btrfs: don't use btrfs_chunk::sub_stripes from disk

[BUG]
There are two reports (the earliest one from LKP, a more recent one from
kernel bugzilla) that we can have some chunks with 0 as sub_stripes.

This will cause divide-by-zero errors at btrfs_rmap_block, which is
introduced by a recent kernel patch ac06773 ("btrfs: merge
calculations for simple striped profiles in btrfs_rmap_block"):

		if (map->type & (BTRFS_BLOCK_GROUP_RAID0 |
				 BTRFS_BLOCK_GROUP_RAID10)) {
			stripe_nr = stripe_nr * map->num_stripes + i;
			stripe_nr = div_u64(stripe_nr, map->sub_stripes); <<<
		}

[CAUSE]
From the more recent report, it has been proven that we have some chunks
with 0 as sub_stripes, mostly caused by older mkfs.

It turns out that the mkfs.btrfs fix is only introduced in 6718ab4d33aa
("btrfs-progs: Initialize sub_stripes to 1 in btrfs_alloc_data_chunk")
which is included in v5.4 btrfs-progs release.

So there would be quite some old filesystems with such 0 sub_stripes.

[FIX]
Just don't trust the sub_stripes values from disk.

We have a trusted btrfs_raid_array[] to fetch the correct sub_stripes
numbers for each profile and that are fixed.

By this, we can keep the compatibility with older filesystems while
still avoid divide-by-zero bugs.

Reported-by: kernel test robot <[email protected]>
Reported-by: Viktor Kuzmin <[email protected]>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=216559
Fixes: ac06773 ("btrfs: merge calculations for simple striped profiles in btrfs_rmap_block")
CC: [email protected] # 6.0
Reviewed-by: Su Yue <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Signed-off-by: Qu Wenruo <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: adam900710

fs/btrfs/volumes.c

@@ -7142,13 +7142,15 @@ static int read_one_chunk(struct btrfs_key *key, struct extent_buffer *leaf,
 	u64 devid;
 	u64 type;
 	u8 uuid[BTRFS_UUID_SIZE];
+	int index;
 	int num_stripes;
 	int ret;
 	int i;
 
 	logical = key->offset;
 	length = btrfs_chunk_length(leaf, chunk);
 	type = btrfs_chunk_type(leaf, chunk);
+	index = btrfs_bg_flags_to_raid_index(type);
 	num_stripes = btrfs_chunk_num_stripes(leaf, chunk);
 
 #if BITS_PER_LONG == 32
@@ -7202,7 +7204,15 @@ static int read_one_chunk(struct btrfs_key *key, struct extent_buffer *leaf,
 	map->io_align = btrfs_chunk_io_align(leaf, chunk);
 	map->stripe_len = btrfs_chunk_stripe_len(leaf, chunk);
 	map->type = type;
-	map->sub_stripes = btrfs_chunk_sub_stripes(leaf, chunk);
+	/*
+	 * We can't use the sub_stripes value, as for profiles other than
+	 * RAID10, they may have 0 as sub_stripes for filesystems created by
+	 * older mkfs (<v5.4).
+	 * In that case, it can cause divide-by-zero errors later.
+	 * Since currently sub_stripes is fixed for each profile, let's
+	 * use the trusted value instead.
+	 */
+	map->sub_stripes = btrfs_raid_array[index].sub_stripes;
 	map->verified_stripes = 0;
 	em->orig_block_len = btrfs_calc_stripe_length(em);
 	for (i = 0; i < num_stripes; i++) {