Currently we allow only one attempt to create init in a new namespace.
If the first fork() fails after alloc_pid() succeeds, free_pid() clears
PIDNS_ADDING and thus disables further PID allocations.

Nowadays this looks like an unnecessary limitation. The original reason
to handle "case PIDNS_ADDING" in free_pid() is gone, most probably after
commit 69879c01a0c3 ("proc: Remove the now unnecessary internal mount of
proc").

Change free_pid() to keep ns->pid_allocated == PIDNS_ADDING, and change
alloc_pid() to reset the cursor early, right after taking pidmap_lock.

Test-case:

	#define _GNU_SOURCE
	#include <linux/sched.h>
	#include <sys/syscall.h>
	#include <sys/wait.h>
	#include <assert.h>
	#include <sched.h>
	#include <errno.h>

	int main(void)
	{
		struct clone_args args = {
			.exit_signal = SIGCHLD,
			.flags	= CLONE_PIDFD,
			.pidfd	= 0,
		};
		unsigned long pidfd;
		int pid;

		assert(unshare(CLONE_NEWPID) == 0);

		pid = syscall(__NR_clone3, &args, sizeof(args));
		assert(pid == -1 && errno == EFAULT);

		args.pidfd = (unsigned long)&pidfd;
		pid = syscall(__NR_clone3, &args, sizeof(args));
		if (pid)
			assert(pid > 0 && wait(NULL) == pid);
		else
			assert(getpid() == 1);

		return 0;
	}

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/pid.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index ebf013f35cb3..1a0d2ac1f4a9 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -131,9 +131,8 @@ void free_pid(struct pid *pid)
 			wake_up_process(ns->child_reaper);
 			break;
 		case PIDNS_ADDING:
-			/* Handle a fork failure of the first process */
-			WARN_ON(ns->child_reaper);
-			ns->pid_allocated = 0;
+			/* Only possible if the 1st fork fails */
+			WARN_ON(READ_ONCE(ns->child_reaper));
 			break;
 		}
 
@@ -230,6 +229,10 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid,
 	retried_preload = false;
 	idr_preload(GFP_KERNEL);
 	spin_lock(&pidmap_lock);
+	/* For the case when the previous attempt to create init failed */
+	if (ns->pid_allocated == PIDNS_ADDING)
+		idr_set_cursor(&ns->idr, 0);
+
 	for (tmp = ns, i = ns->level; i >= 0;) {
 		int tid = set_tid[ns->level - i];
 
@@ -341,10 +344,6 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid,
 		idr_remove(&upid->ns->idr, upid->nr);
 	}
 
-	/* On failure to allocate the first pid, reset the state */
-	if (ns->pid_allocated == PIDNS_ADDING)
-		idr_set_cursor(&ns->idr, 0);
-
 	spin_unlock(&pidmap_lock);
 	idr_preload_end();
 
-- 
2.52.0