Dropping root privileges and still generate coredumps

Question

I've noticed that some of my users didn't get a coredump at all after crashing, even when everything else in their configuration seemed correct.

After reading the core(5) man page a bunch of times I noticed this particular point:

[A core dump file is not produced if] The process is executing a set-user-ID (set-group-ID) program that is owned by a user (group) other than the real user (group) ID of the process.

My daemon isn't setuid root, but in a lot of configurations it's started as root, and if the .conf file specifies a username, it drops privileges, with the usual combination:

setgid(gid);
setuid(uid);

When it does this, coredumps are not generated anymore. Everything else in the environment seems to be correct, removing those calls (and staying as root) gets me coredumps as usual.

I tried to change the "real" uid/gid as the man page seemed to suggest, by calling the less-portable setresgid/uid which seem to be suggested often to drop privileges permanently:

setresgid(gid, gid, gid);
setresuid(uid, uid, uid);

I expected that to solve the problem but... it didn't improve at all. Still no coredumps.

Sooo... now what?

---

Test code:

#include <stdlib.h>

int main(int argc, char **argv) {
        if (argc > 1) {
                setgid(atoi(argv[2]));
                setuid(atoi(argv[1]));
        }
        abort();
}

Usage:

• ./a.out as any user to just abort without setgid/setuid
• ./a.out 1000 100 (where 1000 is uid and 100 is gid) as root to drop privileges and watch coredumps not happen.
• Bonus unintentional feature: pass one parameter, not two, to get SIGSEGV instead of SIGABRT.

I've already tested this in arch linux, centos 6.5 and openbsd 5.3

Answer 1

To force a process to always core dump use the prctl system call.

prctl(PR_SET_DUMPABLE, 1, 0, 0, 0);

Answer 2

You have to enable core dumps for applications that have their privileges changed:

echo 2 > /proc/sys/fs/suid_dumpable

I advice you to put that in /etc/rc.local.

---

For example, here's what I have there:

# This is to enable debugging as a normal user, rather than root
sysctl kernel.yama.ptrace_scope=0

# This is a convenient core file pattern 
# '%e' is the name of the process
# '%p' is the pid of process
sysctl kernel.core_pattern="/tmp/core.%e.%p"

# Enable dump for processes with lowered privileges
echo 2 > /proc/sys/fs/suid_dumpable

# Remove limit for the size of core files
ulimit -c unlimited

---

Edit:

You can take a look at this neat library, that allows you to manually write core dumps to a custom file : https://code.google.com/p/google-coredumper/

I believe it's exactly what you need.