livepatch: Handle arbitrary size names with the list operation

livepatch: Handle arbitrary size names with the list operation

The payloads' name strings can be of arbitrary size (typically small
with an upper bound of XEN_LIVEPATCH_NAME_SIZE).
Current implementation of the list operation interface allows to copy
names in the XEN_LIVEPATCH_NAME_SIZE chunks regardless of its actual
size and enforces space allocation requirements on userland tools.

To unify and simplify the interface, handle the name strings of
arbitrary size by copying them in adhering chunks to the userland.
In order to let the userland allocate enough space for the incoming
data add an auxiliary interface xc_livepatch_list_get_sizes() that
provides the current number of payload entries and the total size of
all name strings. This is achieved by extending the sysctl list
interface with an extra fields: name_total_size.

The xc_livepatch_list_get_sizes() issues the livepatch sysctl list
operation with the nr field set to 0. In this mode the operation
returns the number of payload entries and calculates the total sizes
for all payloads' names.
When the sysctl operation is issued with a non-zero nr field (for
instance with a value obtained earlier with the prior call to the
xc_livepatch_list_get_sizes()) the new field name_total_size provides
the total size of actually copied data.

Extend the libxc to handle the name back-to-back data transfers.

The xen-livepatch tool is modified to start the list operation with a
call to the xc_livepatch_list_get_sizes() to obtain the actual number
of payloads as well as the necessary space for names.
The tool now always requests the actual number of entries and leaves
the preemption handling to the libxc routine. The libxc still returns
'done' and 'left' parameters with the same semantic allowing the tool
to detect anomalies and react to them. At the moment it is expected
that the tool receives the exact number of entries as requested.
The xen-livepatch tool has been also modified to handle the name
back-to-back transfers correctly.

Signed-off-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Reviewed-by: Andra-Irina Paraschiv <andraprs@amazon.com>
Reviewed-by: Bjoern Doebel <doebel@amazon.de>
Reviewed-by: Martin Pohlack <mpohlack@amazon.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>