rbd: Fix .bdrv_get_specific_info implementation

rbd: Fix .bdrv_get_specific_info implementation

qemu_rbd_get_specific_info() has at least two problems:

The first is that it issues a blocking rbd_read() call in order to probe
the encryption format for the image while querying the node. This means
that if the connection to the server goes down, not only I/O is stuck
(which is unavoidable), but query-names-block-nodes will actually make
the whole QEMU instance unresponsive. .bdrv_get_specific_info
implementations shouldn't perform blocking operations, but only return
what is already known.

The second is that the information returned isn't even correct. If the
image is already opened with encryption enabled at the RBD level, we'll
probe for "double encryption", i.e. if the encrypted data contains
another encryption header. If it doesn't (which is the normal case), we
won't return the encryption format. If it does, we return misleading
information because it looks like we're talking about the outer level
(the encryption format of the image itself) while the information is
about an encryption header in the guest data.

Fix this by storing the encryption format in BDRVRBDState when the image
is opened (and we do blocking operations anyway) and returning only the
stored information in qemu_rbd_get_specific_info().

The information we'll store is either the actual encryption format that
we enabled on the RBD level, or if the image is unencrypted, the result
of the same probing as we previously did when querying the node. Probing
image formats based on content that can be modified by the guest has
long been known as problematic, but as long as we only output it to the
user instead of making decisions based on it, it should be okay. It is
undoubtedly useful in the context of 'qemu-img info' when you're trying
to figure out which encryption options you have to use to open the
image successfully.

Fixes: 42e4ac9ef5a6 ("block/rbd: Add support for rbd image encryption")
Buglink: https://issues.redhat.com/browse/RHEL-105440
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20250811134010.81787-1-kwolf@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>