Martin,

The reason why I think there is a possible overflow is that according to
issue 5705, uid/gid overflows are fixed in the following functions:
posix_setegid, posix_setreuid(), posix_setregid(), posix_setgid(). So I
think a similar fix should also be applied to the function posix_lchown.
Or did I misunderstand anything?

And you're right. The previous patch is incorrect. I now submitted
another patch that deals with the *actual* overflow of gid and uid. 

---
Victor,

I agree that all posix_*chown() functions should also be fixed for the
same overflow problem, and it's a good idea to create callback functions
as you described. But if nobody does that, I can at least created more
patches to fix other posix_*chown() functions.