Apr 30, · While performing under-privileged work, the effective UID is changed to some lower privilege value, and the euid is saved to saved userID (suid), so that it can be used for switching back to a privileged account when the task is completed. You can print UID by simply typing id on terminal: # id. Sets the real, effective, or saved set user IDs (UIDs) for the current process to uid. If uid is the same as the real UID or the saved set-user-ID of the process, setuid() always succeeds and sets the effective UID. the real user ID and saved set-user-ID will remain unchanged.
First, in order to really understand how this works, we need to step back for a moment and talk about the Unix user ID concept. The kernel needs to be fast and robust, and data structures better be small, and moving around strings is anything but efficient. So, each user name and group name is mapped to a unique unsigned number, called user and group ID for short, or uid and gid.
Each Unix process has effechive user ID and a group ID associated with it, and when trying to open a file for writing, for instance, these IDs are used to determine whether the process should be granted access or how to organize a event. These IDs constitute the effective privilege of the process, because they determine what a process can do and what it cannot.
Most of the time, these IDs will be referred to as the effective uid and gid. What happens when you invoke the passwd utility is that the effective uid of the process is set to 0, i. So this begs the question, how does it know who invoked it? These IDs are used to track who a user really id, i. This uid value is not changed when you invoke programs such as passwd. So the program simply needs to find out what user name corresponds to its real uid, and refuse to change any other account.
Things start to get interesting when you invoke a setuid application, however. Now you invoke a setuid root application. The real user ID, however, remains unchanged. This allows the application to learn the identity of the user who invoked it, and to continue to access files etc with the privilege of the invoking user. I have to confess that for the sake of simplicity, I have so far avoided to talk about an additional bunch of group IDs a process usually lugs around; these are called the supplementary gids.
A user can be a member of several groups at the same time, for instance a software engineer working on project Fourtytwo may be a effectivw of both the eng and fourtytwo groups at the same time. You are commenting using your WordPress.
You what is a collagen mask commenting using your Google account. You are commenting using your Twitter account. You are commenting using your Facebook account. Notify me of new comments via email. Notify me of new posts via email. Intelligea learn more. Skip to content First, in order to really understand how this works, we need to step back for a moment and talk about the Unix user ID concept.
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So, the real user id is who you really are (the one who owns the process), and the effective user id is what the operating system looks at to make a decision whether or not you are allowed to do something (most of the time, there are some exceptions). Your user ID helps protect the confidentiality of your Account and enables us to verify your identity. Follow these tips to create a good user ID: Create a user ID that others can't guess but is easy for you to remember Use a combination of letters and numbers (e.g., jim14my or my2dog5is). Feb 11, · Because it’s setuid root, the operating system will set the the effective user ID of the process to that of the root user (0). The real user ID, however, remains unchanged. This allows the application to learn the identity of the user who invoked it, and to continue to access files etc with the privilege of the invoking user.
Join Stack Overflow to learn, share knowledge, and build your career. Connect and share knowledge within a single location that is structured and easy to search. I am already aware of the real user id. It is the unique number for a user in the system. And what is the use of effective user id and saved user id and where do we use them in the system? The distinction between a real and an effective user id is made because you may have the need to temporarily take another user's identity most of the time, that would be root , but it could be any user.
If you only had one user id, then there would be no way of changing back to your original user id afterwards other than taking your word for granted, and in case you are root , using root 's privileges to change to any user. So, the real user id is who you really are the one who owns the process , and the effective user id is what the operating system looks at to make a decision whether or not you are allowed to do something most of the time, there are some exceptions.
When you log in, the login shell sets both the real and effective user id to the same value your real user id as supplied by the password file. Now, it also happens that you execute a setuid program, and besides running as another user e.
How does this work? After executing the setuid program, it will have your real id since you're the process owner and the effective user id of the file owner for example root since it is setuid. The program does whatever magic it needs to do with superuser privileges and then wants to do something on your behalf.
That means, attempting to do something that you shouldn't be able to do should fail. How does it do that? Well, obviously by changing its effective user id to the real user id! Now that setuid program has no way of switching back since all the kernel knows is your id and Bang, you're dead.
Every process has an owner and belongs to a group. In our shell, every process that we'll now run will inherit the privileges of my user account and will run with the same UID and GID.
You can see that the owner and the group of the file are root. This is because the ping command needs to open up a socket and the Linux kernel demands root privilege for that. This is a special permission bit for specific binary executable files like ping and sudo which is known as setuid.
The kernel makes the decision whether this process has the privilege by looking on the EUID of the process. Because now the EUID points to root , the operation won't be rejected by the kernel. Notice : On latest Linux releases the output of the ping command will look different because of the fact that they adopted the Linux Capabilities approach instead of this setuid approach - for those who are not familiar - read here.
The Saved user ID SUID is being used when a privileged process is running as root for example and it needs to do some unprivileged tasks. This is how I understand it. The file an user executes equivalent to starting a process will have a RUID equal to that user's id. Important thing to note here is that the uid which created a file is not the same as the uid that executes the file.
They can be the same or different. When a file has the setuid bit on it, whenever an uid executes that file, that uid will temporary be replaced with the file owner's uid. So, if we have a file owned by uid and has the setuid bit on it, whenever uid executes that file, that file will be executed with the uid Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Learn more. Asked 5 years, 7 months ago. Active 1 month ago. Viewed 74k times. Improve this question. Matthias Braun FYI - there is also the file system user id, as outlined on the Wikipedia page: en. I think he didn't mention it because from your wiki link : "Since kernel 2.
Add a comment. Active Oldest Votes. This is what the saved set-user id is for. Improve this answer. Toby Speight Damon Damon For more clarity on that last point about saved-set user id, see Wikipedia. Can you point me to some readings where I can find which sys call check the Real uid instead?
That's Also fork does not check, but can fail if you reach the maximum process quota on the real UID. Google with site:man7. DanielFarrell it depends on how ping is implemented to use ICMP, tcp or udp, for icmp it requires the raw sockets and the privileges — Walid Jan 25 at I'll try to explain step by step with some examples. We'll focus on those. Search for the binary location with the which command then run ls -la : -rwsr-xr-x 1 root root Mar 10 ping You can see that the owner and the group of the file are root.
But how can I use ping if I don't have root privilege? Notice the 's' letter instead of 'x' in the owner part of the file permission. This is all done by the simple fact that this file has the setuid bit. RtmY RtmY 6, 6 6 gold badges 49 49 silver badges 59 59 bronze badges. Clear answer except for the last para on SUID. Got confused with privileged and privileged tasks. Useful if an example is provided. Normally this is not changed, and only root can change it. As an example for that, think about that init process forks your login shell.
During forks, the shell would have root's RUID because the shell's parent is init. So, we can say RUID is of process owner. Sign up or log in Sign up using Google.
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