Re: [libvirt] locking down struct size/layout in remote-protocol.x
Eric Blake wrote:
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:
There's a way out.
/* 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,
Not a problem. We don't send pointers over the wire.
The types are designed to be used portably.
> 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.
Right. There are a couple options:
- maintain two sets of sizeof-verifying "verify" directives,
one for 32-bit, the other for 64-bit. With this, the verification
is performed at compile time and requires no special tools, in general.
However, in the unusual event that we update remote_protocol.x and
need to regenerate the directives, we'll have to run pdwtags on both
i686 and x86_64.
- maintain a textual representation of these structs and their members
(including type names, but not sizes) and make pdwtags a requirement
for running "make syntax-check", which would perform the verification.
I.e., keep a copy of the output of pdwtags, but without the comments.