really - gain privilege or run commands a different user
really [options] [command args... ]
really checks whether the caller is allowed, and if it is it changes its uids and gids (and perhaps root directory) according to the command line options and executes the specified command.
If no options are specified, the uid will be set to 0 and the gids and root directory will be left unchanged.
If no command is specified, really will run $SHELL -i.
A caller is allowed if it has write access to /etc/inittab and is also member of the group root. This is most easily achieved by making inittab group-writeable by some suitable group containing all the appropriate users, and making /etc/inittab group-owned by that group and group-writeable. The root group is perhaps a good choice if it isn’t being used for anything else.
|-u username | --user username|
|Sets the uid, gid, and supplementary group list, according to username’s entry in the password and group databases.|
|-i username | --useronly username|
|Sets only the uid according to username’s entry in the password database.|
|-I uid | --uidonly uid|
|Sets the uid to the numeric value uid (which need not correspond to any existing user in the password database).|
|-g groupname | --group groupname|
|groupname is looked up in the group database and its gid is appended to the process’s supplementary groups list. If this is the first gid specified it will also be set as the primary gid.|
|-G gid | --gid gid|
|gid is appended to the process’s supplementary groups list. (gid need not correspond to any existing group in the group database.) If this is the first gid specified it will also be set as the primary gid.|
|-z | --groupsclear|
|Clears the process’s supplementary groups list. When using this option you must also specify -g or -G. The process’s groups will then be exactly those specified. The relative position of -z in the argument list is not relevant.|
|-R root-dir | --chroot root-dir|
|The program will have its root directory set to root-dir.
Do not use this option unless you know what you are doing: Unlike chroot(8), the current working directory will remain unchanged. This means that if the current directory isn’t underneath the specified new root, the program will still be able to access files outside the new root by using relative pathnames. If this isn’t what you want, please use the chroot utility instead.
|--||Indicates the end of the options. The next argument (if present) will be interpreted as the command name, even if it starts with a hyphen.|
really is designed so that installing it setuid root is extremely unlikely to compromise the security of any system. It will check using access(2) whether the real user is allowed to write to /etc/inittab and if this check fails really will exit without even attempting to parse its command line.
really is not designed to be resistant to malicious command line arguments. Do not allow untrusted processes to pass options to really, or to specify the command to be run. Whether it is safe to allow relatively untrusted processes to pass options to the command which is to be run depends on the behaviour of that command and its security status.
Attempting to use really to drop privilege is dangerous unless the calling environment is very well understood. There are many inherited process properties and resources which might be used by the callee to escalate its privilege to that of the (root-equivalent) caller. For this function, it is usually better to use userv if possible.
really does not manipulate the environment at all. The calling program is run in exactly the same environment as the caller passes to really. In particular, really will not add sbin directories to PATH so really-enabled accounts will usually need to have these directories on their configured PATH to start with.
SHELL is used to find the default shell to use in interactive mode (ie, when no command is specified).
This version of really was written by Ian Jackson <ian>.
It and this manpage are Copyright (C) 1992-5,2004,2013 Ian Jackson <ian>.
really is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.
really is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this file; if not, consult the Free Software Foundation’s website at www.fsf.org, or the GNU Project website at www.gnu.org.
really is currently part of chiark-utils and is available for download from ftp.chiark.greenend.org.uk in /users/ian/chiark-utils/, in source and pre-compiled binary form, and also from Ian Jackson’s cvsweb.