[libvirt] [PATCH v3] qemu: Pass file descriptor when using TPM passthrough

[Stefan Berger]

On 02/06/2015 01:58 PM, Eric Blake wrote:
> On 02/06/2015 10:49 AM, Daniel P. Berrange wrote:
>
>>> I'm still trying to figure out how virCommandReorderFDs() got into the
>>> picture (I didn't write that section of the code); when I originally
>>> worked on virCommand, the only way to pass fds to the child was in
>>> direct positions (same fd in child as in parent), not by remapping one
>>> fd in the parent to another position in the child, except for the three
>>> standard descriptors.  It looks like virCommandReorderFDs was added to
>>> allow remapping other fds and populates the LISTEN_FDS environment
>>> variable with how many fds were thus mapped.  So the two approaches
>>> don't really mix.  Do we ever use virCommandPassListenFDs() on a
>>> virCommand that will also do raw fd passthrough?  Maybe the real thing
>>> to do is to track at virCommand build-up time that only one of the two
>>> passthrough methods can be used, and fail loudly if a programmer tries
>>> to do both methods at once.
>> Or instead of virCommandPassFD() which only takes a source FD number,
>> we could have added a virComandPassFDRemap which takes a source and
>> target FD number. That way we don't have a global "reorder FDs" concept
>> that can break in unexpected ways - we would only ever re-order FDs for
>> which an explicitly target FD was requested.
> But then we have to be careful that there are no collisions between
> inherited-in-place and reordered fds.  My argument is that reordering
> ALWAYS ensures we don't have to worry about collisions between different
> registration styles, because we no longer have different registration
> styles.
>
>>> Or maybe we should ALWAYS prepare to remap fds in the child, so that the
>>> child receives fds in contiguous order with no gaps.  It might simplify
>>> the code base to always reorder things, and have the mapping done up
>>> front.  That is, change the virCommandPassFD() function to return an
>>> integer of which next consecutive fd the child will see, or -1 on
>>> failure.  Callers that then need to alter the command line of the child
>>> will have to pay attention to the return value (something a bit
>>> different than most virCommand build-up, which intentionally defer error
>>> checking to the very end), but it might be worth it.
>> This might be simplest way to go - I'm just wondering if it will cause
>> us any other type of fun problems
> And it sounds like Stefan is looking to me to play with it and see what
> breaks.  Oh well, I guess I walked into that one :)
>
Well, we can think it through.

Are the following changes sufficient or is there much more to it?

1) reorder the FDs all the time ; the function is invoked in virExec and 
we can move the call up 2 lines out of the if () branch.
2a) modify the return value as shown in the other email and propagate 
into virCommandPassFD and return an int here now that can be ignored by 
callers
   OR
2b) possibly have another function that lets one determine the fd the 
child will see given a passed fd

3) possibly adapt test cases...

    Stefan