[PATCH] mirror: Fix missed dirty bitmap writes during startup

[Kevin Wolf <kwolf@redhat.com>]

Currently, mirror disables the block layer's dirty bitmap before its own
replacement is working. This means that during startup, there is a
window in which the allocation status of blocks in the source has
already been checked, but new writes coming in aren't tracked yet,
resulting in a corrupted copy:

1. Dirty bitmap is disabled in mirror_start_job()
2. Some request are started in mirror_top_bs while s->job == NULL
3. mirror_dirty_init() -> bdrv_co_is_allocated_above() runs and because
   the request hasn't completed yet, the block isn't allocated
4. The request completes, still sees s->job == NULL and skips the
   bitmap, and nothing else will mark it dirty either

One ingredient is that mirror_top_opaque->job is only set after the
job is fully initialized. For the rationale, see commit 32125b1460
("mirror: Fix access of uninitialised fields during start").

Fix this by giving mirror_top_bs access to dirty_bitmap and enabling it
to track writes from the beginning. Disabling the block layer's tracking
and enabling the mirror_top_bs one happens in a drained section, so
there is no danger of races with in-flight requests any more. All of
this happens well before the block allocation status is checked, so we
can be sure that no writes will be missed.

Cc: qemu-stable@nongnu.org
Closes: https://gitlab.com/qemu-project/qemu/-/issues/3273
Fixes: 32125b14606a ('mirror: Fix access of uninitialised fields during start')
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
Supersedes: <20260212120411.369498-1-f.ebner@proxmox.com>
---
 block/mirror.c | 48 +++++++++++++++++++++++++++++-------------------
 1 file changed, 29 insertions(+), 19 deletions(-)

diff --git a/block/mirror.c b/block/mirror.c
index b344182c747..f38636e7457 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -99,6 +99,7 @@ typedef struct MirrorBlockJob {
 
 typedef struct MirrorBDSOpaque {
     MirrorBlockJob *job;
+    BdrvDirtyBitmap *dirty_bitmap;
     bool stop;
     bool is_commit;
 } MirrorBDSOpaque;
@@ -1672,9 +1673,9 @@ bdrv_mirror_top_do_write(BlockDriverState *bs, MirrorMethod method,
         abort();
     }
 
-    if (!copy_to_target && s->job && s->job->dirty_bitmap) {
+    if (!copy_to_target) {
         qatomic_set(&s->job->actively_synced, false);
-        bdrv_set_dirty_bitmap(s->job->dirty_bitmap, offset, bytes);
+        bdrv_set_dirty_bitmap(s->dirty_bitmap, offset, bytes);
     }
 
     if (ret < 0) {
@@ -1901,13 +1902,35 @@ static BlockJob *mirror_start_job(
 
     bdrv_drained_begin(bs);
     ret = bdrv_append(mirror_top_bs, bs, errp);
-    bdrv_drained_end(bs);
-
     if (ret < 0) {
+        bdrv_drained_end(bs);
+        bdrv_unref(mirror_top_bs);
+        return NULL;
+    }
+
+    bs_opaque->dirty_bitmap = bdrv_create_dirty_bitmap(mirror_top_bs,
+                                                       granularity,
+                                                       NULL, errp);
+    if (!bs_opaque->dirty_bitmap) {
+        bdrv_drained_end(bs);
         bdrv_unref(mirror_top_bs);
         return NULL;
     }
 
+    /*
+     * The mirror job doesn't use the block layer's dirty tracking because it
+     * needs to be able to switch seemlessly between background copy mode (which
+     * does need dirty tracking) and write blocking mode (which doesn't) and
+     * doing that would require draining the node. Instead, mirror_top_bs takes
+     * care of updating the dirty bitmap as appropriate.
+     *
+     * Note that write blocking mode only becomes effective after mirror_run()
+     * sets mirror_top_opaque->job (see should_copy_to_target()). Until then,
+     * we're still in background copy mode irrespective of @copy_mode.
+     */
+    bdrv_disable_dirty_bitmap(bs_opaque->dirty_bitmap);
+    bdrv_drained_end(bs);
+
     /* Make sure that the source is not resized while the job is running */
     s = block_job_create(job_id, driver, NULL, mirror_top_bs,
                          BLK_PERM_CONSISTENT_READ,
@@ -2002,24 +2025,13 @@ static BlockJob *mirror_start_job(
     s->base_overlay = bdrv_find_overlay(bs, base);
     s->granularity = granularity;
     s->buf_size = ROUND_UP(buf_size, granularity);
+    s->dirty_bitmap = bs_opaque->dirty_bitmap;
     s->unmap = unmap;
     if (auto_complete) {
         s->should_complete = true;
     }
     bdrv_graph_rdunlock_main_loop();
 
-    s->dirty_bitmap = bdrv_create_dirty_bitmap(s->mirror_top_bs, granularity,
-                                               NULL, errp);
-    if (!s->dirty_bitmap) {
-        goto fail;
-    }
-
-    /*
-     * The dirty bitmap is set by bdrv_mirror_top_do_write() when not in active
-     * mode.
-     */
-    bdrv_disable_dirty_bitmap(s->dirty_bitmap);
-
     bdrv_graph_wrlock_drained();
     ret = block_job_add_bdrv(&s->common, "source", bs, 0,
                              BLK_PERM_WRITE_UNCHANGED | BLK_PERM_WRITE |
@@ -2099,9 +2111,6 @@ fail:
         g_free(s->replaces);
         blk_unref(s->target);
         bs_opaque->job = NULL;
-        if (s->dirty_bitmap) {
-            bdrv_release_dirty_bitmap(s->dirty_bitmap);
-        }
         job_early_fail(&s->common.job);
     }
 
@@ -2115,6 +2124,7 @@ fail:
     bdrv_graph_wrunlock();
     bdrv_drained_end(bs);
 
+    bdrv_release_dirty_bitmap(bs_opaque->dirty_bitmap);
     bdrv_unref(mirror_top_bs);
 
     return NULL;
-- 
2.53.0