setreuid(2)

1SETREUID(2)                Linux Programmer's Manual               SETREUID(2)
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NAME

6       setreuid, setregid - set real and/or effective user or group ID
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SYNOPSIS

9       #include <sys/types.h>
10       #include <unistd.h>
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12       int setreuid(uid_t ruid, uid_t euid);
13       int setregid(gid_t rgid, gid_t egid);
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15   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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17       setreuid(), setregid():
18           _XOPEN_SOURCE >= 500
19               || /* Since glibc 2.19: */ _DEFAULT_SOURCE
20               || /* Glibc versions <= 2.19: */ _BSD_SOURCE
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DESCRIPTION

23       setreuid() sets real and effective user IDs of the calling process.
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25       Supplying a value of -1 for either the real or effective user ID forces
26       the system to leave that ID unchanged.
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28       Unprivileged processes may only set the effective user ID to  the  real
29       user ID, the effective user ID, or the saved set-user-ID.
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31       Unprivileged users may only set the real user ID to the real user ID or
32       the effective user ID.
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34       If the real user ID is set (i.e., ruid is not -1) or the effective user
35       ID  is set to a value not equal to the previous real user ID, the saved
36       set-user-ID will be set to the new effective user ID.
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38       Completely analogously, setregid() sets real and effective  group  ID's
39       of the calling process, and all of the above holds with "group" instead
40       of "user".
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RETURN VALUE

43       On success, zero is returned.  On error, -1 is returned, and  errno  is
44       set appropriately.
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46       Note: there are cases where setreuid() can fail even when the caller is
47       UID 0; it is a grave security error to  omit  checking  for  a  failure
48       return from setreuid().
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ERRORS

51       EAGAIN The call would change the caller's real UID (i.e., ruid does not
52              match the caller's real UID), but there was a temporary  failure
53              allocating the necessary kernel data structures.
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55       EAGAIN ruid  does  not  match the caller's real UID and this call would
56              bring the number of processes belonging to the real user ID ruid
57              over the caller's RLIMIT_NPROC resource limit.  Since Linux 3.1,
58              this error case no longer occurs (but robust applications should
59              check  for  this  error);  see  the  description  of  EAGAIN  in
60              execve(2).
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62       EINVAL One or more of the target user or group IDs is not valid in this
63              user namespace.
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65       EPERM  The  calling  process is not privileged (on Linux, does not have
66              the necessary capability in its user  namespace:  CAP_SETUID  in
67              the case of setreuid(), or CAP_SETGID in the case of setregid())
68              and a change other than (i) swapping the effective user  (group)
69              ID  with  the  real  user (group) ID, or (ii) setting one to the
70              value of the other or (iii) setting the effective  user  (group)
71              ID  to  the  value of the saved set-user-ID (saved set-group-ID)
72              was specified.
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CONFORMING TO

75       POSIX.1-2001, POSIX.1-2008, 4.3BSD  (setreuid()  and  setregid()  first
76       appeared in 4.2BSD).
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NOTES

79       Setting  the  effective user (group) ID to the saved set-user-ID (saved
80       set-group-ID) is possible since Linux 1.1.37 (1.1.38).
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82       POSIX.1 does not specify all of the UID changes that Linux permits  for
83       an  unprivileged process.  For setreuid(), the effective user ID can be
84       made the same as the real user ID or the saved set-user-ID, and  it  is
85       unspecified  whether unprivileged processes may set the real user ID to
86       the real user ID, the effective user ID, or the saved set-user-ID.  For
87       setregid(),  the real group ID can be changed to the value of the saved
88       set-group-ID, and the effective group ID can be changed to the value of
89       the  real  group  ID or the saved set-group-ID.  The precise details of
90       what ID changes are permitted vary across implementations.
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92       POSIX.1 makes no specification about the effect of these calls  on  the
93       saved set-user-ID and saved set-group-ID.
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95       The  original  Linux  setreuid()  and setregid() system calls supported
96       only  16-bit  user  and  group  IDs.   Subsequently,  Linux  2.4  added
97       setreuid32()  and  setregid32(),  supporting  32-bit  IDs.   The  glibc
98       setreuid() and setregid() wrapper functions transparently deal with the
99       variations across kernel versions.
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101   C library/kernel differences
102       At the kernel level, user IDs and group IDs are a per-thread attribute.
103       However, POSIX requires that all threads in a process  share  the  same
104       credentials.   The  NPTL  threading  implementation  handles  the POSIX
105       requirements by providing wrapper  functions  for  the  various  system
106       calls  that  change  process  UIDs  and  GIDs.  These wrapper functions
107       (including those for setreuid() and setregid()) employ  a  signal-based
108       technique  to  ensure  that when one thread changes credentials, all of
109       the other threads in the process also change  their  credentials.   For
110       details, see nptl(7).
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COLOPHON

117       This page is part of release 4.16 of the Linux  man-pages  project.   A
118       description  of  the project, information about reporting bugs, and the
119       latest    version    of    this    page,    can     be     found     at
120       https://www.kernel.org/doc/man-pages/.
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124Linux                             2017-09-15                       SETREUID(2)