xfs: correct null checks and error processing in xfs_initialize_perag

If pag cannot be allocated, the current error exit path will trip
a null pointer deference error when calling xfs_buf_hash_destroy
with a null pag.  Fix this by adding a new error exit labels and
jumping to those accordingly, avoiding the hash destroy and
unnecessary kmem_free on pag.

Up to three things need to be properly unwound:

1) pag memory allocation
2) xfs_buf_hash_init
3) radix_tree_insert

For any given iteration through the loop, any of the above which
succeed must be unwound for /this/ pag, and then all prior
initialized pags must be unwound.

Addresses-Coverity-Id: 1397628 ("Dereference after null check")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_mount.c

@@ -187,7 +187,7 @@ xfs_initialize_perag(
 	xfs_agnumber_t	*maxagi)
 {
 	xfs_agnumber_t	index;
-	xfs_agnumber_t	first_initialised = 0;
+	xfs_agnumber_t	first_initialised = NULLAGNUMBER;
 	xfs_perag_t	*pag;
 	int		error = -ENOMEM;
 
@@ -202,22 +202,20 @@ xfs_initialize_perag(
 			xfs_perag_put(pag);
 			continue;
 		}
-		if (!first_initialised)
-			first_initialised = index;
 
 		pag = kmem_zalloc(sizeof(*pag), KM_MAYFAIL);
 		if (!pag)
-			goto out_unwind;
+			goto out_unwind_new_pags;
 		pag->pag_agno = index;
 		pag->pag_mount = mp;
 		spin_lock_init(&pag->pag_ici_lock);
 		mutex_init(&pag->pag_ici_reclaim_lock);
 		INIT_RADIX_TREE(&pag->pag_ici_root, GFP_ATOMIC);
 		if (xfs_buf_hash_init(pag))
-			goto out_unwind;
+			goto out_free_pag;
 
 		if (radix_tree_preload(GFP_NOFS))
-			goto out_unwind;
+			goto out_hash_destroy;
 
 		spin_lock(&mp->m_perag_lock);
 		if (radix_tree_insert(&mp->m_perag_tree, index, pag)) {
@@ -225,10 +223,13 @@ xfs_initialize_perag(
 			spin_unlock(&mp->m_perag_lock);
 			radix_tree_preload_end();
 			error = -EEXIST;
-			goto out_unwind;
+			goto out_hash_destroy;
 		}
 		spin_unlock(&mp->m_perag_lock);
 		radix_tree_preload_end();
+		/* first new pag is fully initialized */
+		if (first_initialised == NULLAGNUMBER)
+			first_initialised = index;
 	}
 
 	index = xfs_set_inode_alloc(mp, agcount);
@@ -239,11 +240,16 @@ xfs_initialize_perag(
 	mp->m_ag_prealloc_blocks = xfs_prealloc_blocks(mp);
 	return 0;
 
-out_unwind:
+out_hash_destroy:
 	xfs_buf_hash_destroy(pag);
+out_free_pag:
 	kmem_free(pag);
-	for (; index > first_initialised; index--) {
+out_unwind_new_pags:
+	/* unwind any prior newly initialized pags */
+	for (index = first_initialised; index < agcount; index++) {
 		pag = radix_tree_delete(&mp->m_perag_tree, index);
+		if (!pag)
+			break;
 		xfs_buf_hash_destroy(pag);
 		kmem_free(pag);
 	}