ipc/sem: use flexible array in 'struct sem_undo'

Turn 'semadj' in 'struct sem_undo' into a flexible array.

The advantages are:
   - save the size of a pointer when the new undo structure is allocated
   - avoid some always ugly pointer arithmetic to get the address of semadj
   - avoid an indirection when the array is accessed

While at it, use struct_size() to compute the size of the new undo
structure.

Link: https://lkml.kernel.org/r/1ba993d443ad7e16ac2b1902adab1f05ebdfa454.1688918791.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Manfred Spraul <manfred@colorfullife.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Cc: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

ipc/sem.c

@@ -152,7 +152,7 @@ struct sem_undo {
 	struct list_head	list_id;	/* per semaphore array list:
 						 * all undos for one array */
 	int			semid;		/* semaphore set identifier */
-	short			*semadj;	/* array of adjustments */
+	short			semadj[];	/* array of adjustments */
 						/* one per semaphore */
 };
 
@@ -1938,8 +1938,7 @@ static struct sem_undo *find_alloc_undo(struct ipc_namespace *ns, int semid)
 	rcu_read_unlock();
 
 	/* step 2: allocate new undo structure */
-	new = kvzalloc(sizeof(struct sem_undo) + sizeof(short)*nsems,
-		       GFP_KERNEL_ACCOUNT);
+	new = kvzalloc(struct_size(new, semadj, nsems), GFP_KERNEL_ACCOUNT);
 	if (!new) {
 		ipc_rcu_putref(&sma->sem_perm, sem_rcu_free);
 		return ERR_PTR(-ENOMEM);
@@ -1967,7 +1966,6 @@ static struct sem_undo *find_alloc_undo(struct ipc_namespace *ns, int semid)
 		goto success;
 	}
 	/* step 5: initialize & link new undo structure */
-	new->semadj = (short *) &new[1];
 	new->ulp = ulp;
 	new->semid = semid;
 	assert_spin_locked(&ulp->lock);