From 6897c2a462dc255302554586bcb8cddfcdca435e Mon Sep 17 00:00:00 2001
From: Anatol Belski <anbelski@linux.microsoft.com>
Date: Thu, 27 Nov 2025 09:39:30 +0100
Subject: [PATCH] block: qcow: Set OFLAG_COPIED bit in L1 entries for spec
 compliance

The OFLAG_COPIED bit (bit 63) indicates a cluster's refcount is exactly
1 and doesn't need copy-on-write. This bit must be set in L1 entries
when their referenced L2 clusters have refcount=1.

Previously, L1 entries were always written as raw addresses without the
OFLAG_COPIED bit, violating the QCOW2 specification and causing qemu-img
check to report errors like

`ERROR OFLAG_COPIED L2 cluster: l1_index=X .... refcount=1`

The implementation queries each L2 cluster's refcount in sync_caches()
and sets OFLAG_COPIED appropriately when writing the L1 table. This
ensures QCOW2 images are specification compliant and maintain correct
COW semantics to avoid data corruption.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>
---
 block/src/qcow/mod.rs | 29 ++++++++++++++++++++++++-----
 1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/block/src/qcow/mod.rs b/block/src/qcow/mod.rs
index 251c1b772..46a938418 100644
--- a/block/src/qcow/mod.rs
+++ b/block/src/qcow/mod.rs
@@ -207,6 +207,11 @@ fn l2_entry_make_std(cluster_addr: u64) -> u64 {
     (cluster_addr & L2_TABLE_OFFSET_MASK) | CLUSTER_USED_FLAG
 }
 
+// Make L1 entry with optional flags
+fn l1_entry_make(cluster_addr: u64, refcount_is_one: bool) -> u64 {
+    (cluster_addr & L1_TABLE_OFFSET_MASK) | (refcount_is_one as u64 * CLUSTER_USED_FLAG)
+}
+
 /// Contains the information from the header of a qcow file.
 #[derive(Clone, Debug)]
 pub struct QcowHeader {
@@ -1571,11 +1576,25 @@ impl QcowFile {
         // Push L1 table and refcount table last as all the clusters they point to are now
         // guaranteed to be valid.
         let mut sync_required = if self.l1_table.dirty() {
-            self.raw_file.write_pointer_table(
-                self.header.l1_table_offset,
-                self.l1_table.get_values(),
-                0,
-            )?;
+            // Build L1 table with OFLAG_COPIED bits set correctly based on L2 cluster refcounts
+            let l1_active: Vec<u64> = self
+                .l1_table
+                .get_values()
+                .iter()
+                .map(|&l2_addr| {
+                    if l2_addr == 0 {
+                        Ok(0)
+                    } else {
+                        let refcount = self
+                            .refcounts
+                            .get_cluster_refcount(&mut self.raw_file, l2_addr)
+                            .map_err(|e| std::io::Error::other(Error::GettingRefcount(e)))?;
+                        Ok(l1_entry_make(l2_addr, refcount == 1))
+                    }
+                })
+                .collect::<io::Result<Vec<u64>>>()?;
+            self.raw_file
+                .write_pointer_table(self.header.l1_table_offset, &l1_active, 0)?;
             self.l1_table.mark_clean();
             true
         } else {