Re: [libvirt] locking down struct size/layout in remote-protocol.x
On 05/06/2010 12:35 PM, Jim Meyering wrote:
This week we noticed that a change to struct remote_error
was causing trouble (new member addition changed the size of
the object being decoded -- bad for the protocol).
In order to ensure that no such changes sneak in again,
I'm planning to do the following.
pdwtags src/libvirt_driver_remote_la-remote_protocol.o
prints, among other things, detailed type info for every struct:
/* 89 */
struct remote_nonnull_domain {
remote_nonnull_string name; /* 0 8 */
remote_uuid uuid; /* 8 16 */
int id; /* 24 4 */
/* size: 32, cachelines: 1, members: 3 */
/* last cacheline: 32 bytes */
/* BRAIN FART ALERT! 32 != 28 + 0(holes), diff = 4 */
}; /* size: 32 */
Ouch. Architecture sizing plays a role. On a 32-bit machine, the first
struct is:
/* 86 */
struct remote_nonnull_domain {
remote_nonnull_string name; /* 0 4 */
remote_uuid uuid; /* 4 16 */
int id; /* 20 4 */
/* size: 24, cachelines: 1, members: 3 */
/* last cacheline: 24 bytes */
}; /* size: 24 */
Are we sure migration between 32-bit and 64-bit hypervisors works? And
if it does, then these structs don't quite match what is actually sent
over the wire. At any rate,
And here's a sample of its output:
verify (sizeof (struct remote_nonnull_domain) == 32);
we'd have to make this output conditional on sizeof(void*) to be
portable to both 32- and 64- bit machines.
--
Eric Blake eblake(a)redhat.com +1-801-349-2682
Libvirt virtualization library http://libvirt.org
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