EUID != root + EGID = root, and CAP_SETGID

[Ivan Zahariev <famzah@icdsoft.com>]

Hi guys,

Currently, if a process is started with EUID which is non-root, and EGID 
which IS root (for example by set-group-ID file permission + file group 
owner "root", or an account in /etc/passwd with group=0), then the 
processes is not granted CAP_SETGID.

As a result, such a process cannot change its EGID to an arbitrary one, 
even though the current EGID is the super-user "root" one. Therefore, 
such a process cannot easily drop its EGID "root" privileges to non-root 
ones, for security reasons.

This is not the case if the process starts with EUID=0. Then the 
processes is granted *both* CAP_SETUID and CAP_SETGID.

Is this an intended behavior? Shouldn't a process which is started with 
EGID=0 get CAP_SETGID too?

Thank you.

Best regads,
Ivan Zahariev

P.S. For more detailed info: 
http://blog.famzah.net/2009/12/11/linux-non-root-user-processes-which-run-with-group-root-cannot-change-their-process-group-to-an-arbitrary-one/