Jim Meyering wrote:
> 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.
FYI, here's code to generate the latter:
pdwtags src/libvirt_driver_remote_la-remote_protocol.o \
|perl -0777 -n \
-e 'foreach my $p (split m!\n\n/\* \d+ \*/\n!)' \
-e ' { if ($p =~ /^struct remote_/) {' \
-e ' $p =~ s!\t*/\*.*?\*/!!sg;' \
-e ' $p =~ s!\s+\n!\n!sg;' \
-e ' print "$p\n" } }' \
> remote-protocol-structs
IMHO, it would be sufficient to enable a check comparing
this output to a version-controlled reference file, and
making it part of "make check".
Of course, this would add a dependency on pdwtags/dwarves,
but it would be fine/easy to skip the check on non-Linux systems.
Any objection to requiring the dwarves package for development on Linux?
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