Re: [Qemu-devel] [PATCH v2 3/8] block: Require auto-read-only for existing fallbacks

[Kevin Wolf <kwolf@redhat.com>]

Am 12.10.2018 um 19:02 hat Eric Blake geschrieben:
> On 10/12/18 6:55 AM, Kevin Wolf wrote:
> > Some block drivers have traditionally changed their node to read-only
> > mode without asking the user. This behaviour has been marked deprecated
> > since 2.11, expecting users to provide an explicit read-only=on option.
> > 
> > Now that we have auto-read-only=on, enable these drivers to make use of
> > the option.
> > 
> > This is the only use of bdrv_set_read_only(), so we can make it a bit
> > more specific and turn it into a bdrv_apply_auto_read_only() that is
> > more convenient for drivers to use.
> > 
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> 
> > +++ b/block.c
> > @@ -266,27 +266,36 @@ int bdrv_can_set_read_only(BlockDriverState *bs, bool read_only,
> >       return 0;
> >   }
> > -/* TODO Remove (deprecated since 2.11)
> > - * Block drivers are not supposed to automatically change bs->read_only.
> > - * Instead, they should just check whether they can provide what the user
> > - * explicitly requested and error out if read-write is requested, but they can
> > - * only provide read-only access. */
> > -int bdrv_set_read_only(BlockDriverState *bs, bool read_only, Error **errp)
> > +/*
> > + * Called by a driver that can only provide a read-only image.
> > + *
> > + * Returns 0 if the node is already read-only or it could switch the node to
> > + * read-only because BDRV_O_AUTO_RDONLY is set.
> > + *
> > + * Returns -EACCES if the node is read-write and BDRV_O_AUTO_RDONLY is not set.
> > + * If @errmsg is not NULL, it is used as the error message for the Error
> > + * object.
> 
> I like it.
> 
> Worth documenting the -EINVAL (copy-on-read prevents setting read-only)
> failure as well?  (The -EPERM failure of bdrv_can_set_read_only() is not
> reachable, since this new function never clears readonly).

In fact, -EINVAL and the error string from bdrv_can_set_read_only() may
be confusing because the user didn't explicitly request a read-only
image. Maybe it would be better to just turn this case into -EACCES with
the same error message.

What do you think?

> > + */
> > +int bdrv_apply_auto_read_only(BlockDriverState *bs, const char *errmsg,
> > +                              Error **errp)
> >   {
> >       int ret = 0;
> > -    ret = bdrv_can_set_read_only(bs, read_only, false, errp);
> > +    if (!(bs->open_flags & BDRV_O_RDWR)) {
> > +        return 0;
> > +    }
> > +    if (!(bs->open_flags & BDRV_O_AUTO_RDONLY)) {
> > +        error_setg(errp, "%s", errmsg ?: "Image is read-only");
> > +        return -EACCES;
> > +    }
> > +
> > +    ret = bdrv_can_set_read_only(bs, true, false, errp);
> >       if (ret < 0) {
> >           return ret;
> >       }
> 
> Makes sense.
> 
> > +++ b/block/vvfat.c
> > @@ -1262,16 +1262,10 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
> >                          "Unable to set VVFAT to 'rw' when drive is read-only");
> >               goto fail;
> >           }
> > -    } else  if (!bdrv_is_read_only(bs)) {
> > -        error_report("Opening non-rw vvfat images without an explicit "
> > -                     "read-only=on option is deprecated. Future versions "
> > -                     "will refuse to open the image instead of "
> > -                     "automatically marking the image read-only.");
> > -        /* read only is the default for safety */
> > -        ret = bdrv_set_read_only(bs, true, &local_err);
> > +    } else {
> > +        ret = bdrv_apply_auto_read_only(bs, NULL, errp);
> >           if (ret < 0) {
> > -            error_propagate(errp, local_err);
> > -            goto fail;
> > +            return ret;
> 
> Don't you still need the goto fail, to avoid leaking opts?

Yes, I do. Thanks.

Kevin