users.texi

一个C源代码分析器

@node Users and Groups, System Information, Job Control, Top
@chapter Users and Groups

Every user who can log in on the system is identified by a unique number
called the @dfn{user ID}.  Each process has an effective user ID which
says which user's access permissions it has.

Users are classified into @dfn{groups} for access control purposes.  Each
process has one or more @dfn{group ID values} which say which groups the
process can use for access to files.

The effective user and group IDs of a process collectively form its
@dfn{persona}.  This determines which files the process can access.
Normally, a process inherits its persona from the parent process, but
under special circumstances a process can change its persona and thus
change its access permissions.

Each file in the system also has a user ID and a group ID.  Access
control works by comparing the user and group IDs of the file with those
of the running process.

The system keeps a database of all the registered users, and another
database of all the defined groups.  There are library functions you
can use to examine these databases.

@menu
* User and Group IDs::          Each user has a unique numeric ID;
				 likewise for groups.
* Process Persona::             The user IDs and group IDs of a process.
* Why Change Persona::          Why a program might need to change
				 its user and/or group IDs.
* How Change Persona::          Changing the user and group IDs.
* Reading Persona::             How to examine the user and group IDs.

* Setting User ID::             Functions for setting the user ID.
* Setting Groups::              Functions for setting the group IDs.

* Enable/Disable Setuid::       Turning setuid access on and off.
* Setuid Program Example::      The pertinent parts of one sample program.
* Tips for Setuid::             How to avoid granting unlimited access.

* Who Logged In::               Getting the name of the user who logged in,
				 or of the real user ID of the current process.

* User Database::               Functions and data structures for
                        	 accessing the user database.
* Group Database::              Functions and data structures for
                        	 accessing the group database.
* Database Example::            Example program showing use of database
				 inquiry functions.
@end menu

@node User and Group IDs
@section User and Group IDs

@cindex login name
@cindex user name
@cindex user ID
Each user account on a computer system is identified by a @dfn{user
name} (or @dfn{login name}) and @dfn{user ID}.  Normally, each user name
has a unique user ID, but it is possible for several login names to have
the same user ID.  The user names and corresponding user IDs are stored
in a data base which you can access as described in @ref{User Database}.

@cindex group name
@cindex group ID
Users are classified in @dfn{groups}.  Each user name also belongs to
one or more groups, and has one @dfn{default group}.  Users who are
members of the same group can share resources (such as files) that are
not accessible to users who are not a member of that group.  Each group
has a @dfn{group name} and @dfn{group ID}.  @xref{Group Database},
for how to find information about a group ID or group name.

@node Process Persona
@section The Persona of a Process
@cindex persona
@cindex effective user ID
@cindex effective group ID

@c !!! bogus; not single ID.  set of effective group IDs (and, in GNU,
@c set of effective UIDs) determines privilege.  lying here and then
@c telling the truth below is confusing.
At any time, each process has a single user ID and a group ID which
determine the privileges of the process.  These are collectively called
the @dfn{persona} of the process, because they determine ``who it is''
for purposes of access control.  These IDs are also called the
@dfn{effective user ID} and @dfn{effective group ID} of the process.

Your login shell starts out with a persona which consists of your user
ID and your default group ID.  
@c !!! also supplementary group IDs.
In normal circumstances, all your other processes inherit these values.

@cindex real user ID
@cindex real group ID
A process also has a @dfn{real user ID} which identifies the user who
created the process, and a @dfn{real group ID} which identifies that
user's default group.  These values do not play a role in access
control, so we do not consider them part of the persona.  But they are
also important.

Both the real and effective user ID can be changed during the lifetime
of a process.  @xref{Why Change Persona}.

@cindex supplementary group IDs
In addition, a user can belong to multiple groups, so the persona
includes @dfn{supplementary group IDs} that also contribute to access
permission.

For details on how a process's effective user IDs and group IDs affect
its permission to access files, see @ref{Access Permission}.

The user ID of a process also controls permissions for sending signals
using the @code{kill} function.  @xref{Signaling Another Process}.

@node Why Change Persona
@section Why Change the Persona of a Process?

The most obvious situation where it is necessary for a process to change
its user and/or group IDs is the @code{login} program.  When
@code{login} starts running, its user ID is @code{root}.  Its job is to
start a shell whose user and group IDs are those of the user who is
logging in.  (To accomplish this fully, @code{login} must set the real
user and group IDs as well as its persona.  But this is a special case.)

The more common case of changing persona is when an ordinary user
program needs access to a resource that wouldn't ordinarily be
accessible to the user actually running it.

For example, you may have a file that is controlled by your program but
that shouldn't be read or modified directly by other users, either
because it implements some kind of locking protocol, or because you want
to preserve the integrity or privacy of the information it contains.
This kind of restricted access can be implemented by having the program
change its effective user or group ID to match that of the resource.

Thus, imagine a game program that saves scores in a file.  The game
program itself needs to be able to update this file no matter who is
running it, but if users can write the file without going through the
game, they can give themselves any scores they like.  Some people
consider this undesirable, or even reprehensible.  It can be prevented
by creating a new user ID and login name (say, @code{games}) to own the
scores file, and make the file writable only by this user.  Then, when
the game program wants to update this file, it can change its effective
user ID to be that for @code{games}.  In effect, the program must
adopt the persona of @code{games} so it can write the scores file.

@node How Change Persona
@section How an Application Can Change Persona
@cindex @code{setuid} programs

The ability to change the persona of a process can be a source of
unintentional privacy violations, or even intentional abuse.  Because of
the potential for problems, changing persona is restricted to special
circumstances.

You can't arbitrarily set your user ID or group ID to anything you want;
only privileged processes can do that.  Instead, the normal way for a
program to change its persona is that it has been set up in advance to
change to a particular user or group.  This is the function of the setuid
and setgid bits of a file's access mode.  @xref{Permission Bits}.

When the setuid bit of an executable file is set, executing that file
automatically changes the effective user ID to the user that owns the
file.  Likewise, executing a file whose setgid bit is set changes the
effective group ID to the group of the file.  @xref{Executing a File}.
Creating a file that changes to a particular user or group ID thus
requires full access to that user or group ID.

@xref{File Attributes}, for a more general discussion of file modes and
accessibility.

A process can always change its effective user (or group) ID back to its
real ID.  Programs do this so as to turn off their special privileges
when they are not needed, which makes for more robustness.

@c !!! talk about _POSIX_SAVED_IDS

@node Reading Persona
@section Reading the Persona of a Process

Here are detailed descriptions of the functions for reading the user and
group IDs of a process, both real and effective.  To use these
facilities, you must include the header files @file{sys/types.h} and
@file{unistd.h}.
@pindex unistd.h
@pindex sys/types.h

@comment sys/types.h
@comment POSIX.1
@deftp {Data Type} uid_t
This is an integer data type used to represent user IDs.  In the GNU
library, this is an alias for @code{unsigned int}.
@end deftp

@comment sys/types.h
@comment POSIX.1
@deftp {Data Type} gid_t
This is an integer data type used to represent group IDs.  In the GNU
library, this is an alias for @code{unsigned int}.
@end deftp

@comment unistd.h
@comment POSIX.1
@deftypefun uid_t getuid (void)
The @code{getuid} function returns the real user ID of the process.
@end deftypefun

@comment unistd.h
@comment POSIX.1
@deftypefun gid_t getgid (void)
The @code{getgid} function returns the real group ID of the process.
@end deftypefun

@comment unistd.h
@comment POSIX.1
@deftypefun uid_t geteuid (void)
The @code{geteuid} function returns the effective user ID of the process.
@end deftypefun

@comment unistd.h
@comment POSIX.1
@deftypefun gid_t getegid (void)
The @code{getegid} function returns the effective group ID of the process.
@end deftypefun

@comment unistd.h
@comment POSIX.1
@deftypefun int getgroups (int @var{count}, gid_t *@var{groups})
The @code{getgroups} function is used to inquire about the supplementary
group IDs of the process.  Up to @var{count} of these group IDs are
stored in the array @var{groups}; the return value from the function is
the number of group IDs actually stored.  If @var{count} is smaller than
the total number of supplementary group IDs, then @code{getgroups}
returns a value of @code{-1} and @code{errno} is set to @code{EINVAL}.

If @var{count} is zero, then @code{getgroups} just returns the total
number of supplementary group IDs.  On systems that do not support
supplementary groups, this will always be zero.

Here's how to use @code{getgroups} to read all the supplementary group
IDs:

@smallexample
@group
gid_t *
read_all_groups (void)
@{
  int ngroups = getgroups (NULL, 0);
  gid_t *groups
    = (gid_t *) xmalloc (ngroups * sizeof (gid_t));
  int val = getgroups (ngroups, groups);
  if (val < 0)
    @{
      free (groups);
      return NULL;
    @}
  return groups;
@}
@end group
@end smallexample
@end deftypefun

@node Setting User ID
@section Setting the User ID

This section describes the functions for altering the user ID (real
and/or effective) of a process.  To use these facilities, you must
include the header files @file{sys/types.h} and @file{unistd.h}.
@pindex unistd.h
@pindex sys/types.h

@comment unistd.h
@comment POSIX.1
@deftypefun int setuid (uid_t @var{newuid})
This function sets both the real and effective user ID of the process
to @var{newuid}, provided that the process has appropriate privileges.
@c !!! also sets saved-id

If the process is not privileged, then @var{newuid} must either be equal
to the real user ID or the saved user ID (if the system supports the
@code{_POSIX_SAVED_IDS} feature).  In this case, @code{setuid} sets only
the effective user ID and not the real user ID.
@c !!! xref to discussion of _POSIX_SAVED_IDS

The @code{setuid} function returns a value of @code{0} to indicate
successful completion, and a value of @code{-1} to indicate an error.
The following @code{errno} error conditions are defined for this
function:

@table @code
@item EINVAL
The value of the @var{newuid} argument is invalid.

@item EPERM
The process does not have the appropriate privileges; you do not
have permission to change to the specified ID.
@end table
@end deftypefun

@comment unistd.h
@comment BSD
@deftypefun int setreuid (uid_t @var{ruid}, uid_t @var{euid})
This function sets the real user ID of the process to @var{ruid} and the
effective user ID to @var{euid}.  If @var{ruid} is @code{-1}, it means
not to change the real user ID; likewise if @var{euid} is @code{-1}, it
means not to change the effective user ID.

The @code{setreuid} function exists for compatibility with 4.3 BSD Unix,
which does not support saved IDs.  You can use this function to swap the
effective and real user IDs of the process.  (Privileged processes are
not limited to this particular usage.)  If saved IDs are supported, you
should use that feature instead of this function.  @xref{Enable/Disable
Setuid}.

The return value is @code{0} on success and @code{-1} on failure.
The following @code{errno} error conditions are defined for this
function:

@table @code
@item EPERM
The process does not have the appropriate privileges; you do not
have permission to change to the specified ID.
@end table
@end deftypefun

@node Setting Groups
@section Setting the Group IDs

This section describes the functions for altering the group IDs (real
and effective) of a process.  To use these facilities, you must include
the header files @file{sys/types.h} and @file{unistd.h}.
@pindex unistd.h
@pindex sys/types.h

@comment unistd.h
@comment POSIX.1
@deftypefun int setgid (gid_t @var{newgid})