Re: [Qemu-devel] [RFC V3 6/7] block: Create authorizations mechanism for external snapshots.

["Benoît Canet" <benoit.canet@irqsave.net>]

Le Wednesday 04 Dec 2013 à 14:12:19 (+0800), Fam Zheng a écrit :
> On 2013年12月04日 13:20, Benoît Canet wrote:
> >Le Wednesday 04 Dec 2013 à 11:47:22 (+0800), Fam Zheng a écrit :
> >>On 2013年12月03日 21:26, Benoît Canet wrote:
> >>>---
> >>>  block.c                   | 64 +++++++++++++++++++++++++++++++++++++++++------
> >>>  block/blkverify.c         |  2 +-
> >>>  include/block/block.h     | 16 +++++++++---
> >>>  include/block/block_int.h |  9 ++++---
> >>>  4 files changed, 75 insertions(+), 16 deletions(-)
> >>>
> >>>diff --git a/block.c b/block.c
> >>>index 8016ff2..0569cb2 100644
> >>>--- a/block.c
> >>>+++ b/block.c
> >>>@@ -4945,21 +4945,69 @@ int bdrv_amend_options(BlockDriverState *bs, QEMUOptionParameter *options)
> >>>      return bs->drv->bdrv_amend_options(bs, options);
> >>>  }
> >>>
> >>>-ExtSnapshotPerm bdrv_check_ext_snapshot(BlockDriverState *bs)
> >>>+/* will be used to recurse on single child block filter until first format
> >>>+ * (single child block filter will store their child in bs->file)
> >>>+ */
> >>>+ExtSnapshotPerm bdrv_generic_check_ext_snapshot(BlockDriverState *bs,
> >>>+                                                BlockDriverState *candidate)
> >>>  {
> >>>-    if (bs->drv->bdrv_check_ext_snapshot) {
> >>>-        return bs->drv->bdrv_check_ext_snapshot(bs);
> >>>+    if (!bs->drv) {
> >>>+        return EXT_SNAPSHOT_FORBIDDEN;
> >>>      }
> >>>
> >>>-    if (bs->file && bs->file->drv && bs->file->drv->bdrv_check_ext_snapshot) {
> >>>-        return bs->file->drv->bdrv_check_ext_snapshot(bs);
> >>>+    if (!bs->drv->authorizations[BS_CANT_SNAPSHOT]) {
> >>
> >>This double negative feels hard to read for me.
> >>
> >>>+        if (bs == candidate) {
> >>>+             return EXT_SNAPSHOT_ALLOWED;
> >>>+        } else {
> >>>+             return EXT_SNAPSHOT_FORBIDDEN;
> >>>+        }
> >>>      }
> >>>
> >>>-    /* external snapshots are allowed by default */
> >>>-    return EXT_SNAPSHOT_ALLOWED;
> >>>+    if (!bs->drv->authorizations[BS_FILTER_PASS_DOWN]) {
> >>>+        return EXT_SNAPSHOT_FORBIDDEN;
> >>>+    }
> >>>+
> >>>+    if (!bs->file) {
> >>>+        return EXT_SNAPSHOT_FORBIDDEN;
> >>>+    }
> >>>+
> >>>+    return bdrv_recurse_check_ext_snapshot(bs->file, candidate);
> >>>  }
> >>>
> >>>-ExtSnapshotPerm bdrv_check_ext_snapshot_forbidden(BlockDriverState *bs)
> >>>+ExtSnapshotPerm bdrv_recurse_check_ext_snapshot(BlockDriverState *bs,
> >>>+                                                BlockDriverState *candidate)
> >>>  {
> >>>+    if (bs->drv && bs->drv->bdrv_check_ext_snapshot) {
> >>>+        return bs->drv->bdrv_check_ext_snapshot(bs, candidate);
> >>>+    }
> >>
> >>Maybe I'm missing something, but if a driver always returns positive
> >>permit, despite of what candidate is (or even it's relevant to bs),
> >>then doesn't it also affect other devices? because...
> >>
> >>>+
> >>>+    return bdrv_generic_check_ext_snapshot(bs, candidate);
> >>>+}
> >>>+
> >>>+/* This function check if the candidate bs has snapshots authorized by going
> >>>+ * down the forest of bs, skipping filters and stopping on the the first bses
> >>>+ * authorizing snapshots
> >>>+ */
> >>>+ExtSnapshotPerm bdrv_check_ext_snapshot(BlockDriverState *candidate)
> >>>+{
> >>>+    BlockDriverState *bs;
> >>>+
> >>>+    /* walk down the bs forest recursively */
> >>>+    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> >>
> >>this iterates through all the known graph trees (device_list),
> >>instead of limiting to only the device that candidate belongs to.
> >
> >The recursion termination success is candidate == bs.
> >This make sure that the scan of the other tree of the forest will not return any
> >spurious success.
> >
>
> But the "candidate == bs" check is in
> bdrv_generic_check_ext_snapshot, which gets short-circuited by
> driver implementation if the driver implements it, in
> bdrv_recurse_check_ext_snapshot.
>
> So if I have an "always yes" drv->bdrv_check_ext_snapshot and it
> happens to be the first one in bdrv_states, I will allow all
> snapshot operations.
>

My bad I forgot to document the drv_>bdrv_check_ext_snapshot.
It meant to be recursive and only for twisted block filter like this one (quorum):

static ExtSnapshotPerm quorum_check_ext_snapshot(BlockDriverState *bs,
                                                 BlockDriverState *candidate)
{
    BDRVQuorumState *s = bs->opaque;
    int i;

    for (i = 0; i < s->total; i++) {
        ExtSnapshotPerm perm = bdrv_recurse_check_ext_snapshot(s->bs[i],
                                                               candidate);
        if (perm == EXT_SNAPSHOT_ALLOWED) {
            return EXT_SNAPSHOT_ALLOWED;
        }
    }

    return EXT_SNAPSHOT_FORBIDDEN;
}

Maybe the callback needs a serious rename.

Best regards

Benoît

> Fam