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

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

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 \
      --blockdev 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
permission.

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>
Message-Id: <20210726122839.822900-1-stefanha@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>