pid: make sub-init creation retryable

pid: make sub-init creation retryable

Patch series "pid: make sub-init creation retryable".

This patch (of 2):

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;
	}

Link: https://lkml.kernel.org/r/aaGHu3ixbw9Y7kFj@redhat.com
Link: https://lkml.kernel.org/r/aaGIHa7vGdwhEc_D@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Andrei Vagin <avagin@gmail.com>
Cc: Adrian Reber <areber@redhat.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Kirill Tkhai <tkhai@ya.ru>
Cc: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>