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geets
Asked: 2020-04-01 22:41:04 +0800 CST

What determines the user a services started

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I thought, that the service files under /lib/systemd/system/ determine as what user a service is started. But apparently that's not (always) the case. Here's an example:

~# ps faux
...
whoopsie  1445  0.0  0.1 388420 12604 ?        Ssl  08:22   0:00 /usr/bin/whoopsie -f
...

From the above output we see, that the process 'whoopsie' runs under the user 'whoopsie'

Now we look at the startupscript:

~# cat /lib/systemd/system/whoopsie.service

Description=crash report submission daemon
After=network-online.target
Wants=network-online.target

[Service]
Environment="CRASH_DB_URL=https://daisy.ubuntu.com"
ExecStart=/usr/bin/whoopsie -f
Restart=always

[Install]
WantedBy=multi-user.target

In that startup script is no user defined.

So what determines as which user a process is started?

linux service users process systemd

3 Answers

  1. While your observation is correct, it is not due to systemd. Whoopsie is run as root and simply drops its privileges.

    • 3
  2. Best Answer

    Additional info, this whoopsie process changes userid from root to whoopsie after started.

    Source code:

    /* Drop privileges */
if (setgroups (1, &pw->pw_gid) < 0 ||
    setresgid (pw->pw_gid, pw->pw_gid, pw->pw_gid) < 0 ||
    setresuid (pw->pw_uid, pw->pw_uid, pw->pw_uid) < 0) {
    g_set_error (error, g_quark_from_static_string ("whoopsie-quark"), 0,
                 "Failed to become user: %s", username);
    return;
}

    From: https://github.com/pexip/os-whoopsie-daisy/blob/master/src/whoopsie.c#L838

    setresuid is the function to change user

    Reference: http://man7.org/linux/man-pages/man2/setresuid.2.html

    • 2

  3. Please refer user or group if nothing is specified some defaults are applicable. The default is "root".

    • 1