On 29.03.2017 03:16, Dong Jia Shi wrote: > raw_open() expects the caller always passing in the right actual > @options parameter. But when trying to applying snapshot on a RBD > image, bdrv_snapshot_goto() calls raw_open() (by calling the > bdrv_open callback on the BlockDriver) with a NULL @options, and > that will result in a Segmentation fault. > > For the other non-raw format drivers, it also makes sense to passing > in the actual options, althought they don't trigger the problem so > far. > > Let's prepare a @options by adding the "file" key-value pair to a > copy of the actual options that were given for the node (i.e. > bs->options), and pass it to the callback. > > BlockDriver.bdrv_open() expects bs->file to be NULL and just > overwrites it with the result from bdrv_open_child(). If that > bdrv_open_child() fails, the field becomes NULL. While we are at > it, we also correct the cleanning up action for a call failure of > BlockDriver.bdrv_open() by replacing bdrv_unref() with > bdrv_unref_child(). > > Suggested-by: Max Reitz <[email protected]> > Signed-off-by: Dong Jia Shi <[email protected]> > --- > block/snapshot.c | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > > diff --git a/block/snapshot.c b/block/snapshot.c > index bf5c2ca..281626c 100644 > --- a/block/snapshot.c > +++ b/block/snapshot.c > @@ -27,6 +27,7 @@ > #include "block/block_int.h" > #include "qapi/error.h" > #include "qapi/qmp/qerror.h" > +#include "qapi/qmp/qstring.h" > > QemuOptsList internal_snapshot_opts = { > .name = "snapshot", > @@ -189,11 +190,20 @@ int bdrv_snapshot_goto(BlockDriverState *bs, > } > > if (bs->file) { > + QDict *options = qdict_clone_shallow(bs->options); > + QDict *file_options; > + > + qdict_extract_subqdict(options, &file_options, "file."); > + QDECREF(file_options); > + qdict_put(options, "file", > + qstring_from_str(bdrv_get_node_name(bs->file->bs))); > + > drv->bdrv_close(bs); > ret = bdrv_snapshot_goto(bs->file->bs, snapshot_id); > - open_ret = drv->bdrv_open(bs, NULL, bs->open_flags, NULL); > + open_ret = drv->bdrv_open(bs, options, bs->open_flags, NULL); > + QDECREF(options); > if (open_ret < 0) { > - bdrv_unref(bs->file->bs); > + bdrv_unref_child(bs, bs->file); > bs->drv = NULL; > return open_ret; > }
I just noticed another issue (sorry I did not before...):
In drv->bdrv_open(), the block driver will generally overwrite bs->file
without looking at it because it assumes that it's NULL. That means we
should probably actually make sure it's NULL because otherwise we will
the child BDS will have a reference count that is 1 too high.
(bdrv_open_inherit() (ultimately called from bdrv_open_child()) invokes
bdrv_ref() for a child BDS specified by node-name reference.)
That means we should unconditionally invoke
bdrv_unref_child(bs, bs->file) before calling drv->bdrv_open(). But we
have to wrap everything in bdrv_ref()/bdrv_unref() so the BDS isn't
deleted in the meantime.
So I think it should look something like this:
BlockDriverState *file;
QDict *options = ...;
QDict *file_options;
file = bs->file->bs;
/* Prevent it from getting deleted when detached from bs */
bdrv_ref(file);
qdict_extract_subqdict(...);
QDECREF(file_options);
qdict_put(..., qstring_from_str(bdrv_get_node_name(file)));
drv->bdrv_close(bs);
bdrv_unref_child(bs, bs->file);
bs->file = NULL;
ret = bdrv_snapshot_goto(file, snapshot_id);
open_ret = drv->bdrv_open(...);
if (open_ret < 0) {
bdrv_unref(file);
bs->drv = NULL;
return open_ret;
}
assert(bs->file->bs == file);
bdrv_unref(file);
Max
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