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Switch user without creating an intermediate process (3)

I'm able to use sudo or su to execute a command as another user. By combining with exec, I'm able to replace the current process with sudo or su, and a child process running the command. But I want to replace the current process with the command running as another user. How do I do that?

Testing with sleep inf as the command, and someguy as the user:

exec su someguy -c 'sleep inf'

This gives me from pstree:

bash───su───sleep

And

exec sudo -u someguy sleep inf

gives

bash───sudo───sleep

In both cases I just want the sleep command, with bash as the parent.

I expect I could do this from C with something some sequence of setuid() and exec().


In order to free a command, you have to give him std io:

For this,you could either close all stdin, stdout and stderr or let them point elsewhere.

Try this:

su - someguy -c 'exec nohup sleep 60 >/tmp/sleep.log 2>/tmp/sleep.err <<<"" &'

Note:

su - someguy -c 'exec nohup sleep 60 &'

Is enough, and

su - someguy -c 'exec sleep 60 >/tmp/sleep.log 2>/tmp/sleep.err <<<"" &'

Will work too.

Consider having a look at man nohup

Note 2: Under bash, you could use:

su - someguy -c 'exec sleep 60 & disown -h'

... And read help disown or man bash.

Little demo showing how to close all IOs:

su - someguy -c 'exec 0<&- ; exec 1>&- ; exec 2>&- ; exec sleep 60 &'

quick test:

pstree $(ps -C sleep ho pid)
sleep

I am not sure if this can be done using sudo or su. But you can easily achieve this using a simple c program. I will be showing a very bare minimal one with hard coded command and user id, but you can always customize it to your liking

test.c

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <stddef.h>

 int runAs(int gid, int uid, char *command[]) {
    setgid(gid);
    setuid(uid);
    char *args[]={"sleep","inf",NULL};
    execvp(args[0],args);
 }


 int main(int argc, char *argv[] )
 {
     runAs(1000, 65534, argv);
     return 0;
 }

Note: on my machine 1000 is the uid/gid of vagrant user and group. 65534 is uid and gid of nobody user and group

build.sh

#!/bin/bash

sudo gcc test.c -o sosu
sudo chown root:root sosu
sudo chmod u+s sosu

Now time for a test

$ pstree $$ -p
bash(23251)───pstree(28627)

$ ./sosu

Now from another terminal

$ pstree -p 23251
bash(23251)───sleep(28687)

$ ps aux | grep [2]8687
nobody   28687  0.0  0.0   7288   700 pts/0    S+   11:40   0:00 sleep inf

As you can see the process is run as nobody and its a child of bash


The difference between sudo sleep and exec sudo sleep is that in the second command sudo process replaces bash image and calling shell process exits when sleep exits

pstree -p $$
bash(8765)───pstree(8943)

((sleep 1; pstree -p $$ )&); sudo -u user sleep 2
bash(8765)───sudo(8897)───sleep(8899)

((sleep 1; pstree -p $$ )&); exec sudo -u user sleep 2
sudo(8765)───sleep(8993)

however the fact that sudo or su fork a new process depends on design and their implementation (some sources found here).

From sudo man page :

Process model

When sudo runs a command, it calls fork(2), sets up the execution environment as described above, and calls the execve system call in the child process. The main sudo process waits until the command has completed, then passes the command's exit status to the security policy's close function and exits. If an I/O logging plugin is config- ured or if the security policy explicitly requests it, a new pseudo-terminal (“pty”) is created and a second sudo process is used to relay job control signals between the user's existing pty and the new pty the command is being run in. This extra process makes it possible to, for example, suspend and resume the command. Without it, the com- mand would be in what POSIX terms an “orphaned process group” and it would not receive any job control signals. As a special case, if the policy plugin does not define a close function and no pty is required, sudo will execute the command directly instead of calling fork(2) first. The sudoers policy plugin will only define a close function when I/O logging is enabled, a pty is required, or the pam_session or pam_setcred options are enabled. Note that pam_session and pam_setcred are enabled by default on sys- tems using PAM.





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