1SETUID(P) POSIX Programmer's Manual SETUID(P)
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6 setuid - set user ID
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9 #include <unistd.h>
10 int setuid(uid_t uid);
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15 If the process has appropriate privileges, setuid() shall set the real
16 user ID, effective user ID, and the saved set-user-ID of the calling
17 process to uid.
18 If the process does not have appropriate privileges, but uid is equal
20 to the real user ID or the saved set-user-ID, setuid() shall set the
21 effective user ID to uid; the real user ID and saved set-user-ID shall
22 remain unchanged.
23 The setuid() function shall not affect the supplementary group list in
25 any way.
26
28 Upon successful completion, 0 shall be returned. Otherwise, -1 shall be
29 returned and errno set to indicate the error.
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32 The setuid() function shall fail, return -1, and set errno to the cor‐
33 responding value if one or more of the following are true:
34 EINVAL The value of the uid argument is invalid and not supported by
36 the implementation.
37 EPERM The process does not have appropriate privileges and uid does
39 not match the real user ID or the saved set-user-ID.
40 The following sections are informative.
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45 None.
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48 None.
49
51 The various behaviors of the setuid() and setgid() functions when
52 called by non-privileged processes reflect the behavior of different
53 historical implementations. For portability, it is recommended that new
54 non-privileged applications use the seteuid() and setegid() functions
55 instead.
56 The saved set-user-ID capability allows a program to regain the effec‐
58 tive user ID established at the last exec call. Similarly, the saved
59 set-group-ID capability allows a program to regain the effective group
60 ID established at the last exec call. These capabilities are derived
61 from System V. Without them, a program might have to run as superuser
62 in order to perform the same functions, because superuser can write on
63 the user's files. This is a problem because such a program can write on
64 any user's files, and so must be carefully written to emulate the per‐
65 missions of the calling process properly. In System V, these capabili‐
66 ties have traditionally been implemented only via the setuid() and set‐
67 gid() functions for non-privileged processes. The fact that the behav‐
68 ior of those functions was different for privileged processes made them
69 difficult to use. The POSIX.1-1990 standard defined the setuid() func‐
70 tion to behave differently for privileged and unprivileged users. When
71 the caller had the appropriate privilege, the function set the calling
72 process' real user ID, effective user ID, and saved set-user ID on
73 implementations that supported it. When the caller did not have the
74 appropriate privilege, the function set only the effective user ID,
75 subject to permission checks. The former use is generally needed for
76 utilities like login and su, which are not conforming applications and
77 thus outside the scope of IEEE Std 1003.1-2001. These utilities wish
78 to change the user ID irrevocably to a new value, generally that of an
79 unprivileged user. The latter use is needed for conforming applica‐
80 tions that are installed with the set-user-ID bit and need to perform
81 operations using the real user ID.
82 IEEE Std 1003.1-2001 augments the latter functionality with a mandatory
84 feature named _POSIX_SAVED_IDS. This feature permits a set-user-ID
85 application to switch its effective user ID back and forth between the
86 values of its exec-time real user ID and effective user ID. Unfortu‐
87 nately, the POSIX.1-1990 standard did not permit a conforming applica‐
88 tion using this feature to work properly when it happened to be exe‐
89 cuted with the (implementation-defined) appropriate privilege. Further‐
90 more, the application did not even have a means to tell whether it had
91 this privilege. Since the saved set-user-ID feature is quite desirable
92 for applications, as evidenced by the fact that NIST required it in
93 FIPS 151-2, it has been mandated by IEEE Std 1003.1-2001. However,
94 there are implementors who have been reluctant to support it given the
95 limitation described above.
96 The 4.3BSD system handles the problem by supporting separate functions:
98 setuid() (which always sets both the real and effective user IDs, like
99 setuid() in IEEE Std 1003.1-2001 for privileged users), and seteuid()
100 (which always sets just the effective user ID, like setuid() in
101 IEEE Std 1003.1-2001 for non-privileged users). This separation of
102 functionality into distinct functions seems desirable. 4.3BSD does not
103 support the saved set-user-ID feature. It supports similar functional‐
104 ity of switching the effective user ID back and forth via setreuid(),
105 which permits reversing the real and effective user IDs. This model
106 seems less desirable than the saved set-user-ID because the real user
107 ID changes as a side effect. The current 4.4BSD includes saved effec‐
108 tive IDs and uses them for seteuid() and setegid() as described above.
109 The setreuid() and setregid() functions will be deprecated or removed.
110 The solution here is:
112 * Require that all implementations support the functionality of the
114 saved set-user-ID, which is set by the exec functions and by privi‐
115 leged calls to setuid().
116 * Add the seteuid() and setegid() functions as portable alternatives
118 to setuid() and setgid() for non-privileged and privileged pro‐
119 cesses.
120 Historical systems have provided two mechanisms for a set-user-ID
122 process to change its effective user ID to be the same as its real user
123 ID in such a way that it could return to the original effective user
124 ID: the use of the setuid() function in the presence of a saved set-
125 user-ID, or the use of the BSD setreuid() function, which was able to
126 swap the real and effective user IDs. The changes included in
127 IEEE Std 1003.1-2001 provide a new mechanism using seteuid() in con‐
128 junction with a saved set-user-ID. Thus, all implementations with the
129 new seteuid() mechanism will have a saved set-user-ID for each process,
130 and most of the behavior controlled by _POSIX_SAVED_IDS has been
131 changed to agree with the case where the option was defined. The kill()
132 function is an exception. Implementors of the new seteuid() mechanism
133 will generally be required to maintain compatibility with the older
134 mechanisms previously supported by their systems. However, compatibil‐
135 ity with this use of setreuid() and with the _POSIX_SAVED_IDS behavior
136 of kill() is unfortunately complicated. If an implementation with a
137 saved set-user-ID allows a process to use setreuid() to swap its real
138 and effective user IDs, but were to leave the saved set-user-ID unmodi‐
139 fied, the process would then have an effective user ID equal to the
140 original real user ID, and both real and saved set-user-ID would be
141 equal to the original effective user ID. In that state, the real user
142 would be unable to kill the process, even though the effective user ID
143 of the process matches that of the real user, if the kill() behavior of
144 _POSIX_SAVED_IDS was used. This is obviously not acceptable. The alter‐
145 native choice, which is used in at least one implementation, is to
146 change the saved set-user-ID to the effective user ID during most calls
147 to setreuid(). The standard developers considered that alternative to
148 be less correct than the retention of the old behavior of kill() in
149 such systems. Current conforming applications shall accommodate either
150 behavior from kill(), and there appears to be no strong reason for
151 kill() to check the saved set-user-ID rather than the effective user
152 ID.
153
155 None.
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158 exec() , getegid() , geteuid() , getgid() , getuid() , setegid() ,
159 seteuid() , setgid() , setregid() , setreuid() , the Base Definitions
160 volume of IEEE Std 1003.1-2001, <sys/types.h>, <unistd.h>
161
163 Portions of this text are reprinted and reproduced in electronic form
164 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
165 -- Portable Operating System Interface (POSIX), The Open Group Base
166 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
167 Electrical and Electronics Engineers, Inc and The Open Group. In the
168 event of any discrepancy between this version and the original IEEE and
169 The Open Group Standard, the original IEEE and The Open Group Standard
170 is the referee document. The original Standard can be obtained online
171 at http://www.opengroup.org/unix/online.html .
172IEEE/The Open Group 2003 SETUID(P)