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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 redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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