CODEWITHSUNDEEP
c++coredump
Nico
Nico

Reputation: 1545

Coredump file not generated when changing user

The following code generates coredump file:

#include <iostream>
#include <string>
#include <pwd.h>
#include <grp.h>
#include <sys/resource.h>

int main() {
    int b = 0;
    int a = 140/b;

    return 0;
}

Output: Floating point exception (core dumped)

Coredump is generated in /opt/cores

$ ls -al /opt/cores
total 188
drwxrwxrwx  2 root root   4096 Jan 13 16:46 .
drwxr-xr-x 28 root root   4096 Jan 12 11:57 ..
-rw-------  1 root root 344064 Jan 13 16:46 core.prueba.6776.8

However, this doesn't generate coredump file:

#include <iostream>
#include <string>
#include <pwd.h>
#include <grp.h>
#include <sys/resource.h>

int main() {
    std::string usr = "nobody";
    std::string grp = "oinstall";

    group* gp = getgrnam(grp.data());
    passwd* user = getpwnam(usr.data());
    if (gp && user && setgid(gp->gr_gid) == 0 && setuid(user->pw_uid) == 0) {
        std::cout << "changed!" << std::endl;
    } else {
        std::cout << "not changed =(" << std::endl;
    }   
    struct rlimit rlim;
    rlim.rlim_cur = RLIM_INFINITY;
    rlim.rlim_max = RLIM_INFINITY;
    if (setrlimit(RLIMIT_CORE, &rlim) != 0) {
        std::cout << "setrlimit error" << std::endl;
    }   

    getrlimit(RLIMIT_CORE, &rlim);

    std::cout << "rlim_cur: " << (int)rlim.rlim_cur <<", rlim_max: " << (int)rlim.rlim_max << std::endl;
    int b = 0;
    int a = 140/b;

    return 0;
}

Output: 

changed!
rlim_cur: -1, rlim_max: -1
Floating point exception

I've run the first piece of code with the changed user and it generates the coredump file, so the directory has the right permissions. The problem is when I change user in the code. Any clues?

This happens on Linux (CentOS 6, CentOS 7, RHEL 6).

In Solaris works fine.

Upvotes: 0

Views: 203

Answers (2)

Nico
Nico

Reputation: 1545

nos was right about the security mechanism.

The solution was to add this after changing user:

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

Now the coredump is generated.

Upvotes: 0

nos
nos

Reputation: 229108

The setuid() manpage has this note:

If uid is different from the old effective UID, the process will be forbidden from leaving core dumps.

This is a security mechanism, you can read more as to why here

You need to enable the fs.suid_dumpable to have your process coredump by doing:

 sysctl -w fs.suid_dumpable=2

Upvotes: 2

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