attempts to
improve this see e.g. [1] and most recently [2].
We regressed returning ENOMEM in [3] and [2] restored it. Let's add a
comment on top explaining that this is historic and documented behavior and
cannot easily be changed.

[1]: 35f71bc0a09a ("fork: report pid reservation failure properly")
[2]: b26ebfe12f34 ("pid: Fix error return value in some cases")
[3]: 49cb2fc42ce4 ("fork: extend clone3() to support setting a PID")
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/kernel/pid.c b/kernel/pid.c
index 19645b25b77c..647b4bb457b5 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -247,6 +247,14 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 		tmp = tmp->parent;
 	}
 
+	/*
+	 * ENOMEM is not the most obvious choice especially for the case
+	 * where the child subreaper has already exited and the pid
+	 * namespace denies the creation of any new processes. But ENOMEM
+	 * is what we have exposed to userspace for a long time and it is
+	 * documented behavior for pid namespaces. So we can't easily
+	 * change it even if there were an error code better suited.
+	 */
 	retval = -ENOMEM;
 
 	if (unlikely(is_child_reaper(pid))) {