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int setuid(uid_t uid);
Under Linux, setuid() is implemented like the POSIX version with the _POSIX_SAVED_IDS feature. This allows a set-user-ID (other than root) program to drop all of its user privileges, do some un-privileged work, and then re-engage the original effective user ID in a secure manner.
If the user is root or the program is set-user-ID-root, special care must be taken. The setuid() function checks the effective user ID of the caller and if it is the superuser, all process related user IDs are set to uid. After this has occurred, it is impossible for the program to regain root privileges.
Thus, a set-user-ID-root program wishing to temporarily drop root privileges, assume the identity of a non-root user, and then regain root privileges afterwards cannot use setuid(). You can accomplish this with the (non-POSIX, BSD) call seteuid().
Tag | Description |
---|---|
EAGAIN | The uid does not match the current uid and uid brings process over its NPROC rlimit. |
EPERM | The user is not privileged (Linux: does not have the CAP_SETUID capability) and uid does not match the real UID or saved set-user-ID of the calling process. |
If uid is different from the old effective uid, the process will be forbidden from leaving core dumps.
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