erofs: handle NONHEAD !delta[1] lclusters gracefully

syzbot reported a WARNING in iomap_iter_done:
 iomap_fiemap+0x73b/0x9b0 fs/iomap/fiemap.c:80
 ioctl_fiemap fs/ioctl.c:220 [inline]

Generally, NONHEAD lclusters won't have delta[1]==0, except for crafted
images and filesystems created by pre-1.0 mkfs versions.

Previously, it would immediately bail out if delta[1]==0, which led to
inadequate decompressed lengths (thus FIEMAP is impacted).  Treat it as
delta[1]=1 to work around these legacy mkfs versions.

`lclusterbits > 14` is illegal for compact indexes, error out too.

Reported-by: syzbot+6c0b301317aa0156f9eb@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/r/67373c0c.050a0220.2a2fcc.0079.GAE@google.com
Tested-by: syzbot+6c0b301317aa0156f9eb@syzkaller.appspotmail.com
Fixes: d95ae5e ("erofs: add support for the full decompressed length")
Fixes: 001b8cc ("erofs: fix compact 4B support for 16k block size")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20241115173651.3339514-1-hsiangkao@linux.alibaba.com

Author: hsiangkao

fs/erofs/zmap.c

@@ -219,7 +219,7 @@ static int z_erofs_load_compact_lcluster(struct z_erofs_maprecorder *m,
 	unsigned int amortizedshift;
 	erofs_off_t pos;
 
-	if (lcn >= totalidx)
+	if (lcn >= totalidx || vi->z_logical_clusterbits > 14)
 		return -EINVAL;
 
 	m->lcn = lcn;
@@ -390,7 +390,7 @@ static int z_erofs_get_extent_decompressedlen(struct z_erofs_maprecorder *m)
 	u64 lcn = m->lcn, headlcn = map->m_la >> lclusterbits;
 	int err;
 
-	do {
+	while (1) {
 		/* handle the last EOF pcluster (no next HEAD lcluster) */
 		if ((lcn << lclusterbits) >= inode->i_size) {
 			map->m_llen = inode->i_size - map->m_la;
@@ -402,14 +402,16 @@ static int z_erofs_get_extent_decompressedlen(struct z_erofs_maprecorder *m)
 			return err;
 
 		if (m->type == Z_EROFS_LCLUSTER_TYPE_NONHEAD) {
-			DBG_BUGON(!m->delta[1] &&
-				  m->clusterofs != 1 << lclusterbits);
+			/* work around invalid d1 generated by pre-1.0 mkfs */
+			if (unlikely(!m->delta[1])) {
+				m->delta[1] = 1;
+				DBG_BUGON(1);
+			}
 		} else if (m->type == Z_EROFS_LCLUSTER_TYPE_PLAIN ||
 			   m->type == Z_EROFS_LCLUSTER_TYPE_HEAD1 ||
 			   m->type == Z_EROFS_LCLUSTER_TYPE_HEAD2) {
-			/* go on until the next HEAD lcluster */
 			if (lcn != headlcn)
-				break;
+				break;	/* ends at the next HEAD lcluster */
 			m->delta[1] = 1;
 		} else {
 			erofs_err(inode->i_sb, "unknown type %u @ lcn %llu of nid %llu",
@@ -418,8 +420,7 @@ static int z_erofs_get_extent_decompressedlen(struct z_erofs_maprecorder *m)
 			return -EOPNOTSUPP;
 		}
 		lcn += m->delta[1];
-	} while (m->delta[1]);
-
+	}
 	map->m_llen = (lcn << lclusterbits) + m->clusterofs - map->m_la;
 	return 0;
 }