erofs: fix 1 lcluster-sized pcluster for big pcluster

If the 1st NONHEAD lcluster of a pcluster isn't CBLKCNT lcluster type
rather than a HEAD or PLAIN type instead, which means its pclustersize
_must_ be 1 lcluster (since its uncompressed size < 2 lclusters),
as illustrated below:

       HEAD     HEAD / PLAIN    lcluster type
   ____________ ____________
  |_:__________|_________:__|   file data (uncompressed)
   .                .
  .____________.
  |____________|                pcluster data (compressed)

Such on-disk case was explained before [1] but missed to be handled
properly in the runtime implementation.

It can be observed if manually generating 1 lcluster-sized pcluster
with 2 lclusters (thus CBLKCNT doesn't exist.) Let's fix it now.

[1] https://lore.kernel.org/r/20210407043927.10623-1-xiang@kernel.org

Link: https://lore.kernel.org/r/20210510064715.29123-1-xiang@kernel.org
Fixes: cec6e93beadf ("erofs: support parsing big pcluster compress indexes")
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <xiang@kernel.org>

diff --git a/fs/erofs/zmap.c b/fs/erofs/zmap.c
index e62d813..efaf325 100644 (file)
--- a/fs/erofs/zmap.c
+++ b/fs/erofs/zmap.c
@@ -450,14 +450,31 @@ static int z_erofs_get_extent_compressedlen(struct z_erofs_maprecorder *m,
        lcn = m->lcn + 1;
        if (m->compressedlcs)
                goto out;
-       if (lcn == initial_lcn)
-               goto err_bonus_cblkcnt;
 
        err = z_erofs_load_cluster_from_disk(m, lcn);
        if (err)
                return err;
 
+       /*
+        * If the 1st NONHEAD lcluster has already been handled initially w/o
+        * valid compressedlcs, which means at least it mustn't be CBLKCNT, or
+        * an internal implemenatation error is detected.
+        *
+        * The following code can also handle it properly anyway, but let's
+        * BUG_ON in the debugging mode only for developers to notice that.
+        */
+       DBG_BUGON(lcn == initial_lcn &&
+                 m->type == Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD);
+
        switch (m->type) {
+       case Z_EROFS_VLE_CLUSTER_TYPE_PLAIN:
+       case Z_EROFS_VLE_CLUSTER_TYPE_HEAD:
+               /*
+                * if the 1st NONHEAD lcluster is actually PLAIN or HEAD type
+                * rather than CBLKCNT, it's a 1 lcluster-sized pcluster.
+                */
+               m->compressedlcs = 1;
+               break;
        case Z_EROFS_VLE_CLUSTER_TYPE_NONHEAD:
                if (m->delta[0] != 1)
                        goto err_bonus_cblkcnt;