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26 .\" Modified, 2003-05-26, Michael Kerrisk, <mtk.manpages@gmail.com>
27 .TH SETRESUID 2 2021-03-22 "Linux" "Linux Programmer's Manual"
28 .SH NAME
29 setresuid, setresgid \- set real, effective, and saved user or group ID
30 .SH SYNOPSIS
31 .nf
32 .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
33 .B #include <unistd.h>
34 .PP
35 .BI "int setresuid(uid_t " ruid ", uid_t " euid ", uid_t " suid );
36 .BI "int setresgid(gid_t " rgid ", gid_t " egid ", gid_t " sgid );
37 .fi
38 .SH DESCRIPTION
39 .BR setresuid ()
40 sets the real user ID, the effective user ID, and the
41 saved set-user-ID of the calling process.
42 .PP
43 An unprivileged process may change its real UID,
44 effective UID, and saved set-user-ID, each to one of:
45 the current real UID, the current effective UID or the
46 current saved set-user-ID.
47 .PP
48 A privileged process (on Linux, one having the \fBCAP_SETUID\fP capability)
49 may set its real UID, effective UID, and
50 saved set-user-ID to arbitrary values.
51 .PP
52 If one of the arguments equals \-1, the corresponding value is not changed.
53 .PP
54 Regardless of what changes are made to the real UID, effective UID,
55 and saved set-user-ID, the filesystem UID is always set to the same
56 value as the (possibly new) effective UID.
57 .PP
58 Completely analogously,
59 .BR setresgid ()
60 sets the real GID, effective GID, and saved set-group-ID
61 of the calling process (and always modifies the filesystem GID
62 to be the same as the effective GID),
63 with the same restrictions for unprivileged processes.
64 .SH RETURN VALUE
65 On success, zero is returned.
66 On error, \-1 is returned, and
67 .I errno
68 is set to indicate the error.
69 .PP
70 .IR Note :
71 there are cases where
72 .BR setresuid ()
73 can fail even when the caller is UID 0;
74 it is a grave security error to omit checking for a failure return from
75 .BR setresuid ().
76 .SH ERRORS
77 .TP
78 .B EAGAIN
79 The call would change the caller's real UID (i.e.,
80 .I ruid
81 does not match the caller's real UID),
82 but there was a temporary failure allocating the
83 necessary kernel data structures.
84 .TP
85 .B EAGAIN
86 .I ruid
87 does not match the caller's real UID and this call would
88 bring the number of processes belonging to the real user ID
89 .I ruid
90 over the caller's
91 .B RLIMIT_NPROC
92 resource limit.
93 Since Linux 3.1, this error case no longer occurs
94 (but robust applications should check for this error);
95 see the description of
96 .B EAGAIN
97 in
98 .BR execve (2).
99 .TP
100 .B EINVAL
101 One or more of the target user or group IDs
102 is not valid in this user namespace.
103 .TP
104 .B EPERM
105 The calling process is not privileged (did not have the necessary
106 capability in its user namespace)
107 and tried to change the IDs to values that are not permitted.
108 For
109 .BR setresuid (),
110 the necessary capability is
111 .BR CAP_SETUID ;
112 for
113 .BR setresgid (),
114 it is
115 .BR CAP_SETGID .
116 .SH VERSIONS
117 These calls are available under Linux since Linux 2.1.44.
118 .SH CONFORMING TO
119 These calls are nonstandard;
120 they also appear on HP-UX and some of the BSDs.
121 .SH NOTES
122 Under HP-UX and FreeBSD, the prototype is found in
123 .IR <unistd.h> .
124 Under Linux, the prototype is provided by glibc since version 2.3.2.
125 .PP
126 The original Linux
127 .BR setresuid ()
128 and
129 .BR setresgid ()
130 system calls supported only 16-bit user and group IDs.
131 Subsequently, Linux 2.4 added
132 .BR setresuid32 ()
133 and
134 .BR setresgid32 (),
135 supporting 32-bit IDs.
136 The glibc
137 .BR setresuid ()
138 and
139 .BR setresgid ()
140 wrapper functions transparently deal with the variations across kernel versions.
141 .\"
142 .SS C library/kernel differences
143 At the kernel level, user IDs and group IDs are a per-thread attribute.
144 However, POSIX requires that all threads in a process
145 share the same credentials.
146 The NPTL threading implementation handles the POSIX requirements by
147 providing wrapper functions for
148 the various system calls that change process UIDs and GIDs.
149 These wrapper functions (including those for
150 .BR setresuid ()
151 and
152 .BR setresgid ())
153 employ a signal-based technique to ensure
154 that when one thread changes credentials,
155 all of the other threads in the process also change their credentials.
156 For details, see
157 .BR nptl (7).
158 .SH SEE ALSO
159 .BR getresuid (2),
160 .BR getuid (2),
161 .BR setfsgid (2),
162 .BR setfsuid (2),
163 .BR setreuid (2),
164 .BR setuid (2),
165 .BR capabilities (7),
166 .BR credentials (7),
167 .BR user_namespaces (7)