Frustrated with capabilities..

[Markku Savela <msa@moth.iki.fi>]

I just want to run an exectable with limited capabilities and assumed
the following approach would work fine:

 1) fork process
 2) in child

    2.1 set current capabilities (eip) using cap_set_proc
    2.2 execve the executable.

But it frigging does not work! Just before the execve, the result of
cap_to_text is

    = cap_net_bind_service+eip

but, in the execve executable, the result is suddenly

    = cap_net_bind_service+i

Why does the execve clear the effective and permitted capabities,
against my clear instructions? (I also have the prctl KEEP_CAPS set,
though in this case it should be irrelevant).

- The kernel is from ubuntu distro, 2.6.24.

- the executable *does* *not* have any setuid/setgid bits

- the upcoming file capabities will not be any help, because I will
  need to start the same executable with different capabilities
  depending on context.

If this is not a bug in kernel, it is a misdesign, which makes the
obvious use of cap_set_proc rather useless...

-- 
Markku Savela