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| <h4 class="subsection">24.6.3 Permission for using <code>kill</code></h4> |
| |
| <p>There are restrictions that prevent you from using <code>kill</code> to send |
| signals to any random process. These are intended to prevent antisocial |
| behavior such as arbitrarily killing off processes belonging to another |
| user. In typical use, <code>kill</code> is used to pass signals between |
| parent, child, and sibling processes, and in these situations you |
| normally do have permission to send signals. The only common exception |
| is when you run a setuid program in a child process; if the program |
| changes its real UID as well as its effective UID, you may not have |
| permission to send a signal. The <code>su</code> program does this. |
| |
| <p>Whether a process has permission to send a signal to another process |
| is determined by the user IDs of the two processes. This concept is |
| discussed in detail in <a href="Process-Persona.html#Process-Persona">Process Persona</a>. |
| |
| <p>Generally, for a process to be able to send a signal to another process, |
| either the sending process must belong to a privileged user (like |
| ‘<samp><span class="samp">root</span></samp>’), or the real or effective user ID of the sending process |
| must match the real or effective user ID of the receiving process. If |
| the receiving process has changed its effective user ID from the |
| set-user-ID mode bit on its process image file, then the owner of the |
| process image file is used in place of its current effective user ID. |
| In some implementations, a parent process might be able to send signals |
| to a child process even if the user ID's don't match, and other |
| implementations might enforce other restrictions. |
| |
| <p>The <code>SIGCONT</code> signal is a special case. It can be sent if the |
| sender is part of the same session as the receiver, regardless of |
| user IDs. |
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