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SETUID(2) System Calls Manual SETUID(2) NAME setuid, seteuid, setgid, setegid -- set user and group ID LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <unistd.h> int setuid(uid_t uid); int seteuid(uid_t euid); int setgid(gid_t gid); int setegid(gid_t egid); DESCRIPTION The setuid() system call sets the real and effective user IDs and the saved set-user-ID of the current process to the specified value. The setuid() system call is permitted if the specified ID is equal to the real user ID or the effective user ID of the process, or if the effec- tive user ID is that of the super user. The setgid() system call sets the real and effective group IDs and the saved set-group-ID of the current process to the specified value. The setgid() system call is permitted if the specified ID is equal to the real group ID or the effective group ID of the process, or if the ef- fective user ID is that of the super user. The seteuid() system call (setegid()) sets the effective user ID (group ID) of the current process. The effective user ID may be set to the value of the real user ID or the saved set-user-ID (see intro(2) and execve(2)); in this way, the effective user ID of a set-user-ID exe- cutable may be toggled by switching to the real user ID, then re-en- abled by reverting to the set-user-ID value. Similarly, the effective group ID may be set to the value of the real group ID or the saved set- group-ID. RETURN VALUES Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error. ERRORS The system calls will fail if: [EPERM] The user is not the super user and the ID specified is not the real, effective ID, or saved ID. SEE ALSO getgid(2), getuid(2), issetugid(2), setregid(2), setreuid(2) STANDARDS The setuid() and setgid() system calls are compliant with the ISO/IEC 9945-1:1990 ("POSIX.1") specification with _POSIX_SAVED_IDS not defined with the permitted extensions from Appendix B.4.2.2. The seteuid() and setegid() system calls are extensions based on the POSIX concept of _POSIX_SAVED_IDS, and have been proposed for a future revision of the standard. HISTORY The setuid() function appeared in Version 1 AT&T UNIX. The setgid() function appeared in Version 4 AT&T UNIX. SECURITY CONSIDERATIONS Read and write permissions to files are determined upon a call to open(2). Once a file descriptor is open, dropping privilege does not affect the process's read/write permissions, even if the user ID speci- fied has no read or write permissions to the file. These files nor- mally remain open in any new process executed, resulting in a user be- ing able to read or modify potentially sensitive data. To prevent these files from remaining open after an exec(3) call, be sure to set the close-on-exec flag: void pseudocode(void) { int fd; /* ... */ fd = open("/path/to/sensitive/data", O_RDWR | O_CLOEXEC); if (fd == -1) err(1, "open"); /* ... */ execve(path, argv, environ); } FreeBSD 13.2 December 15, 2015 SETUID(2)
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY | SECURITY CONSIDERATIONS
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