block/io: accept NULL qiov in bdrv_pad_request

Some operations, e.g. block-stream, perform reads while discarding the
results (only copy-on-read matters). In this case, they will pass NULL
as the target QEMUIOVector, which will however trip bdrv_pad_request,
since it wants to extend its passed vector. In particular, this is the
case for the blk_co_preadv() call in stream_populate().

If there is no qiov, no operation can be done with it, but the bytes
and offset still need to be updated, so the subsequent aligned read
will actually be aligned and not run into an assertion failure.

In particular, this can happen when the request alignment of the top
node is larger than the allocated part of the bottom node, in which
case padding becomes necessary. For example:

> ./qemu-img create /tmp/backing.qcow2 -f qcow2 64M -o cluster_size=32768
> ./qemu-io -c "write -P42 0x0 0x1" /tmp/backing.qcow2
> ./qemu-img create /tmp/top.qcow2 -f qcow2 64M -b /tmp/backing.qcow2 -F qcow2
> ./qemu-system-x86_64 --qmp stdio \
> --blockdev qcow2,node-name=node0,file.driver=file,file.filename=/tmp/top.qcow2 \
> <<EOF
> {"execute": "qmp_capabilities"}
> {"execute": "blockdev-add", "arguments": { "driver": "compress", "file": "node0", "node-name": "node1" } }
> {"execute": "block-stream", "arguments": { "job-id": "stream0", "device": "node1" } }
> EOF

Originally-by: Stefan Reiter <s.reiter@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
[FE: do update bytes and offset in any case
     add reproducer to commit message]
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20240322095009.346989-2-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 3f93481)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

block/io.c

@@ -1756,22 +1756,29 @@ static int bdrv_pad_request(BlockDriverState *bs,
         return 0;
     }
 
-    sliced_iov = qemu_iovec_slice(*qiov, *qiov_offset, *bytes,
-                                  &sliced_head, &sliced_tail,
-                                  &sliced_niov);
-
-    /* Guaranteed by bdrv_check_request32() */
-    assert(*bytes <= SIZE_MAX);
-    ret = bdrv_create_padded_qiov(bs, pad, sliced_iov, sliced_niov,
-                                  sliced_head, *bytes);
-    if (ret < 0) {
-        bdrv_padding_finalize(pad);
-        return ret;
+    /*
+     * For prefetching in stream_populate(), no qiov is passed along, because
+     * only copy-on-read matters.
+     */
+    if (qiov && *qiov) {
+        sliced_iov = qemu_iovec_slice(*qiov, *qiov_offset, *bytes,
+                                      &sliced_head, &sliced_tail,
+                                      &sliced_niov);
+
+        /* Guaranteed by bdrv_check_request32() */
+        assert(*bytes <= SIZE_MAX);
+        ret = bdrv_create_padded_qiov(bs, pad, sliced_iov, sliced_niov,
+                                      sliced_head, *bytes);
+        if (ret < 0) {
+            bdrv_padding_finalize(pad);
+            return ret;
+        }
+        *qiov = &pad->local_qiov;
+        *qiov_offset = 0;
     }
+
     *bytes += pad->head + pad->tail;
     *offset -= pad->head;
-    *qiov = &pad->local_qiov;
-    *qiov_offset = 0;
     if (padded) {
         *padded = true;
     }