1SETREUID(2) Linux Programmer's Manual SETREUID(2)
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6 setreuid, setregid - set real and/or effective user or group ID
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9 #include <unistd.h>
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11 int setreuid(uid_t ruid, uid_t euid);
12 int setregid(gid_t rgid, gid_t egid);
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14 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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16 setreuid(), setregid():
17 _XOPEN_SOURCE >= 500
18 || /* Since glibc 2.19: */ _DEFAULT_SOURCE
19 || /* Glibc <= 2.19: */ _BSD_SOURCE
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22 setreuid() sets real and effective user IDs of the calling process.
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24 Supplying a value of -1 for either the real or effective user ID forces
25 the system to leave that ID unchanged.
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27 Unprivileged processes may only set the effective user ID to the real
28 user ID, the effective user ID, or the saved set-user-ID.
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30 Unprivileged users may only set the real user ID to the real user ID or
31 the effective user ID.
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33 If the real user ID is set (i.e., ruid is not -1) or the effective user
34 ID is set to a value not equal to the previous real user ID, the saved
35 set-user-ID will be set to the new effective user ID.
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37 Completely analogously, setregid() sets real and effective group ID's
38 of the calling process, and all of the above holds with "group" instead
39 of "user".
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42 On success, zero is returned. On error, -1 is returned, and errno is
43 set to indicate the error.
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45 Note: there are cases where setreuid() can fail even when the caller is
46 UID 0; it is a grave security error to omit checking for a failure re‐
47 turn from setreuid().
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50 EAGAIN The call would change the caller's real UID (i.e., ruid does not
51 match the caller's real UID), but there was a temporary failure
52 allocating the necessary kernel data structures.
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54 EAGAIN ruid does not match the caller's real UID and this call would
55 bring the number of processes belonging to the real user ID ruid
56 over the caller's RLIMIT_NPROC resource limit. Since Linux 3.1,
57 this error case no longer occurs (but robust applications should
58 check for this error); see the description of EAGAIN in ex‐
59 ecve(2).
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61 EINVAL One or more of the target user or group IDs is not valid in this
62 user namespace.
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64 EPERM The calling process is not privileged (on Linux, does not have
65 the necessary capability in its user namespace: CAP_SETUID in
66 the case of setreuid(), or CAP_SETGID in the case of setregid())
67 and a change other than (i) swapping the effective user (group)
68 ID with the real user (group) ID, or (ii) setting one to the
69 value of the other or (iii) setting the effective user (group)
70 ID to the value of the saved set-user-ID (saved set-group-ID)
71 was specified.
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74 POSIX.1-2001, POSIX.1-2008, 4.3BSD (setreuid() and setregid() first ap‐
75 peared in 4.2BSD).
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78 Setting the effective user (group) ID to the saved set-user-ID (saved
79 set-group-ID) is possible since Linux 1.1.37 (1.1.38).
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81 POSIX.1 does not specify all of the UID changes that Linux permits for
82 an unprivileged process. For setreuid(), the effective user ID can be
83 made the same as the real user ID or the saved set-user-ID, and it is
84 unspecified whether unprivileged processes may set the real user ID to
85 the real user ID, the effective user ID, or the saved set-user-ID. For
86 setregid(), the real group ID can be changed to the value of the saved
87 set-group-ID, and the effective group ID can be changed to the value of
88 the real group ID or the saved set-group-ID. The precise details of
89 what ID changes are permitted vary across implementations.
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91 POSIX.1 makes no specification about the effect of these calls on the
92 saved set-user-ID and saved set-group-ID.
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94 The original Linux setreuid() and setregid() system calls supported
95 only 16-bit user and group IDs. Subsequently, Linux 2.4 added se‐
96 treuid32() and setregid32(), supporting 32-bit IDs. The glibc se‐
97 treuid() and setregid() wrapper functions transparently deal with the
98 variations across kernel versions.
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100 C library/kernel differences
101 At the kernel level, user IDs and group IDs are a per-thread attribute.
102 However, POSIX requires that all threads in a process share the same
103 credentials. The NPTL threading implementation handles the POSIX re‐
104 quirements by providing wrapper functions for the various system calls
105 that change process UIDs and GIDs. These wrapper functions (including
106 those for setreuid() and setregid()) employ a signal-based technique to
107 ensure that when one thread changes credentials, all of the other
108 threads in the process also change their credentials. For details, see
109 nptl(7).
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112 getgid(2), getuid(2), seteuid(2), setgid(2), setresuid(2), setuid(2),
113 capabilities(7), credentials(7), user_namespaces(7)
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116 This page is part of release 5.13 of the Linux man-pages project. A
117 description of the project, information about reporting bugs, and the
118 latest version of this page, can be found at
119 https://www.kernel.org/doc/man-pages/.
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123Linux 2021-03-22 SETREUID(2)