rbd: fixes in rbd_header_from_disk()

This fixes a few issues in rbd_header_from_disk():
    - There is a check intended to catch overflow, but it's wrong in
      two ways.
	- First, the type we don't want to overflow is size_t, not
	  unsigned int, and there is now a SIZE_MAX we can use for
	  use with that type.
	- Second, we're allocating the snapshot ids and snapshot
	  image sizes separately (each has type u64; on disk they
          grouped together as a rbd_image_header_ondisk structure).
	  So we can use the size of u64 in this overflow check.
    - If there are no snapshots, then there should be no snapshot
      names.  Enforce this, and issue a warning if we encounter a
      header with no snapshots but a non-zero snap_names_len.
    - When saving the snapshot names into the header, be more direct
      in defining the offset in the on-disk structure from which
      they're being copied by using "snap_count" rather than "i"
      in the array index.
    - If an error occurs, the "snapc" and "snap_names" fields are
      freed at the end of the function.  Make those fields be null
      pointers after they're freed, to be explicit that they are
      no longer valid.
    - Finally, move the definition of the local variable "i" to the
      innermost scope in which it's needed.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -494,14 +494,14 @@ static int rbd_header_from_disk(struct rbd_image_header *header,
 				 struct rbd_image_header_ondisk *ondisk,
 				 u32 allocated_snaps)
 {
-	u32 i, snap_count;
+	u32 snap_count;
 
 	if (!rbd_dev_ondisk_valid(ondisk))
 		return -ENXIO;
 
 	snap_count = le32_to_cpu(ondisk->snap_count);
-	if (snap_count > (UINT_MAX - sizeof(struct ceph_snap_context))
-			 / sizeof (*ondisk))
+	if (snap_count > (SIZE_MAX - sizeof(struct ceph_snap_context))
+				 / sizeof (u64))
 		return -EINVAL;
 	header->snapc = kmalloc(sizeof(struct ceph_snap_context) +
 				snap_count * sizeof(u64),
@@ -509,8 +509,8 @@ static int rbd_header_from_disk(struct rbd_image_header *header,
 	if (!header->snapc)
 		return -ENOMEM;
 
-	header->snap_names_len = le64_to_cpu(ondisk->snap_names_len);
 	if (snap_count) {
+		header->snap_names_len = le64_to_cpu(ondisk->snap_names_len);
 		header->snap_names = kmalloc(header->snap_names_len,
 					     GFP_KERNEL);
 		if (!header->snap_names)
@@ -520,6 +520,8 @@ static int rbd_header_from_disk(struct rbd_image_header *header,
 		if (!header->snap_sizes)
 			goto err_names;
 	} else {
+		WARN_ON(ondisk->snap_names_len);
+		header->snap_names_len = 0;
 		header->snap_names = NULL;
 		header->snap_sizes = NULL;
 	}
@@ -544,6 +546,8 @@ static int rbd_header_from_disk(struct rbd_image_header *header,
 	header->total_snaps = snap_count;
 
 	if (snap_count && allocated_snaps == snap_count) {
+		int i;
+
 		for (i = 0; i < snap_count; i++) {
 			header->snapc->snaps[i] =
 				le64_to_cpu(ondisk->snaps[i].id);
@@ -552,7 +556,7 @@ static int rbd_header_from_disk(struct rbd_image_header *header,
 		}
 
 		/* copy snapshot names */
-		memcpy(header->snap_names, &ondisk->snaps[i],
+		memcpy(header->snap_names, &ondisk->snaps[snap_count],
 			header->snap_names_len);
 	}
 
@@ -560,10 +564,14 @@ static int rbd_header_from_disk(struct rbd_image_header *header,
 
 err_sizes:
 	kfree(header->snap_sizes);
+	header->snap_sizes = NULL;
 err_names:
 	kfree(header->snap_names);
+	header->snap_names = NULL;
 err_snapc:
 	kfree(header->snapc);
+	header->snapc = NULL;
+
 	return -ENOMEM;
 }