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>

diff --git a/kernel/pid.c b/kernel/pid.c
index 19645b25b77c2b96efab4b48a0d2ebda51cc4ce9..647b4bb457b5945d16f217ab7ad785cd2b2f84d1 100644 (file)
--- 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))) {