Re: [Qemu-block] [Qemu-devel] [PATCH for-4.1] block/copy-on-read: Fix permissions for inactive node

[Kevin Wolf]

Am 29.07.2019 um 15:35 hat Eric Blake geschrieben:
> On 7/29/19 5:53 AM, Kevin Wolf wrote:
> > The copy-on-read drive must not request the WRITE_UNCHANGED permission
> > for its child if the node is inactive, otherwise starting a migration
> > destination with -incoming will fail because the child cannot provide
> > write access yet:
> > 
> >   qemu-system-x86_64: -blockdev copy-on-read,file=img,node-name=cor: Block 
> > node is read-only
> > 
> > Earlier QEMU versions additionally ran into an abort() on the migration
> > source side: bdrv_inactivate_recurse() failed to update permissions.
> > This is silently ignored today because it was only supposed to loosen
> > restrictions. This is the symptom that was originally reported here:
> > 
> >   https://bugzilla.redhat.com/show_bug.cgi?id=1733022
> > 
> > Signed-off-by: Kevin Wolf <address@hidden>
> > ---
> >  block/copy-on-read.c | 16 +++++++---------
> >  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> Do any of the iotests cover this?  Should they, especially if you are
> trying to get this in for -rc3 tomorrow?

No, we don't have any iotests for migration with filter drivers yet. We
probably should, but I didn't want to miss -rc3 with the fix because I
was busy writing a test case.

> > 
> > diff --git a/block/copy-on-read.c b/block/copy-on-read.c
> > index 22f24fd0db..6631f30205 100644
> > --- a/block/copy-on-read.c
> > +++ b/block/copy-on-read.c
> > @@ -56,16 +56,14 @@ static void cor_child_perm(BlockDriverState *bs, 
> > BdrvChild *c,
> >                             uint64_t perm, uint64_t shared,
> >                             uint64_t *nperm, uint64_t *nshared)
> >  {
> > -    if (c == NULL) {
> > -        *nperm = (perm & PERM_PASSTHROUGH) | BLK_PERM_WRITE_UNCHANGED;
> > -        *nshared = (shared & PERM_PASSTHROUGH) | PERM_UNCHANGED;
> > -        return;
> > -    }
> > +    *nperm = perm & PERM_PASSTHROUGH;
> > +    *nshared = (shared & PERM_PASSTHROUGH) | PERM_UNCHANGED;
> >  
> > -    *nperm = (perm & PERM_PASSTHROUGH) |
> > -             (c->perm & PERM_UNCHANGED);
> > -    *nshared = (shared & PERM_PASSTHROUGH) |
> > -               (c->shared_perm & PERM_UNCHANGED);
> 
> The old code unconditionally returned one set of permissions when c ==
> NULL, or made a choice based on c's existing permissions on whether to
> pass in those two bits.
> 
> > +    /* We must not request write permissions for an inactive node, the 
> > child
> > +     * cannot provide it. */
> > +    if (!(bs->open_flags & BDRV_O_INACTIVE)) {
> > +        *nperm |= BLK_PERM_WRITE_UNCHANGED;
> > +    }
> 
> The new code changes the condition for or'ing in WRITE_UNCHANGED to
> *nperm (it is no longer dependent on whether c == NULL, but whether the
> drive is inactive), which matches your commit message.
> 
> But the new code also changes to always pass in the PERM_UNCHANGED to
> *nshared; that used to be skipped if c was non-NULL and did not already
> have the permission.  I don't follow that change from the commit
> message, am I missing something?

The old code didn't actually do anything that should have a different
result (apart from WRITE_UNCHANGED for inactive images), just everything
in a more complicated way for no apparent reason. Or at least that's
what Max and I concluded after looking at this.

Taking the PERM_UNCHANGED bits from the old value effectively means that
they are taken from the very first call, which had c == NULL. So we can
just use the same code to set them instead of referring to the old
values of c->perm and c->shared_perm (which is really something a
.bdrv_child_perm implementation shouldn't do - there are more cases, but
we can clean them up for 4.2).

Not cleaning this up would mean that I'd have to explicitly clear the
WRITE_UNCHANGED bit after uselessly copying from the old state. This
would be further complication of already unnecessarily complicated code,
so I decided that cleaning it up so that its correctness becomes very
obvious (request everything the parent nodes need, plus WRITE_UNCHANGED
for the copy on read functionality if the node is active) makes more
sense.

Kevin

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