Date: Sun, 17 Apr 2011 13:07:22 -0500
From: "Serge E. Hallyn" <serge@hallyn.com>
To: crocket <crockabiscuit@gmail.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Linux capabilities shouldn't be lost during setuid to non-root
 from root or to another non-root uid from a non-root uid.
Message-ID: <20110417180722.GA21112@hallyn.com>
References: <BANLkTikTxCFHs2xb4b5QFQp3SJAEjgyRJg@mail.gmail.com>
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You need

	prctl(PR_SET_KEEPCAPS, 1);

man prctl for details.  In particular, see the following example:

#include <stdio.h>
#include <sys/prctl.h>
#include <sys/capability.h>

int main()
{
	cap_t c;
	c = cap_get_proc();
	printf("uid %d, current caps: %s\n", getuid(), cap_to_text(c, NULL));
	cap_free(c);
	prctl(PR_SET_KEEPCAPS, 1);
	setuid(1001, 1001, 1001);
	c = cap_get_proc();
	printf("uid %d, current caps: %s\n", getuid(), cap_to_text(c, NULL));
	cap_free(c);
}

You can either run that as root, or you can do

	sudo setcap cap_setuid,cap_setgid,cap_sys_admin=eip captest

and then run it as non-root.

HTH,
-serge
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