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.

remote-protocol-structs