Re: [PATCH 2/6] block: truncate: Don't make backing file data visible

[Kevin Wolf <kwolf@redhat.com>]

Am 20.11.2019 um 15:47 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 20.11.2019 17:03, Kevin Wolf wrote:
> > When extending the size of an image that has a backing file larger than
> > its old size, make sure that the backing file data doesn't become
> > visible in the guest, but the added area is properly zeroed out.
> > 
> > The old behaviour made a difference in 'block_resize' (where showing the
> > backing file data from an old snapshot rather than zeros is
> > questionable) as well as in commit block jobs (both from active and
> > intermediate nodes) and HMP 'commit', where committing to a short
> > backing file would incorrectly omit writing zeroes for unallocated
> > blocks on the top layer after the EOF of the short backing file.
> > 
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> >   block/io.c | 25 +++++++++++++++++++++++++
> >   1 file changed, 25 insertions(+)
> > 
> > diff --git a/block/io.c b/block/io.c
> > index 003f4ea38c..8683f7a4bd 100644
> > --- a/block/io.c
> > +++ b/block/io.c
> > @@ -3382,6 +3382,31 @@ int coroutine_fn bdrv_co_truncate(BdrvChild *child, int64_t offset, bool exact,
> >           goto out;
> >       }
> >   
> > +    /*
> > +     * If the image has a backing file that is large enough that it would
> > +     * provide data for the new area, we cannot leave it unallocated because
> > +     * then the backing file content would become visible. Instead, zero-fill
> > +     * the area where backing file and new area overlap.
> > +     */
> 
> Should we mention that, still, we don't care if user for some reason will change
> backing file in future?

This should be obvious, but I can add something.

> > +    if (new_bytes && bs->backing && prealloc == PREALLOC_MODE_OFF) {
> > +        int64_t backing_len;
> > +
> > +        backing_len = bdrv_getlength(backing_bs(bs));
> > +        if (backing_len < 0) {
> > +            ret = backing_len;
> > +            goto out;
> > +        }
> > +
> > +        if (backing_len > old_size) {
> > +            ret = bdrv_co_do_pwrite_zeroes(bs, old_size,
> > +                                           MIN(new_bytes, backing_len - old_size),
> > +                                           BDRV_REQ_ZERO_WRITE | BDRV_REQ_MAY_UNMAP);
> 
> two over-80 lines

Will fix.

> > +            if (ret < 0) {
> > +                goto out;
> > +            }
> > +        }
> > +    }
> 
> should we improve "off" mode specification in qapi?

I don't think we're changing the semantics of "off". We're merely fixing
a bug that happens not to exist with preallocation.

> > +
> >       ret = refresh_total_sectors(bs, offset >> BDRV_SECTOR_BITS);
> >       if (ret < 0) {
> >           error_setg_errno(errp, -ret, "Could not refresh total sector count");
> > 
> 
> Hmm. is it correct to call write_zeroes before refresh_total_sectors?
> Note that qcow2_co_pwrite_zeroes rely on bs->total_sectors...

Hm... I placed the code where I did because I didn't want to make the
new area valid before it isn't zeroed. But you're probably right that
we shouldn't run requests with inconsistent bs->total_sectors, so I'll
switch the order.

Kevin