btrfs-progs: image: fix the bug that filename sanitization not working

[BUG]
There is a bug report that image dump taken by "btrfs-image -s" doesn't
really sanitize the filenames:

  # truncates -s 1G source.raw
  # mkfs.btrfs -f source.raw
  # mount source.raw $mnt
  # touch $mnt/top_secret_filename
  # touch $mnt/secret_filename
  # umount $mnt
  # btrfs-image -s source.raw dump.img
  # string dump.img | grep filename
  top_secret_filename
  secret_filename
  top_secret_filename
  secret_filename
  top_secret_filename

[CAUSE]
Using above image to store the fs, and we got the following result in fs
tree:

	item 0 key (256 INODE_ITEM 0) itemoff 16123 itemsize 160
		generation 3 transid 7 size 68 nbytes 16384
		block group 0 mode 40755 links 1 uid 0 gid 0 rdev 0
		sequence 2 flags 0x0(none)
	item 1 key (256 INODE_REF 256) itemoff 16111 itemsize 12
		index 0 namelen 2 name: ..
	item 2 key (256 DIR_ITEM 439756795) itemoff 16062 itemsize 49
		location key (257 INODE_ITEM 0) type FILE
		transid 7 data_len 0 name_len 19
		name: top_secret_filename
	item 3 key (256 DIR_ITEM 693462946) itemoff 16017 itemsize 45
		location key (258 INODE_ITEM 0) type FILE
		transid 7 data_len 0 name_len 15
		name: secret_filename
	item 4 key (256 DIR_INDEX 2) itemoff 15968 itemsize 49
		location key (257 INODE_ITEM 0) type FILE
		transid 7 data_len 0 name_len 19
		name: top_secret_filename
	item 5 key (256 DIR_INDEX 3) itemoff 15923 itemsize 45
		location key (258 INODE_ITEM 0) type FILE
		transid 7 data_len 0 name_len 15
		name: secret_filename
	item 6 key (257 INODE_ITEM 0) itemoff 15763 itemsize 160
		generation 7 transid 7 size 0 nbytes 0
		block group 0 mode 100644 links 1 uid 0 gid 0 rdev 0
		sequence 1 flags 0x0(none)
	item 7 key (257 INODE_REF 256) itemoff 15734 itemsize 29
		index 2 namelen 19 name: top_secret_filename
	item 8 key (258 INODE_ITEM 0) itemoff 15574 itemsize 160
		generation 7 transid 7 size 0 nbytes 0
		block group 0 mode 100644 links 1 uid 0 gid 0 rdev 0
		sequence 1 flags 0x0(none)
	item 9 key (258 INODE_REF 256) itemoff 15549 itemsize 25
		index 3 namelen 15 name: 1���'�gc*&R

The result shows, only the last INODE_REF got sanitized, all the
remaining are not touched at all.

This is caused by how we sanitize the filenames:

 copy_buffer()
 |- memcpy(dst, src->data, src->len);
 |  This means we copy the whole eb into our buffer already.
 |
 |- zero_items()
    |- sanitize_name()
       |- eb = alloc_dummy_eb();
       |- memcpy(eb->data, src->data, src->len);
       |  This means we generate a dummy eb with the same contents of
       |  the source eb.
       |
       |- sanitize_dir_item();
       |  We override the dir item of the given item (specified by the
       |  slot number) inside our dummy eb.
       |
       |- memcpy(dst, eb->data, eb->lem);

The last one copy the dummy eb into our buffer, with only the slot
corrupted.

But when the whole work flow hits the next slot, we only corrupt the
next slot, but still copy the whole dummy eb back to buffer.

This means the previous slot would be overwritten by the old unsanitized
data.

Resulting only the last slot is corrupted.

[FIX]
Fix the bug by only copying back the corrupted item to the buffer.
So that other slots won't be overwritten by unsanitized data.

Reported-by: Andrea Gelmini <andrea.gelmini@gmail.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>

image/sanitize.c

@@ -449,6 +449,8 @@ void sanitize_name(enum sanitize_mode sanitize, struct rb_root *name_tree,
 		int slot)
 {
 	struct extent_buffer *eb;
+	u32 item_ptr_off = btrfs_item_ptr_offset(src, slot);
+	u32 item_ptr_size = btrfs_item_size(src, slot);
 
 	eb = alloc_dummy_eb(src->start, src->len);
 	if (!eb) {
@@ -476,7 +478,11 @@ void sanitize_name(enum sanitize_mode sanitize, struct rb_root *name_tree,
 		break;
 	}
 
-	memcpy(dst, eb->data, eb->len);
+	/*
+	 * Only overwrite the sanitized part, or we can overwrite previous
+	 * sanitized value with the old values from @src.
+	 */
+	memcpy(dst + item_ptr_off, eb->data + item_ptr_off, item_ptr_size);
 	free(eb);
 }