iomap: reject delalloc mappings during writeback

Filesystems should never provide a delayed allocation mapping to
writeback; they're supposed to allocate the space before replying.
This can lead to weird IO errors and crashes in the block layer if the
filesystem is being malicious, or if it hadn't set iomap->dev because
it's a delalloc mapping.

Fix this by failing writeback on delalloc mappings.  Currently no
filesystems actually misbehave in this manner, but we ought to be
stricter about things like that.

Cc: [email protected] # v5.5
Fixes: 598ecfb ("iomap: lift the xfs writeback code to iomap")
Signed-off-by: Darrick J. Wong <[email protected]>
Link: https://patch.msgid.link/20260302173002.GL13829@frogsfrogsfrogs
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Carlos Maiolino <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: Darrick J. Wong

fs/iomap/ioend.c

@@ -215,17 +215,18 @@ ssize_t iomap_add_to_ioend(struct iomap_writepage_ctx *wpc, struct folio *folio,
 	WARN_ON_ONCE(!folio->private && map_len < dirty_len);
 
 	switch (wpc->iomap.type) {
-	case IOMAP_INLINE:
-		WARN_ON_ONCE(1);
-		return -EIO;
+	case IOMAP_UNWRITTEN:
+		ioend_flags |= IOMAP_IOEND_UNWRITTEN;
+		break;
+	case IOMAP_MAPPED:
+		break;
 	case IOMAP_HOLE:
 		return map_len;
 	default:
-		break;
+		WARN_ON_ONCE(1);
+		return -EIO;
 	}
 
-	if (wpc->iomap.type == IOMAP_UNWRITTEN)
-		ioend_flags |= IOMAP_IOEND_UNWRITTEN;
 	if (wpc->iomap.flags & IOMAP_F_SHARED)
 		ioend_flags |= IOMAP_IOEND_SHARED;
 	if (folio_test_dropbehind(folio))