Re: [Qemu-devel] [PATCH v3 1/1] block: pass the right options for BlockDriver.bdrv_open()

[Max Reitz]

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 <address@hidden>
> Signed-off-by: Dong Jia Shi <address@hidden>
> ---
>  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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