Switch user without creating an intermediate process

Question

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().

Answer 1

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.

Answer 2

I do not share the observation and the conclusions. See below:

I created two shellscripts:

$ cat just_sudo.sh
#!/bin/bash
sudo sleep inf
$ cat exec_sudo.sh
#!/bin/bash
exec sudo sleep inf

So, one with an exec, one without. If I do a pstree to see the starting situation, I get:

$ pstree $$
bash───pstree
$ echo $$
17250

This gives me the baseline. Next I launched both scripts:

$ bash just_sudo.sh &
[1] 1218
$ bash exec_sudo.sh &
[2] 1220

And then, pstree gives:

$ pstree $$
bash─┬─bash───sleep
     ├─pstree
     └─sleep

the first being the just_sudo, the second is the exec_sudo. Both run as root:

$ ps -ef | grep sleep
root      1219  1218  0 14:01 pts/4    00:00:00 sleep inf
root      1220 17250  0 14:01 pts/4    00:00:00 sleep inf

once again the first is the just_sudo and the second the exec_sudo. You can see that the parent-PID for the sleep in the exec_sudo is the interactive shell from which the scripts are launched and the PID is 1220, which was the PID we saw when the script was launched in the background.

If you use two terminal windows and do not put it in the background, this will work also:

terminal 1                            terminal 2
$ echo $$
16053                                 $ pstree 16053
                                      bash
$ sudo sleep inf
                                      $ pstree 16053
                                      bash───sleep
^C
$ exec sudo sleep inf
                                      $ pstree 16053
                                      sleep
^C
 ( window is closed )

So, on my linux system, the behavior is not as you suggest.The only way that the sudo may remain in the process-tree is if it runs in the existing tty (so without an exec), or if it is invoked with a pseudo-terminal, for example as exec sudoedit.