Experimental Physics and
Industrial Control System

Gerry Swislow <[email protected]> · Wed, 27 Jan 2016 21:52:49 -0500

Hi Tech-Talkers,

I'm having a problem with EPICS 3.15 in the "spec" set-user-id-root executable.  

Why do I need a set-user-id-root binary?  Many sites use spec on Linux platforms with PC board motor controllers, counter/timers, GPIB interfaces, etc.  For many years now, spec has eschewed kernel drivers in favor of user-level support.  The user-level support works on all Linux distributions over all iterations of the Linux kernel with no effort needed by the users. (Try that with kernel drivers!)

User level support requires access to I/O ports or on-board memory, which requires super-user privilege.  Thus, in most installations, spec is installed as a set-user-id root program.  The privilege escalation is immediately reset to that of the real user first thing when spec starts, and is only reactivated for the needed system calls, along these lines:

	if (setreuid(-1, 0) == 0) {
		iopl(3);
		setreuid(-1, getuid());
	}

There is no issue with users having root access while running spec with respect to permissions to do anything else that would not normally be permitted.

The problem is that in such an environment, certain library or system calls fail in a strange way, but only when spec is linked with EPICS 3.15.  There is no issue with EPICS 3.14.  When run as a normal user, the following set-user-id-root test program fails with the messages:

 Successfully locked memory using mlockAll
 pthread_create error Resource temporarily unavailable

Of course, if run as root, there is no error.  Using the command "sudo strace -u gerry ./a.out", I see that the error is associated with a mmap() call:

mmap(NULL, 2101248, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK, -1, 0) = -1 EAGAIN (Resource temporarily unavailable)

In spec, the errors occur with shmat(), getpwuid(), which uses mmap(), and malloc().  spec fails for good when malloc() fails.

I should also point out that the EPICS call isn't even needed.  Just the act of linking with the EPICS 3.15 libraries breaks the code.

To be specific, I used EPICS 3.15.3 and EPICS 3.14.12.  I tested on CentOS 7.2 and Linux Mint 17.2, both 64 bits.  And I've spent all day trying to figure this one out.

Test code and a build script follow.  Does anybody have a clue?

Thanks,

Gerry

-------------------------------------
/* file foo.c */#include <cadef.h>

int
main() {
	char    *p;

	setreuid(-1, getuid());
	ca_context_create(ca_disable_preemptive_callback);
}

-------------------------------------
#!/bin/bash

BASE=/usr/local/epics/base-3.14.12
BASE=/usr/local/epics/base-3.15.3

cc -I$BASE/include/os/Linux -I$BASE/include  -I$BASE/include/compiler/gcc \
   foo.c -L$BASE/lib/linux-x86_64 -lca -lCom

sudo chown root a.out
sudo chmod u+s a.out
./a.out

-------------------------------------

-------------------------------------
    Gerry Swislow
    Certified Scientific Software
    PO Box 390640
    Cambridge, MA  02139-0007

  phone:  (617) 576-1610
     fax:  (617) 497-4242
    email:  [email protected]
       web:  http://www.certif.com

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System