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SETUID(2) FreeBSD 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 effective
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
effective 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
executable may be toggled by switching to the real user ID, then re-
enabled 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)
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
specified has no read or write permissions to the file. These files
normally remain open in any new process executed, resulting in a user
being 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 14.0-RELEASE-p11 December 15, 2015 FreeBSD 14.0-RELEASE-p11