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.