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root can kill any program run by any user. Is it possible for a user to kill another user's program without using sudo, etc.?

Example: How can make user foo kill program x which is run by user bar?

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Is there any reason why you want to do this? –  Nils Oct 7 '11 at 20:48
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up vote 7 down vote accepted

This is from the kill(2) manpage:

For a process to have permission to send a signal it must either be privileged (under Linux: have the CAP_KILL capability), or the real or effective user ID of the sending process must equal the real or saved set-user-ID of the target process. In the case of SIGCONT it suffices when the send‐ ing and receiving processes belong to the same session.

So essentially, under normal circumstances, if you are not root (and have no way to become root or that other user, via sudo or setuid programs), you cannot kill another user's process.

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It's only possible for root and bar to kill a process owned by bar. Sudo/setuid programs change foo's privileges to those of root.

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