setuid(1) [php man page]

SETUID(1)						      General Commands Manual							 SETUID(1)

NAME

       setuid - run a command with a different uid.

SYNOPSIS

       setuid username|uid   command [ args ]

DESCRIPTION

       Setuid  changes	user id, then executes the specified command.  Unlike some versions of su(1), this program doesn't ever ask for a password
       when executed with effective uid=root.  This program doesn't change the environment; it only changes the uid and then uses execvp() to find
       the command in the path, and execute it.  (If the command is a script, execvp() passes the command name to /bin/sh for processing.)

       For example,
	      setuid  some_user  $SHELL
       can be used to start a shell running as another user.

       Setuid  is  useful  inside  scripts that are being run by a setuid-root user -- such as a script invoked with super, so that the script can
       execute some commands using the uid of the original user, instead of root.  This allows unsafe commands (such as editors and pagers) to	be
       used  in  a  non-root  mode inside a super script.  For example, an operator with permission to modify a certain protected_file could use a
       super command that simply does:
	      cp protected_file temp_file
	      setuid $ORIG_USER ${EDITOR:-/bin/vi} temp_file
	      cp temp_file protected_file
       (Note: don't use this example directly.	If the temp_file can somehow be replaced by another user, as might be the case if it's kept  in  a
       temporary  directory,  there  will  be a race condition in the time between editing the temporary file and copying it back to the protected
       file.)

AUTHOR

       Will Deich

								       local								 SETUID(1)

SETUID(2)						     Linux Programmer's Manual							 SETUID(2)

NAME

       setuid - set user identity

SYNOPSIS

       #include <sys/types.h>
       #include <unistd.h>

       int setuid(uid_t uid);

DESCRIPTION

       setuid  sets the effective user ID of the current process.  If the effective userid of the caller is root, the real and saved user ID's are
       also set.

       Under Linux, setuid is implemented like the POSIX version with the _POSIX_SAVED_IDS feature.  This allows a setuid (other than  root)  pro-
       gram 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 setuid root, special care must be taken. The setuid function checks the effective uid of the caller
       and if it is the superuser, all process related user ID's are set to uid.  After this has occurred, it is impossible  for  the  program	to
       regain root privileges.

       Thus, a setuid-root program wishing to temporarily drop root privileges, assume the identity of a non-root user, and then regain root priv-
       ileges afterwards cannot use setuid.  You can accomplish this with the (non-POSIX, BSD) call seteuid.

RETURN VALUE

       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS

       EPERM  The user is not the super-user, and uid does not match the real or saved user ID of the calling process.

CONFORMING TO

       SVr4, SVID, POSIX.1.  Not quite compatible with the 4.4BSD call, which sets all of the real, saved, and effective user IDs.  SVr4 documents
       an additional EINVAL error condition.

LINUX-SPECIFIC REMARKS
       Linux  has the concept of filesystem user ID, normally equal to the effective user ID.  The setuid call also sets the filesystem user ID of
       the current process.  See setfsuid(2).

       If uid is different from the old effective uid, the process will be forbidden from leaving core dumps.

SEE ALSO

       getuid(2), setreuid(2), seteuid(2), setfsuid(2)

Linux 1.1.36							    1994-07-29								 SETUID(2)