[Qemu-devel] [PATCH] block: do not allow read-only=on and snapshot=on to be used together

[Qemu-devel] [PATCH] block: do not allow read-only=on and snapshot=on to be used together

[Jeff Cody]

Having both read-only=on and snapshot=on together does not make sense;
currently, the read-only argument is effectively ignored for the
temporary snapshot.  To prevent confusion, disallow the usage of both
'snapshot=on' and 'read-only=on'.

Signed-off-by: Jeff Cody <jcody@redhat.com>
---
 blockdev.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/blockdev.c b/blockdev.c
index e457494..845ff8a 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -352,6 +352,13 @@ static DriveInfo *blockdev_init(QDict *bs_opts,
     /* extract parameters */
     snapshot = qemu_opt_get_bool(opts, "snapshot", 0);
     ro = qemu_opt_get_bool(opts, "read-only", 0);
+
+    /* having ro and snapshot together does not make sense */
+    if (ro && snapshot) {
+        error_setg(errp, "invalid option combination: read-only and snapshot");
+        goto early_err;
+    }
+
     copy_on_read = qemu_opt_get_bool(opts, "copy-on-read", false);
 
     file = qemu_opt_get(opts, "file");
-- 
1.8.3.1

Re: [Qemu-devel] [PATCH] block: do not allow read-only=on and snapshot=on to be used together

[Eric Blake]

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On 01/14/2014 12:12 PM, Jeff Cody wrote:
> Having both read-only=on and snapshot=on together does not make sense;
> currently, the read-only argument is effectively ignored for the
> temporary snapshot.  To prevent confusion, disallow the usage of both
> 'snapshot=on' and 'read-only=on'.
> 
> Signed-off-by: Jeff Cody <jcody@redhat.com>
> ---
>  blockdev.c | 7 +++++++
>  1 file changed, 7 insertions(+)

Reviewed-by: Eric Blake <eblake@redhat.com>

No impact to libvirt, which (intentionally) doesn't use snapshot=on.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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Re: [Qemu-devel] [PATCH] block: do not allow read-only=on and snapshot=on to be used together

[Stefan Hajnoczi]

On Tue, Jan 14, 2014 at 02:12:19PM -0500, Jeff Cody wrote:
> Having both read-only=on and snapshot=on together does not make sense;
> currently, the read-only argument is effectively ignored for the
> temporary snapshot.  To prevent confusion, disallow the usage of both
> 'snapshot=on' and 'read-only=on'.
> 
> Signed-off-by: Jeff Cody <jcody@redhat.com>
> ---
>  blockdev.c | 7 +++++++
>  1 file changed, 7 insertions(+)

Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan

Re: [Qemu-devel] [PATCH] block: do not allow read-only=on and snapshot=on to be used together

[Kevin Wolf]

Am 14.01.2014 um 20:12 hat Jeff Cody geschrieben:
> Having both read-only=on and snapshot=on together does not make sense;
> currently, the read-only argument is effectively ignored for the
> temporary snapshot.  To prevent confusion, disallow the usage of both
> 'snapshot=on' and 'read-only=on'.
> 
> Signed-off-by: Jeff Cody <jcody@redhat.com>

I believe the reason why this was allowed was so that you can use a
read-only file with -snapshot. It might not be necessary any more since
I switched -snapshot implementation to modify the options QDict instead
of manually doing a second bdrv_open().

Did you test that this still works now?

The other question is about this code in bdrv_open_flags():

    /*
     * Snapshots should be writable.
     */
    if (bs->is_temporary) {
        open_flags |= BDRV_O_RDWR;
    }

Is this dead code now because the flag is always already set?

Kevin

Re: [Qemu-devel] [PATCH] block: do not allow read-only=on and snapshot=on to be used together

[Jeff Cody]

On Thu, Jan 16, 2014 at 10:39:30AM +0100, Kevin Wolf wrote:
> Am 14.01.2014 um 20:12 hat Jeff Cody geschrieben:
> > Having both read-only=on and snapshot=on together does not make sense;
> > currently, the read-only argument is effectively ignored for the
> > temporary snapshot.  To prevent confusion, disallow the usage of both
> > 'snapshot=on' and 'read-only=on'.
> > 
> > Signed-off-by: Jeff Cody <jcody@redhat.com>
> 
> I believe the reason why this was allowed was so that you can use a
> read-only file with -snapshot. It might not be necessary any more since
> I switched -snapshot implementation to modify the options QDict instead
> of manually doing a second bdrv_open().
> 
> Did you test that this still works now?
> 

Yes, that still works.

> The other question is about this code in bdrv_open_flags():
> 
>     /*
>      * Snapshots should be writable.
>      */
>     if (bs->is_temporary) {
>         open_flags |= BDRV_O_RDWR;
>     }
> 
> Is this dead code now because the flag is always already set?
>

Yes, that ends up being dead code.  From later in blockdev_init():

    bdrv_flags |= ro ? 0 : BDRV_O_RDWR;
    
    QINCREF(bs_opts);
    ret = bdrv_open(dinfo->bdrv, file, bs_opts, bdrv_flags, drv, &error);


And ro is set from the read-only QemuOpts, that we check in this
patch in conjunction with snapshot.  So if read-only=on is not
specified, it is opened r/w by default.

Re: [Qemu-devel] [PATCH] block: do not allow read-only=on and snapshot=on to be used together

[Kevin Wolf]

Am 16.01.2014 um 20:20 hat Jeff Cody geschrieben:
> On Thu, Jan 16, 2014 at 10:39:30AM +0100, Kevin Wolf wrote:
> > Am 14.01.2014 um 20:12 hat Jeff Cody geschrieben:
> > > Having both read-only=on and snapshot=on together does not make sense;
> > > currently, the read-only argument is effectively ignored for the
> > > temporary snapshot.  To prevent confusion, disallow the usage of both
> > > 'snapshot=on' and 'read-only=on'.
> > > 
> > > Signed-off-by: Jeff Cody <jcody@redhat.com>
> > 
> > I believe the reason why this was allowed was so that you can use a
> > read-only file with -snapshot. It might not be necessary any more since
> > I switched -snapshot implementation to modify the options QDict instead
> > of manually doing a second bdrv_open().
> > 
> > Did you test that this still works now?
> 
> Yes, that still works.

Great. :-)

> > The other question is about this code in bdrv_open_flags():
> > 
> >     /*
> >      * Snapshots should be writable.
> >      */
> >     if (bs->is_temporary) {
> >         open_flags |= BDRV_O_RDWR;
> >     }
> > 
> > Is this dead code now because the flag is always already set?
> >
> 
> Yes, that ends up being dead code.  From later in blockdev_init():
> 
>     bdrv_flags |= ro ? 0 : BDRV_O_RDWR;
>     
>     QINCREF(bs_opts);
>     ret = bdrv_open(dinfo->bdrv, file, bs_opts, bdrv_flags, drv, &error);
> 
> 
> And ro is set from the read-only QemuOpts, that we check in this
> patch in conjunction with snapshot.  So if read-only=on is not
> specified, it is opened r/w by default.

Good, that was my impression as well. Can you send a follow-up patch to
remove the dead code?

Kevin

Re: [Qemu-devel] [PATCH] block: do not allow read-only=on and snapshot=on to be used together

[Kevin Wolf]

Am 14.01.2014 um 20:12 hat Jeff Cody geschrieben:
> Having both read-only=on and snapshot=on together does not make sense;
> currently, the read-only argument is effectively ignored for the
> temporary snapshot.  To prevent confusion, disallow the usage of both
> 'snapshot=on' and 'read-only=on'.
> 
> Signed-off-by: Jeff Cody <jcody@redhat.com>

This breaks in a surprising way:

$ softmmu/qemu-system-x86_64 -drive file=~/images/hd.img,snapshot=on
... works fine ...

$ qemu-system-x86_64 -drive file=~/images/hd.img -snapshot
qemu-system-x86_64: invalid option combination: read-only and snapshot

qemu-iotests case 051 catches this. I'll have to remove this patch and
the follow-up from the queue for now.

Kevin

Re: [Qemu-devel] [PATCH] block: do not allow read-only=on and snapshot=on to be used together

[Jeff Cody]

On Fri, Jan 24, 2014 at 02:33:19PM +0100, Kevin Wolf wrote:
> Am 14.01.2014 um 20:12 hat Jeff Cody geschrieben:
> > Having both read-only=on and snapshot=on together does not make sense;
> > currently, the read-only argument is effectively ignored for the
> > temporary snapshot.  To prevent confusion, disallow the usage of both
> > 'snapshot=on' and 'read-only=on'.
> > 
> > Signed-off-by: Jeff Cody <jcody@redhat.com>
> 
> This breaks in a surprising way:
> 
> $ softmmu/qemu-system-x86_64 -drive file=~/images/hd.img,snapshot=on
> ... works fine ...
> 
> $ qemu-system-x86_64 -drive file=~/images/hd.img -snapshot
> qemu-system-x86_64: invalid option combination: read-only and snapshot
> 
> qemu-iotests case 051 catches this. I'll have to remove this patch and
> the follow-up from the queue for now.
>

Odd - OK, I'll follow up on this and submit a series with both patches
(well, likely squashed together), and whatever is needed to fix this
as well.

Re: [Qemu-devel] [PATCH] block: do not allow read-only=on and snapshot=on to be used together

[Kevin Wolf]

Am 24.01.2014 um 14:48 hat Jeff Cody geschrieben:
> On Fri, Jan 24, 2014 at 02:33:19PM +0100, Kevin Wolf wrote:
> > Am 14.01.2014 um 20:12 hat Jeff Cody geschrieben:
> > > Having both read-only=on and snapshot=on together does not make sense;
> > > currently, the read-only argument is effectively ignored for the
> > > temporary snapshot.  To prevent confusion, disallow the usage of both
> > > 'snapshot=on' and 'read-only=on'.
> > > 
> > > Signed-off-by: Jeff Cody <jcody@redhat.com>
> > 
> > This breaks in a surprising way:
> > 
> > $ softmmu/qemu-system-x86_64 -drive file=~/images/hd.img,snapshot=on
> > ... works fine ...
> > 
> > $ qemu-system-x86_64 -drive file=~/images/hd.img -snapshot
> > qemu-system-x86_64: invalid option combination: read-only and snapshot
> > 
> > qemu-iotests case 051 catches this. I'll have to remove this patch and
> > the follow-up from the queue for now.
> >
> 
> Odd - OK, I'll follow up on this and submit a series with both patches
> (well, likely squashed together), and whatever is needed to fix this
> as well.

Jeff, what happened with this? I found the two patches in an old git
branch and wondered why they didn't disappear in the rebase. But
apparently we still allow read-only and snapshot at the same time.

Kevin

Re: [Qemu-devel] [PATCH] block: do not allow read-only=on and snapshot=on to be used together

[Jeff Cody]

On Wed, Mar 12, 2014 at 12:16:04PM +0100, Kevin Wolf wrote:
> Am 24.01.2014 um 14:48 hat Jeff Cody geschrieben:
> > On Fri, Jan 24, 2014 at 02:33:19PM +0100, Kevin Wolf wrote:
> > > Am 14.01.2014 um 20:12 hat Jeff Cody geschrieben:
> > > > Having both read-only=on and snapshot=on together does not make sense;
> > > > currently, the read-only argument is effectively ignored for the
> > > > temporary snapshot.  To prevent confusion, disallow the usage of both
> > > > 'snapshot=on' and 'read-only=on'.
> > > > 
> > > > Signed-off-by: Jeff Cody <jcody@redhat.com>
> > > 
> > > This breaks in a surprising way:
> > > 
> > > $ softmmu/qemu-system-x86_64 -drive file=~/images/hd.img,snapshot=on
> > > ... works fine ...
> > > 
> > > $ qemu-system-x86_64 -drive file=~/images/hd.img -snapshot
> > > qemu-system-x86_64: invalid option combination: read-only and snapshot
> > > 
> > > qemu-iotests case 051 catches this. I'll have to remove this patch and
> > > the follow-up from the queue for now.
> > >
> > 
> > Odd - OK, I'll follow up on this and submit a series with both patches
> > (well, likely squashed together), and whatever is needed to fix this
> > as well.
> 
> Jeff, what happened with this? I found the two patches in an old git
> branch and wondered why they didn't disappear in the rebase. But
> apparently we still allow read-only and snapshot at the same time.
>

The test case 051 failed because when -snapshot was specified, it was
enabled for all the default drives as well, which included
default_cdrom.

The solutions for that seemed a little hacky, and I wasn't sure it was
actually worth it, given that this is probably not a real problem to
begin with.  If you think it is worth it, I can resurrect the patch
and make sure it works with both snapshot=on and -snapshot.