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

[Kevin Wolf]

Am 26.07.2021 um 16:41 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 26.07.2021 15:28, Stefan Hajnoczi wrote:
> > 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>
> > ---
> > 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)
> >       bdrv_cancel_in_flight(bs->file->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,
> > 
> 
> I think it's OK:
> 
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> 
> 
> Still, did you consider an alternative of making
> bdrv_filter_default_perm() function public and just do
> ".bdrv_child_perm = bdrv_filter_default_perm," here?
> 
> raw_format is not considered to be filter, but for it's permissions I
> think it works exactly like filter.

I had the same thought, but then commit 69dca43d6b6 explicitly made the
opposite change. I seem to remember that Max never liked raw being
treated like a filter much.

Kevin