[PATCH v2] Btrfs: correctly calculate item size used when item key collision happends

[ethanwu <ethanwu@synology.com>]

Item key collision is allowed for some item types, like dir item and
inode refs, but the overall item size is limited by the leafsize.

item size(ins_len) passed from btrfs_insert_empty_items to
btrfs_search_slot already contains size of btrfs_item.

When btrfs_search_slot reaches leaf, we'll see if we need to split leaf.
The check incorrectly reports that split leaf is required, because
it treats the space required by the newly inserted item as
btrfs_item + item data. But in item key collision case, only item data
is actually needed, the newly inserted item could merge into the existing
one. No new btrfs_item will be inserted.

And split_leaf return -EOVERFLOW from following code:
if (extend && data_size + btrfs_item_size_nr(l, slot) +
    sizeof(struct btrfs_item) > BTRFS_LEAF_DATA_SIZE(fs_info))
    return -EOVERFLOW;

In most cases, when callers receive -EOVERFLOW, they either return
this error or handle in different ways. For example, in normal dir item
creation the userspace will get errno EOVERFLOW; in inode ref case
INODE_EXTREF is used instead.

However, this is not the case for rename. To avoid the unrecoverable
situation in rename, btrfs_check_dir_item_collision is called in
early phase of rename. In this function, when item key collision is
detected leaf space is checked:

data_size = sizeof(*di) + name_len;
if (data_size + btrfs_item_size_nr(leaf, slot) +
    sizeof(struct btrfs_item) > BTRFS_LEAF_DATA_SIZE(root->fs_info))

the sizeof(struct btrfs_item) + btrfs_item_size_nr(leaf, slot) here
refers to existing item size, the condition here correctly calculate
the needed size for collision case rather than the wrong case at
above.

The consequence of inconsistent condition check between
btrfs_check_dir_item_collision and btrfs_search_slot when item key
collision happens is that we might pass check here but fail
later at btrfs_search_slot. Rename fails and volume is forced readonly

[436149.586170] ------------[ cut here ]------------
[436149.586173] BTRFS: Transaction aborted (error -75)
[436149.586196] WARNING: CPU: 0 PID: 16733 at fs/btrfs/inode.c:9870 btrfs_rename2+0x1938/0x1b70 [btrfs]
[436149.586197] Modules linked in: btrfs zstd_compress xor raid6_pq ufs qnx4 hfsplus hfs minix ntfs msdos jfs xfs libcrc32c rpcsec_gss_krb5 coretemp crct10dif_pclmul crc32_pclmul ghash_clmulni_intel pcbc aesni_intel aes_x86_64 vmw_balloon crypto_simd cryptd glue_helper joydev input_leds intel_rapl_perf serio_raw vmw_vmci mac_hid sch_fq_codel nfsd auth_rpcgss nfs_acl lockd grace sunrpc parport_pc ppdev lp parport ip_tables x_tables autofs4 vmwgfx ttm drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops drm psmouse mptspi mptscsih mptbase scsi_transport_spi ahci vmxnet3 libahci i2c_piix4 floppy pata_acpi
[436149.586227] CPU: 0 PID: 16733 Comm: python Tainted: G      D           4.18.0-rc5+ #1
[436149.586228] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 04/05/2016
[436149.586238] RIP: 0010:btrfs_rename2+0x1938/0x1b70 [btrfs]
[436149.586238] Code: 50 f0 48 0f ba a8 10 ce 00 00 02 72 27 41 83 f8 fb 74 6f 44 89 c6 48 c7 c7 48 09 85 c0 44 89 55 80 44 89 45 98 e8 f8 5e 4c c5 <0f> 0b 44 8b 45 98 44 8b 55 80 44 89 55 80 44 89 c1 44 89 45 98 ba
[436149.586254] RSP: 0018:ffffa327043a7ce0 EFLAGS: 00010286
[436149.586255] RAX: 0000000000000000 RBX: ffff8d8a17d13340 RCX: 0000000000000006
[436149.586256] RDX: 0000000000000007 RSI: 0000000000000096 RDI: ffff8d8a7fc164b0
[436149.586257] RBP: ffffa327043a7da0 R08: 0000000000000560 R09: 7265282064657472
[436149.586258] R10: 0000000000000000 R11: 6361736e61725420 R12: ffff8d8a0d4c8b08
[436149.586258] R13: ffff8d8a17d13340 R14: ffff8d8a33e0a540 R15: 00000000000001fe
[436149.586260] FS:  00007fa313933740(0000) GS:ffff8d8a7fc00000(0000) knlGS:0000000000000000
[436149.586261] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[436149.586262] CR2: 000055d8d9c9a720 CR3: 000000007aae0003 CR4: 00000000003606f0
[436149.586295] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[436149.586296] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[436149.586296] Call Trace:
[436149.586311]  vfs_rename+0x383/0x920
[436149.586313]  ? vfs_rename+0x383/0x920
[436149.586315]  do_renameat2+0x4ca/0x590
[436149.586317]  __x64_sys_rename+0x20/0x30
[436149.586324]  do_syscall_64+0x5a/0x120
[436149.586330]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[436149.586332] RIP: 0033:0x7fa3133b1d37
[436149.586332] Code: 75 12 48 89 df e8 89 60 09 00 85 c0 0f 95 c0 0f b6 c0 f7 d8 5b c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 b8 52 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 09 f3 c3 0f 1f 80 00 00 00 00 48 8b 15 19 f1
[436149.586348] RSP: 002b:00007fffd3e43908 EFLAGS: 00000246 ORIG_RAX: 0000000000000052
[436149.586349] RAX: ffffffffffffffda RBX: 00007fa3133b1d30 RCX: 00007fa3133b1d37
[436149.586350] RDX: 000055d8da06b5e0 RSI: 000055d8da225d60 RDI: 000055d8da2c4da0
[436149.586351] RBP: 000055d8da2252f0 R08: 00007fa313782000 R09: 00000000000177e0
[436149.586351] R10: 000055d8da010680 R11: 0000000000000246 R12: 00007fa313840b00

Thanks to Hans van Kranenburg for information about crc32 hash collision tools,
I was able to reproduce the dir item collision with following python script.
https://github.com/wutzuchieh/misc_tools/blob/master/crc32_forge.py
Run it under a btrfs volume will trigger the abort transaction.
It simply creates files and rename them to forged names that leads to
hash collision.

There are two ways to fix this. One is to simply revert the patch
"878f2d2cb355 Btrfs: fix max dir item size calculation"
to make the condition consistent although that patch is correct
about the size.

The other way is to handle the leaf space check correctly when
collision happens. I prefer the second one since it correct leaf
space check in collision case. This fix needs unify the usage of ins_len
in btrfs_search_slot to contain btrfs_item anyway and adjust all callers
of btrfs_search_slot that intentionally pass ins_len without btrfs_item
size to add size of btrfs_item from now.

Fixes: 878f2d2cb355 Btrfs: fix max dir item size calculation
Signed-off-by: ethanwu <ethanwu@synology.com>
---

v2: modify change log, add call trace and way to reproduce it

 fs/btrfs/ctree.c       | 15 +++++++++++++--
 fs/btrfs/extent-tree.c |  5 +++--
 fs/btrfs/file-item.c   |  2 +-
 3 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 4bc326d..3614dd9 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -2678,8 +2678,8 @@ static struct extent_buffer *btrfs_search_slot_get_root(struct btrfs_root *root,
  * @p:		Holds all btree nodes along the search path
  * @root:	The root node of the tree
  * @key:	The key we are looking for
- * @ins_len:	Indicates purpose of search, for inserts it is 1, for
- *		deletions it's -1. 0 for plain searches
+ * @ins_len:	Indicates purpose of search, for inserts it is item size
+ *		including btrfs_item, for deletions it's -1. 0 for plain searches
  * @cow:	boolean should CoW operations be performed. Must always be 1
  *		when modifying the tree.
  *
@@ -2893,6 +2893,17 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 			}
 		} else {
 			p->slots[level] = slot;
+			/*
+			 * item key collision happens. In this case, if we are allow
+			 * to insert the item(for example, in dir_item case, item key
+			 * collision is allowed), it will be merged with the original
+			 * item. Only the item size grows, no new btrfs item will be
+			 * added. Since the ins_len already accounts the size btrfs_item,
+			 * this value is counted twice. Duduct this value here so the
+			 * leaf space check will be correct.
+			 */
+			if (ret == 0 && ins_len > 0)
+				ins_len -= sizeof(struct btrfs_item);
 			if (ins_len > 0 &&
 			    btrfs_leaf_free_space(fs_info, b) < ins_len) {
 				if (write_lock_level < 1) {
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 3d9fe58..4e3aa09 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -1094,7 +1094,7 @@ static int convert_extent_item_v0(struct btrfs_trans_handle *trans,
 
 	new_size -= sizeof(*ei0);
 	ret = btrfs_search_slot(trans, root, &key, path,
-				new_size + extra_size, 1);
+				new_size + extra_size + sizeof(struct btrfs_item), 1);
 	if (ret < 0)
 		return ret;
 	BUG_ON(ret); /* Corruption */
@@ -1644,7 +1644,8 @@ int lookup_inline_extent_backref(struct btrfs_trans_handle *trans,
 	}
 
 again:
-	ret = btrfs_search_slot(trans, root, &key, path, extra_size, 1);
+	ret = btrfs_search_slot(trans, root, &key, path,
+			    extra_size + sizeof(struct btrfs_item), 1);
 	if (ret < 0) {
 		err = ret;
 		goto out;
diff --git a/fs/btrfs/file-item.c b/fs/btrfs/file-item.c
index f9dd6d1..0b6c581 100644
--- a/fs/btrfs/file-item.c
+++ b/fs/btrfs/file-item.c
@@ -804,7 +804,7 @@ int btrfs_csum_file_blocks(struct btrfs_trans_handle *trans,
 	 */
 	btrfs_release_path(path);
 	ret = btrfs_search_slot(trans, root, &file_key, path,
-				csum_size, 1);
+				csum_size + sizeof(struct btrfs_item), 1);
 	if (ret < 0)
 		goto fail_unlock;
 
-- 
1.9.1