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| <h3 class="section">29.4 How an Application Can Change Persona</h3> |
| |
| <p><a name="index-g_t_0040code_007bsetuid_007d-programs-3301"></a><a name="index-saved-set_002duser_002dID-3302"></a><a name="index-saved-set_002dgroup_002dID-3303"></a><a name="index-g_t_0040code_007b_005fPOSIX_005fSAVED_005fIDS_007d-3304"></a> |
| The ability to change the persona of a process can be a source of |
| unintentional privacy violations, or even intentional abuse. Because of |
| the potential for problems, changing persona is restricted to special |
| circumstances. |
| |
| <p>You can't arbitrarily set your user ID or group ID to anything you want; |
| only privileged processes can do that. Instead, the normal way for a |
| program to change its persona is that it has been set up in advance to |
| change to a particular user or group. This is the function of the setuid |
| and setgid bits of a file's access mode. See <a href="Permission-Bits.html#Permission-Bits">Permission Bits</a>. |
| |
| <p>When the setuid bit of an executable file is on, executing that file |
| gives the process a third user ID: the <dfn>file user ID</dfn>. This ID is |
| set to the owner ID of the file. The system then changes the effective |
| user ID to the file user ID. The real user ID remains as it was. |
| Likewise, if the setgid bit is on, the process is given a <dfn>file |
| group ID</dfn> equal to the group ID of the file, and its effective group ID |
| is changed to the file group ID. |
| |
| <p>If a process has a file ID (user or group), then it can at any time |
| change its effective ID to its real ID and back to its file ID. |
| Programs use this feature to relinquish their special privileges except |
| when they actually need them. This makes it less likely that they can |
| be tricked into doing something inappropriate with their privileges. |
| |
| <p><strong>Portability Note:</strong> Older systems do not have file IDs. |
| To determine if a system has this feature, you can test the compiler |
| define <code>_POSIX_SAVED_IDS</code>. (In the POSIX standard, file IDs are |
| known as saved IDs.) |
| |
| <p>See <a href="File-Attributes.html#File-Attributes">File Attributes</a>, for a more general discussion of file modes and |
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