Understanding-Linux: Process

Hi Y’ALL, glad to meet you in this blog post. This post will cover the basic concepts on process mainly in linux system and we will have an hands-on with linux process and how it works. Understanding the concepts of Operating System will help in developing robust & perfomance-oriented applications. It also helps in penetesting (cybersecurity).

What is a process?

I hate definition but no other go buddy :) . Process is a program in execution. Then you may have a question, what is a program?. In this context, a program is an executable file. In every operating system, there is a format for a file to be executable.

• Windows - PE format

• Linux - ELF format

• Mac - MACH-O

The executable files should be in one of the format, respective to the operating system.

Parent & Child

Process ID aka PID is used to identify the running process. For example , in a linux machine, ps is the command used to find all the running processes.

pwn@ubuntu:~$ ps auxUSER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMANDroot 1 0.0 0.4 169020 8496 ? Ss Aug01 0:14 /sbin/init auto nopromptroot 2 0.0 0.0 0 0 ? S Aug01 0:00 [kthreadd]root 3 0.0 0.0 0 0 ? I< Aug01 0:00 [rcu_gp]root 4 0.0 0.0 0 0 ? I< Aug01 0:00 [rcu_par_gp]root 6 0.0 0.0 0 0 ? I< Aug01 0:00 [kworker/0:0H-kblockd]root 9 0.0 0.0 0 0 ? I< Aug01 0:00 [mm_percpu_wq]root 10 0.0 0.0 0 0 ? S Aug01 0:02 [ksoftirqd/0]root 11 0.0 0.0 0 0 ? I Aug01 0:04 [rcu_sched]root 12 0.0 0.0 0 0 ? S Aug01 0:00 [migration/0]root 13 0.0 0.0 0 0 ? S Aug01 0:00 [idle_inject/0]root 14 0.0 0.0 0 0 ? S Aug01 0:00 [cpuhp/0]root 15 0.0 0.0 0 0 ? S Aug01 0:00 [kdevtmpfs]root 16 0.0 0.0 0 0 ? I< Aug01 0:00 [netns]root 17 0.0 0.0 0 0 ? S Aug01 0:00 [rcu_tasks_kthre]root 18 0.0 0.0 0 0 ? S Aug01 0:00 [kauditd]root 19 0.0 0.0 0 0 ? S Aug01 0:00 [khungtaskd]root 20 0.0 0.0 0 0 ? S Aug01 0:00 [oom_reaper]root 21 0.0 0.0 0 0 ? I< Aug01 0:00 [writeback]root 22 0.0 0.0 0 0 ? S Aug01 0:01 [kcompactd0]root 23 0.0 0.0 0 0 ? SN Aug01 0:00 [ksmd]root 24 0.0 0.0 0 0 ? SN Aug01 0:00 [khugepaged]root 116 0.0 0.0 0 0 ? I< Aug01 0:00 [kintegrityd]root 117 0.0 0.0 0 0 ? I< Aug01 0:00 [kblockd]root 118 0.0 0.0 0 0 ? I< Aug01 0:00 [blkcg_punt_bio]root 119 0.0 0.0 0 0 ? I< Aug01 0:00 [tpm_dev_wq]root 120 0.0 0.0 0 0 ? I< Aug01 0:00 [ata_sff]root 121 0.0 0.0 0 0 ? I< Aug01 0:00 [md]root 122 0.0 0.0 0 0 ? I< Aug01 0:00 [edac-poller]root 123 0.0 0.0 0 0 ? I< Aug01 0:00 [devfreq_wq]root 124 0.0 0.0 0 0 ? S Aug01 0:00 [watchdogd]root 127 0.0 0.0 0 0 ? S Aug01 0:07 [kswapd0]root 128 0.0 0.0 0 0 ? S Aug01 0:00 [ecryptfs-kthrea]root 131 0.0 0.0 0 0 ? I< Aug01 0:00 [kthrotld]..............................................................................................

I have just mentioned a few process in the ps command output above but we will get a lot. Whoa wait!, How does these many process are there in the first place?. For that, we will go to its root. When you boot into your linux, the kernel is the one first to be loaded, after finishing all the loadings of drivers and kernel modules it will spawn the systemd(or init) process. The systemd process is the first process that will be executed. The systemd process will start to run all the configured daemons(usually process names ends with d, example : ssh -> sshd ) by creating a child process from systemd process for individual daemons. Well, we can visualize this process forking as a tree.

pstree - process visualisation tool

pwn@ubuntu:~$ pstree -psystemd(1)─┬─ModemManager(747)─┬─{ModemManager}(761) │ └─{ModemManager}(765) ├─NetworkManager(670)─┬─{NetworkManager}(718) │ └─{NetworkManager}(736) ├─VGAuthService(1959) ├─accounts-daemon(657)─┬─{accounts-daemon}(663) │ └─{accounts-daemon}(732) ├─acpid(658) ├─avahi-daemon(661)───avahi-daemon(719) ├─bluetoothd(662) ├─colord(4313)─┬─{colord}(4314) │ └─{colord}(4316) ├─containerd(71023)─┬─containerd-shim(74834)─┬─sh(74860) │ │ ├─{containerd-shim}(74835) │ │ ├─{containerd-shim}(74836) │ │ ├─{containerd-shim}(74837) │ │ ├─{containerd-shim}(74838) │ │ ├─{containerd-shim}(74839) │ │ ├─{containerd-shim}(74840) │ │ ├─{containerd-shim}(74841) │ │ ├─{containerd-shim}(74843) │ │ └─{containerd-shim}(74883) │ ├─{containerd}(71031) │ ├─{containerd}(71032) │ ├─{containerd}(71033) │ ├─{containerd}(71034) │ ├─{containerd}(71035) │ ├─{containerd}(71037) │ ├─{containerd}(71040) │ ├─{containerd}(71041) │ └─{containerd}(74696) ├─cron(667) ├─cups-browsed(69655)─┬─{cups-browsed}(69722) │ └─{cups-browsed}(69723) ├─cupsd(69648) ├─dbus-daemon(669) ├─dockerd(73293)─┬─{dockerd}(73305) │ ├─{dockerd}(73306) │ ├─{dockerd}(73307) │ ├─{dockerd}(73308) │ ├─{dockerd}(73313) │ ├─{dockerd}(73314) │ ├─{dockerd}(73318) │ ├─{dockerd}(74542) │ └─{dockerd}(74543) ├─gdm3(3982)─┬─gdm-session-wor(4451)─┬─gdm-x-session(4516)─┬─Xorg(4525)───{Xorg}(4565) │ │ │ ├─gnome-session-b(4568)─┬─ssh-agent(4662) │ │ │ │ ├─{gnome-session-b}(4682) │ │ │ │ └─{gnome-session-b}(4683) │ │ │ ├─{gdm-x-session}(4524) │ │ │ └─{gdm-x-session}(4564) │ │ ├─{gdm-session-wor}(4452) │ │ └─{gdm-session-wor}(4453) │ ├─{gdm3}(3983) │ └─{gdm3}(3984) ├─gnome-keyring-d(4475)─┬─{gnome-keyring-d}(4476) │ ├─{gnome-keyring-d}(4477) │ └─{gnome-keyring-d}(4736) ├─ibus-daemon(4680)─┬─ibus-engine-sim(4769)─┬─{ibus-engine-sim}(4770) │ │ └─{ibus-engine-sim}(4772) │ ├─ibus-extension-(4702)─┬─{ibus-extension-}(4717) │ │ ├─{ibus-extension-}(4718) │ │ └─{ibus-extension-}(4783) │ ├─ibus-memconf(4695)─┬─{ibus-memconf}(4701) │ │ └─{ibus-memconf}(4705) │ ├─ibus-ui-gtk3(4696)─┬─{ibus-ui-gtk3}(4721) │ │ ├─{ibus-ui-gtk3}(4722) │ │ └─{ibus-ui-gtk3}(4726) │ ├─{ibus-daemon}(4689) │ └─{ibus-daemon}(4690) ├─ibus-x11(4706)─┬─{ibus-x11}(4713) │ └─{ibus-x11}(4714) ├─kerneloops(829) ├─kerneloops(836) ├─networkd-dispat(689) ├─polkitd(690)─┬─{polkitd}(694) │ └─{polkitd}(733) ├─rsyslogd(696)─┬─{rsyslogd}(720) │ ├─{rsyslogd}(721) │ └─{rsyslogd}(722) ├─rtkit-daemon(4072)─┬─{rtkit-daemon}(4073) │ └─{rtkit-daemon}(4074) ├─snapd(59489)─┬─{snapd}(59498) │ ├─{snapd}(59499) │ ├─{snapd}(59500) │ ├─{snapd}(59501) │ ├─{snapd}(59505) │ ├─{snapd}(59506) │ ├─{snapd}(59507) │ ├─{snapd}(59560) │ └─{snapd}(60102) ├─switcheroo-cont(701)─┬─{switcheroo-cont}(712) │ └─{switcheroo-cont}(734) .............................................................................

As you can see from the ouput of pstree, the systemd is the first process that has pid 1 (but i told lies ,haha! there will be a process with pid 0 that takes care of process scheduling so for now we dont consider the pid 0 process) and then we have ModemManager(747),NetworkManager(670),accounts-daemon(657), etc are child processes of systemd and these child processes will create other child processes also. If we see bash i.e linux terminal, it will have the following visualisation

pwn@ubuntu:~$ pstree | grep bash | |-gnome-terminal--+-bash---bash-+-grep

We can see the flow for bash process as systemd->gnome-terminal->bash->grep(the command that is executed , the grep part of pstree). And there comes the concept of forking.

fork vs exec

There are two significant syscalls(syscalls are the one that connects userspace program and the kernel), fork and exec to create child process. The fork syscall will create a clone of the parent process and both, the parent and child, will have the same virtual address space whereas in exec the parent process will be replaced by the child process and parent process will get control only after child process finishes its execution.

example for fork call

pwn@ubuntu:~$ cat forkpython.py import osdef child(): print('\nA new child ', os.getpid()) input("in child") os._exit(0) def parent(): while True: newpid = os.fork() if newpid == 0: child() else: pids = (os.getpid(), newpid) print("parent: %d, child: %d\n" % pids) reply = input("q for quit / c for new fork") if reply == 'c': continue else: breakparent()pwn@ubuntu:~$ python3 forkpython.py parent: 82224, child: 82225q for quit / c for new forkA new child 82225in child

To see the virtual address space, we can use procfs information. Find the process id of both parent and child using ps command. Use diff to see any changes in virtual address space

pwn@ubuntu:~$ ps aux | grep forkpython.pypwn 82224 0.0 0.4 26092 8628 pts/2 S+ 09:36 0:00 python3 forkpython.pypwn 82225 0.0 0.3 26092 6300 pts/2 S+ 09:36 0:00 python3 forkpython.pypwn 82263 0.0 0.0 17668 732 pts/5 S+ 09:40 0:00 grep --color=auto forkpython.pypwn@ubuntu:~$ diff /proc/82224/maps /proc/82225/mapspwn@ubuntu:~$

As you can see, there are two process that runs python3 forkpython.py with PID’s 82224 and 82225. In this, one of them will be parent and other will be child. The /proc/PID/maps pseudo file will contain the memory map for the process that is what are all the memory regions are used by the process. And there is no output for the diff command, because both the parent and the child will have the same memory map.

Let’s see in the case of exec,

pwn@ubuntu:~$ cat execpython.py #!/usr/bin/pythonimport osargs=['']input("press enter")os.execvp("/bin/sh",args)pwn@ubuntu:~$ python execpython.py press enter

In new tab, execute ps

pwn@ubuntu:~$ ps aux | grep execpython.pypwn 82461 0.0 0.3 24308 6912 pts/2 S+ 09:50 0:00 python execpython.pypwn 82486 0.0 0.0 17668 732 pts/1 S+ 09:51 0:00 grep --color=auto execpython.py

The pid for the process is 82461 . Now press enter in the previous tab where the program was executed and run the tail command and see the ps command output

pwn@ubuntu:~$ python execpython.py press enter$ tail -f /etc/hosts127.0.0.1 localhost127.0.1.1 ubuntu# The following lines are desirable for IPv6 capable hosts::1 ip6-localhost ip6-loopbackfe00::0 ip6-localnetff00::0 ip6-mcastprefixff02::1 ip6-allnodesff02::2 ip6-allrouters_

pwn@ubuntu:~$ ps aux | grep 82461pwn 82461 0.0 0.0 16744 520 pts/2 S+ 09:55 0:00 tail -f /etc/hostspwn 82572 0.0 0.0 17664 732 pts/1 S+ 09:56 0:00 grep --color=auto 82461

Now you can see that the previous python execpython.py has been replace by tail command but having the same PID. So , exec does not create new process but it overwrites the old process with the new one.

PCB

Process Control Blocks aka PCB is a kernel data structure that maintains information on the processes that is of any state (state can be running,queued etc). Some of the data structures of the PCB are accessible through procfs. procfs is a pesudo file system. Below explains the procfs.

procfs can be found at /proc directory. To find the information about the process, we should find its PID from ps command.

For more read the man pages procfs