ipc: get rid of ids->tables_initialized hack

In sysvipc we have an ids->tables_initialized regarding the rhashtable,
introduced in 0cfb6aee70bd ("ipc: optimize semget/shmget/msgget for lots
of keys")

It's there, specifically, to prevent nil pointer dereferences, from using
an uninitialized api.  Considering how rhashtable_init() can fail
(probably due to ENOMEM, if anything), this made the overall ipc
initialization capable of failure as well.  That alone is ugly, but fine,
however I've spotted a few issues regarding the semantics of
tables_initialized (however unlikely they may be):

- There is inconsistency in what we return to userspace: ipc_addid()
  returns ENOSPC which is certainly _wrong_, while ipc_obtain_object_idr()
  returns EINVAL.

- After we started using rhashtables, ipc_findkey() can return nil upon
  !tables_initialized, but the caller expects nil for when the ipc
  structure isn't found, and can therefore call into ipcget() callbacks.

Now that rhashtable initialization cannot fail, we can properly get rid of
the hack altogether.

[manfred@colorfullife.com: commit id extended to 12 digits]
Link: http://lkml.kernel.org/r/20180712185241.4017-10-manfred@colorfullife.com
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Kees Cook <keescook@chromium.org>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/ipc_namespace.h

@@ -16,7 +16,6 @@ struct user_namespace;
 struct ipc_ids {
 	int in_use;
 	unsigned short seq;
-	bool tables_initialized;
 	struct rw_semaphore rwsem;
 	struct idr ipcs_idr;
 	int max_id;

ipc/util.c

@@ -126,7 +126,6 @@ int ipc_init_ids(struct ipc_ids *ids)
 	if (err)
 		return err;
 	idr_init(&ids->ipcs_idr);
-	ids->tables_initialized = true;
 	ids->max_id = -1;
 #ifdef CONFIG_CHECKPOINT_RESTORE
 	ids->next_id = -1;
@@ -179,19 +178,16 @@ void __init ipc_init_proc_interface(const char *path, const char *header,
  */
 static struct kern_ipc_perm *ipc_findkey(struct ipc_ids *ids, key_t key)
 {
-	struct kern_ipc_perm *ipcp = NULL;
+	struct kern_ipc_perm *ipcp;
 
-	if (likely(ids->tables_initialized))
-		ipcp = rhashtable_lookup_fast(&ids->key_ht, &key,
+	ipcp = rhashtable_lookup_fast(&ids->key_ht, &key,
 					      ipc_kht_params);
+	if (!ipcp)
+		return NULL;
 
-	if (ipcp) {
-		rcu_read_lock();
-		ipc_lock_object(ipcp);
-		return ipcp;
-	}
-
-	return NULL;
+	rcu_read_lock();
+	ipc_lock_object(ipcp);
+	return ipcp;
 }
 
 /*
@@ -269,7 +265,7 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
 	if (limit > IPCMNI)
 		limit = IPCMNI;
 
-	if (!ids->tables_initialized || ids->in_use >= limit)
+	if (ids->in_use >= limit)
 		return -ENOSPC;
 
 	idr_preload(GFP_KERNEL);
@@ -578,9 +574,6 @@ struct kern_ipc_perm *ipc_obtain_object_idr(struct ipc_ids *ids, int id)
 	struct kern_ipc_perm *out;
 	int lid = ipcid_to_idx(id);
 
-	if (unlikely(!ids->tables_initialized))
-		return ERR_PTR(-EINVAL);
-
 	out = idr_find(&ids->ipcs_idr, lid);
 	if (!out)
 		return ERR_PTR(-EINVAL);