It's that time of the year again...
Someone just mailed me about a problem he was having with
unfsd, and
in passing he mentioned that Linux refuses setfsuid(-2).

Unfortunately, unfsd relies on this for root squashing
(unless you've
defined a different anon uid/gid).

The cause of the problem is glibc's move to 32 bit uid_t,
while the kernel remains at 16 bit.  To compensate for this,
the
glibc setfsuid() implementation has some hideously `clever'
code
(sysdeps/unix/sysv/linux/i386/setfsuid.c):

int
setfsuid (uid_t uid)
{
  if (uid != (uid_t) ((__kernel_uid_t) uid))
    {
      __set_errno (EINVAL);
      return -1;
    }

  return INLINE_SYSCALL (setfsuid, 1, uid);
}

It is entirely possible that other applications are affected
by these
symptoms as well, because seteuid exhibits the same
properties.

A patched unfsd (2.2.beta44) will be available shortly from
ftp://linux.mathematik.tu-darmstadt.de/pub/linux/people/okir