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[PATCH] raw-format: drop WRITE and RESIZE child perms when possible

From: Stefan Hajnoczi
Subject: [PATCH] raw-format: drop WRITE and RESIZE child perms when possible
Date: Mon, 26 Jul 2021 13:28:39 +0100

The following command-line fails due to a permissions conflict:

  $ qemu-storage-daemon \
      --blockdev driver=nvme,node-name=nvme0,device=0000:08:00.0,namespace=1 \
      --blockdev driver=raw,node-name=l1-1,file=nvme0,offset=0,size=1073741824 \
driver=raw,node-name=l1-2,file=nvme0,offset=1073741824,size=1073741824 \
      --nbd-server addr.type=unix,addr.path=/tmp/nbd.sock,max-connections=2 \
      --export type=nbd,id=nbd-l1-1,node-name=l1-1,name=l1-1,writable=on \
      --export type=nbd,id=nbd-l1-2,node-name=l1-2,name=l1-2,writable=on

  qemu-storage-daemon: --export 
type=nbd,id=nbd-l1-1,node-name=l1-1,name=l1-1,writable=on: Permission conflict 
on node 'nvme0': permissions 'resize' are both required by node 'l1-1' (uses 
node 'nvme0' as 'file' child) and unshared by node 'l1-2' (uses node 'nvme0' as 
'file' child).

The problem is that block/raw-format.c relies on bdrv_default_perms() to
set permissions on the nvme node. The default permissions add RESIZE in
anticipation of a format driver like qcow2 that needs to grow the image
file. This fails because RESIZE is unshared, so we cannot get the RESIZE

Max Reitz pointed out that block/crypto.c already handles this case by
implementing a custom ->bdrv_child_perm() function that adjusts the
result of bdrv_default_perms().

This patch takes the same approach in block/raw-format.c so that RESIZE
is only required if it's actually necessary (e.g. the parent is qcow2).

Cc: Max Reitz <mreitz@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This is not a bug fix, so I didn't mark it for QEMU 6.1. It's new
behavior that hasn't been supported before. I want to split an NVMe
drive using the raw format's offset=/size= feature.
 block/raw-format.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/block/raw-format.c b/block/raw-format.c
index 7717578ed6..c26f493688 100644
--- a/block/raw-format.c
+++ b/block/raw-format.c
@@ -580,6 +580,25 @@ static void raw_cancel_in_flight(BlockDriverState *bs)
+static void raw_child_perm(BlockDriverState *bs, BdrvChild *c,
+                           BdrvChildRole role,
+                           BlockReopenQueue *reopen_queue,
+                           uint64_t parent_perm, uint64_t parent_shared,
+                           uint64_t *nperm, uint64_t *nshared)
+    bdrv_default_perms(bs, c, role, reopen_queue, parent_perm,
+                       parent_shared, nperm, nshared);
+    /*
+     * bdrv_default_perms() may add WRITE and/or RESIZE (see comment in
+     * bdrv_default_perms_for_storage() for an explanation) but we only need
+     * them if they are in parent_perm. Drop WRITE and RESIZE whenever possible
+     * to avoid permission conflicts.
+     */
+    *nperm &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
+    *nperm |= parent_perm & (BLK_PERM_WRITE | BLK_PERM_RESIZE);
 BlockDriver bdrv_raw = {
     .format_name          = "raw",
     .instance_size        = sizeof(BDRVRawState),
@@ -588,7 +607,7 @@ BlockDriver bdrv_raw = {
     .bdrv_reopen_commit   = &raw_reopen_commit,
     .bdrv_reopen_abort    = &raw_reopen_abort,
     .bdrv_open            = &raw_open,
-    .bdrv_child_perm      = bdrv_default_perms,
+    .bdrv_child_perm      = raw_child_perm,
     .bdrv_co_create_opts  = &raw_co_create_opts,
     .bdrv_co_preadv       = &raw_co_preadv,
     .bdrv_co_pwritev      = &raw_co_pwritev,

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