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Every user who can log in on the system is identified by a unique number called the user ID. Each process has an effective user ID which says which user's access permissions it has.
Users are classified into groups for access control purposes. Each process has one or more 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 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.
Each user account on a computer system is identified by a user name (or login name) and 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 section User Database.
Users are classified in groups. Each user name also belongs to one or more groups, and has one 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 group name and group ID. See section Group Database, for how to find information about a group ID or group name.
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 persona of the process, because they determine "who it is" for purposes of access control. These IDs are also called the effective user ID and 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. In normal circumstances, all your other processes inherit these values.
A process also has a real user ID which identifies the user who created the process, and a 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. See section Why Change the Persona of a Process?.
In addition, a user can belong to multiple groups, so the persona includes 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 section How Your Access to a File is Decided.
The user ID of a process also controls permissions for sending signals
using the kill function. See section Signaling Another Process.
The most obvious situation where it is necessary for a process to change
its user and/or group IDs is the login program. When
login starts running, its user ID is 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, 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, 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 games. In effect, the program must
adopt the persona of games so it can write the scores file.
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. See section The Mode Bits for Access Permission.
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. See section 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.
See section 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.
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 `sys/types.h' and `unistd.h'.
This is an integer data type used to represent user IDs. In the GNU
library, this is an alias for unsigned int.
This is an integer data type used to represent group IDs. In the GNU
library, this is an alias for unsigned int.
The getuid function returns the real user ID of the process.
The getgid function returns the real group ID of the process.
Function: uid_t geteuid (void)
The geteuid function returns the effective user ID of the process.
Function: gid_t getegid (void)
The getegid function returns the effective group ID of the process.
Function: int getgroups (int count, gid_t *groups)
The getgroups function is used to inquire about the supplementary
group IDs of the process. Up to count of these group IDs are
stored in the array groups; the return value from the function is
the number of group IDs actually stored. If count is smaller than
the total number of supplementary group IDs, then getgroups
returns a value of -1 and errno is set to EINVAL.
If count is zero, then 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 getgroups to read all the supplementary group
IDs:
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;
}
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 `sys/types.h' and `unistd.h'.
Function: int setuid (uid_t newuid)
This function sets both the real and effective user ID of the process to newuid, provided that the process has appropriate privileges.
If the process is not privileged, then newuid must either be equal
to the real user ID or the saved user ID (if the system supports the
_POSIX_SAVED_IDS feature). In this case, setuid sets only
the effective user ID and not the real user ID.
The setuid function returns a value of 0 to indicate
successful completion, and a value of -1 to indicate an error.
The following errno error conditions are defined for this
function:
EINVAL
EPERM
Function: int setreuid (uid_t ruid, uid_t euid)
This function sets the real user ID of the process to ruid and the effective user ID to euid.
The 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. See section Enabling and Disabling Setuid Access.
The return value is 0 on success and -1 on failure.
The following errno error conditions are defined for this
function:
EPERM
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 `sys/types.h' and `unistd.h'.
Function: int setgid (gid_t newgid)
This function sets both the real and effective group ID of the process to newgid, provided that the process has appropriate privileges.
If the process is not privileged, then newgid must either be equal
to the real group ID or the saved group ID. In this case, setgid
sets only the effective group ID and not the real group ID.
The return values and error conditions for setgid are the same
as those for setuid.
Function: int setregid (gid_t rgid, fid_t egid)
This function sets the real group ID of the process to rgid and the effective group ID to egid.
The setregid function is provided for compatibility with 4.3 BSD
Unix, which does not support saved IDs. You can use this function to
swap the effective and real group IDs of the process. (Privileged
processes are not limited to this usage.) If saved IDs are supported,
you should use that feature instead of using this function.
See section Enabling and Disabling Setuid Access.
The return values and error conditions for setregid are the same
as those for setreuid.
The GNU system also lets privileged processes change their supplementary
group IDs. To use setgroups or initgroups, your programs
should include the header file `grp.h'.
Function: int setgroups (size_t count, gid_t *groups)
This function sets the process's supplementary group IDs. It can only be called from privileged processes. The count argument specifies the number of group IDs in the array groups.
This function returns 0 if successful and -1 on error.
The following errno error conditions are defined for this
function:
EPERM
Function: int initgroups (const char *user, gid_t gid)
The initgroups function effectively calls setgroups to
set the process's supplementary group IDs to be the normal default for
the user name user. The group ID gid is also included.
A typical setuid program does not need its special access all of the time. It's a good idea to turn off this access when it isn't needed, so it can't possibly give unintended access.
If the system supports the saved user ID feature, you can accomplish
this with setuid. When the game program starts, its real user ID
is jdoe, its effective user ID is games, and its saved
user ID is also games. The program should record both user ID
values once at the beginning, like this:
user_user_id = getuid (); game_user_id = geteuid ();
Then it can turn off game file access with
setuid (user_user_id);
and turn it on with
setuid (game_user_id);
Throughout this process, the real user ID remains jdoe and the
saved user ID remains games, so the program can always set its
effective user ID to either one.
On other systems that don't support the saved user ID feature, you can
turn setuid access on and off by using setreuid to swap the real
and effective user IDs of the process, as follows:
setreuid (geteuid (), getuid ());
This special case is always allowed--it cannot fail.
Why does this have the effect of toggling the setuid access? Suppose a
game program has just started, and its real user ID is jdoe while
its effective user ID is games. In this state, the game can
write the scores file. If it swaps the two uids, the real becomes
games and the effective becomes jdoe; now the program has
only jdoe access. Another swap brings games back to
the effective user ID and restores access to the scores file.
In order to handle both kinds of systems, test for the saved user ID feature with a preprocessor conditional, like this:
#ifdef _POSIX_SAVED_IDS setuid (user_user_id); #else setreuid (geteuid (), getuid ()); #endif
Here's an example showing how to set up a program that changes its effective user ID.
This is part of a game program called caber-toss that
manipulates a file `scores' that should be writable only by the game
program itself. The program assumes that its executable
file will be installed with the set-user-ID bit set and owned by the
same user as the `scores' file. Typically, a system
administrator will set up an account like games for this purpose.
The executable file is given mode 4755, so that doing an
`ls -l' on it produces output like:
-rwsr-xr-x 1 games 184422 Jul 30 15:17 caber-toss
The set-user-ID bit shows up in the file modes as the `s'.
The scores file is given mode 644, and doing an `ls -l' on
it shows:
-rw-r--r-- 1 games 0 Jul 31 15:33 scores
Here are the parts of the program that show how to set up the changed
user ID. This program is conditionalized so that it makes use of the
saved IDs feature if it is supported, and otherwise uses setreuid
to swap the effective and real user IDs.
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
/* Save the effective and real UIDs. */
static uid_t euid, ruid;
/* Restore the effective UID to its original value. */
void
do_setuid (void)
{
int status;
#ifdef _POSIX_SAVED_IDS
status = setuid (euid);
#else
status = setreuid (ruid, euid);
#endif
if (status < 0) {
fprintf (stderr, "Couldn't set uid.\n");
exit (status);
}
}
/* Set the effective UID to the real UID. */
void
undo_setuid (void)
{
int status;
#ifdef _POSIX_SAVED_IDS
status = setuid (ruid);
#else
status = setreuid (euid, ruid);
#endif
if (status < 0) {
fprintf (stderr, "Couldn't set uid.\n");
exit (status);
}
}
/* Main program. */
int
main (void)
{
/* Save the real and effective user IDs. */
ruid = getuid ();
euid = geteuid ();
undo_setuid ();
/* Do the game and record the score. */
...
}
Notice how the first thing the main function does is to set the
effective user ID back to the real user ID. This is so that any other
file accesses that are performed while the user is playing the game use
the real user ID for determining permissions. Only when the program
needs to open the scores file does it switch back to the original
effective user ID, like this:
/* Record the score. */
int
record_score (int score)
{
FILE *stream;
char *myname;
/* Open the scores file. */
do_setuid ();
stream = fopen (SCORES_FILE, "a");
undo_setuid ();
/* Write the score to the file. */
if (stream)
{
myname = cuserid (NULL);
if (score < 0)
fprintf (stream, "%10s: Couldn't lift the caber.\n", myname);
else
fprintf (stream, "%10s: %d feet.\n", myname, score);
fclose (stream);
return 0;
}
else
return -1;
}
It is easy for setuid programs to give the user access that isn't intended--in fact, if you want to avoid this, you need to be careful. Here are some guidelines for preventing unintended access and minimizing its consequences when it does occur:
setuid programs with privileged user IDs such as
root unless it is absolutely necessary. If the resource is
specific to your particular program, it's better to define a new,
nonprivileged user ID or group ID just to manage that resource.
system and exec functions in
combination with changing the effective user ID. Don't let users of
your program execute arbitrary programs under a changed user ID.
Executing a shell is especially bad news. Less obviously, the
execlp and execvp functions are a potential risk (since
the program they execute depends on the user's PATH environment
variable).
If you must exec another program under a changed ID, specify an
absolute file name (see section File Name Resolution) for the executable,
and make sure that the protections on that executable and all
containing directories are such that ordinary users cannot replace it
with some other program.
setuid part of your program needs to access other files
besides the controlled resource, it should verify that the real user
would ordinarily have permission to access those files. You can use the
access function (see section How Your Access to a File is Decided) to check this; it
uses the real user and group IDs, rather than the effective IDs.
You can use the functions listed in this section to determine the login
name of the user who is running a process, and the name of the user who
logged in the current session. See also the function getuid and
friends (see section Reading the Persona of a Process).
The getlogin function is declared in `unistd.h', while
cuserid and L_cuserid are declared in `stdio.h'.
Function: char * getlogin (void)
The getlogin function returns a pointer to a string containing the
name of the user logged in on the controlling terminal of the process,
or a null pointer if this information cannot be determined. The string
is statically allocated and might be overwritten on subsequent calls to
this function or to cuserid.
Function: char * cuserid (char *string)
The cuserid function returns a pointer to a string containing a
user name associated with the effective ID of the process. If
string is not a null pointer, it should be an array that can hold
at least L_cuserid characters; the string is returned in this
array. Otherwise, a pointer to a string in a static area is returned.
This string is statically allocated and might be overwritten on
subsequent calls to this function or to getlogin.
An integer constant that indicates how long an array you might need to store a user name.
These functions let your program identify positively the user who is running or the user who logged in this session. (These can differ when setuid programs are involved; See section The Persona of a Process.) The user cannot do anything to fool these functions.
For most purposes, it is more useful to use the environment variable
LOGNAME to find out who the user is. This is more flexible
precisely because the user can set LOGNAME arbitrarily.
See section Standard Environment Variables.
This section describes all about now to search and scan the database of registered users. The database itself is kept in the file `/etc/passwd' on most systems, but on some systems a special network server gives access to it.
The functions and data structures for accessing the system user database are declared in the header file `pwd.h'.
The passwd data structure is used to hold information about
entries in the system user data base. It has at least the following members:
char *pw_name
char *pw_passwd.
uid_t pw_uid
gid_t pw_gid
char *pw_gecos
char *pw_dir
char *pw_shell
You can search the system user database for information about a
specific user using getpwuid or getpwnam. These
functions are declared in `pwd.h'.
Function: struct passwd * getpwuid (uid_t uid)
This function returns a pointer to a statically-allocated structure
containing information about the user whose user ID is uid. This
structure may be overwritten on subsequent calls to getpwuid.
A null pointer value indicates there is no user in the data base with user ID uid.
Function: struct passwd * getpwnam (const char *name)
This function returns a pointer to a statically-allocated structure
containing information about the user whose user name is name.
This structure may be overwritten on subsequent calls to
getpwnam.
A null pointer value indicates there is no user named name.
This section explains how a program can read the list of all users in the system, one user at a time. The functions described here are declared in `pwd.h'.
The recommended way to scan the users is to open the user file and
then call fgetpwent for each successive user:
Function: struct passwd * fgetpwent (FILE *stream)
This function reads the next user entry from stream and returns a
pointer to the entry. The structure is statically allocated and is
rewritten on subsequent calls to getpwent. You must copy the
contents of the structure if you wish to save the information.
This stream must correspond to a file in the same format as the standard password database file. This function comes from System V.
Another way to scan all the entries in the group database is with
setpwent, getpwent, and endpwent. But this method
is less robust than fgetpwent, so we provide it only for
compatibility with SVID. In particular, these functions are not
reentrant and are not suitable for use in programs with multiple threads
of control.
Function: void setpwent (void)
This function initializes a stream which getpwent uses to read
the user database.
Function: struct passwd * getpwent (void)
The getpwent function reads the next entry from the stream
initialized by setpwent. It returns a pointer to the entry. The
structure is statically allocated and is rewritten on subsequent calls
to getpwent. You must copy the contents of the structure if you
wish to save the information.
Function: void endpwent (void)
This function closes the internal stream used by getpwent.
Function: int putpwent (const struct passwd *p, FILE *stream)
This function writes the user entry *p to the stream
stream, in the format used for the standard user database
file. The return value is zero on success and nonzero on failure.
This function exists for compatibility with SVID. We recommend that you
avoid using it, because it makes sense only on the assumption that the
struct passwd structure has no members except the standard ones;
on a system which merges the traditional Unix data base with other
extended information about users, adding an entry using this function
would inevitably leave out much of the important information.
The function putpwent is declared in `pwd.h'.
This section describes all about how to search and scan the database of registered groups. The database itself is kept in the file `/etc/group' on most systems, but on some systems a special network service provides access to it.
The functions and data structures for accessing the system group database are declared in the header file `grp.h'.
The group structure is used to hold information about an entry in
the system group database. It has at least the following members:
char *gr_name
gid_t gr_gid
char **gr_mem
You can search the group database for information about a specific
group using getgrgid or getgrnam. These functions are
declared in `grp.h'.
Function: struct group * getgrgid (gid_t gid)
This function returns a pointer to a statically-allocated structure
containing information about the group whose group ID is gid.
This structure may be overwritten by subsequent calls to
getgrgid.
A null pointer indicates there is no group with ID gid.
Function: struct group * getgrnam (const char *name)
This function returns a pointer to a statically-allocated structure
containing information about the group whose group name is name.
This structure may be overwritten by subsequent calls to
getgrnam.
A null pointer indicates there is no group named name.
This section explains how a program can read the list of all groups in the system, one group at a time. The functions described here are declared in `grp.h'.
The recommended way to scan the groups is to open the group file and
then call fgetgrent for each successive group:
Function: struct group * fgetgrent (FILE *stream)
The fgetgrent function reads the next entry from stream.
It returns a pointer to the entry. The structure is statically
allocated and is rewritten on subsequent calls to getgrent. You
must copy the contents of the structure if you wish to save the
information.
The stream must correspond to a file in the same format as the standard group database file.
Another way to scan all the entries in the group database is with
setgrent, getgrent, and endgrent. But this method
is less robust than fgetgrent, so we provide it only for
compatibility with SVID. In particular, these functions are not
reentrant and are not suitable for use in programs with multiple threads
of control.
Function: void setgrent (void)
This function initializes a stream for reading from the group data base.
You use this stream by calling getgrent.
Function: struct group * getgrent (void)
The getgrent function reads the next entry from the stream
initialized by setgrent. It returns a pointer to the entry. The
structure is statically allocated and is rewritten on subsequent calls
to getgrent. You must copy the contents of the structure if you
wish to save the information.
Function: void endgrent (void)
This function closes the internal stream used by getgrent.
Here is an example program showing the use of the system database inquiry functions. The program prints some information about the user running the program.
#include <grp.h>
#include <pwd.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
int
main (void)
{
uid_t me;
struct passwd *my_passwd;
struct group *my_group;
char **members;
/* Get information about the user ID. */
me = getuid ();
my_passwd = getpwuid (me);
if (!my_passwd)
{
printf ("Couldn't find out about user %d.\n", (int) me);
exit (EXIT_FAILURE);
}
/* Print the information. */
printf ("I am %s.\n", my_passwd->pw_gecos);
printf ("My login name is %s.\n", my_passwd->pw_name);
printf ("My uid is %d.\n", (int) (my_passwd->pw_uid));
printf ("My home directory is %s.\n", my_passwd->pw_dir);
printf ("My default shell is %s.\n", my_passwd->pw_shell);
/* Get information about the default group ID. */
my_group = getgrgid (my_passwd->pw_gid);
if (!my_group)
{
printf ("Couldn't find out about group %d.\n", (int) my_passwd->pw_gid);
exit (EXIT_FAILURE);
}
/* Print the information. */
printf ("My default group is %s (%d).\n",
my_group->gr_name, (int) (my_passwd->pw_gid));
printf ("The members of this group are:\n");
members = my_group->gr_mem;
while (*members)
{
printf (" %s\n", *(members));
members++;
}
return EXIT_SUCCESS;
}
Here is some output from this program:
I am Throckmorton Snurd. My login name is snurd. My uid is 31093. My home directory is /home/fsg/snurd. My default shell is /bin/sh. My default group is guest (12). The members of this group are: friedman tami
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