Re: ERROR: The effective user ID of staprun must be set to the root user.

[Roland McGrath <roland at redhat dot com>]

Hmm.  No clues here, sorry.  I can think of two approaches.

One is to debug the exec path in the kernel and track down
what is or isn't happening differently.  Look at current->cred
and figure out where those values change, or where the new
struct gets installed.  (I presume you'd do this with stap.)

The other is to strip down staprun/stapio, or build up your test program,
until you hit the difference.  Clearly it's doing something other than what
your test program does, since it behaves differently.  It might be that it
has used some syscalls that set some magic bit in the kernel affecting
later setuid.  Narrow down by process of elimination.