man2/setresuid.2

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  26 .\" Modified, 2003-05-26, Michael Kerrisk, <mtk.manpages@gmail.com>
  27 .TH SETRESUID 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  28 .SH NAME
  29 setresuid, setresgid \- set real, effective and saved user or group ID
  30 .SH SYNOPSIS
  31 .BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
  32 .br
  33 .B #include <unistd.h>
  34 .PP
  35 .BI "int setresuid(uid_t " ruid ", uid_t " euid ", uid_t " suid );
  36 .br
  37 .BI "int setresgid(gid_t " rgid ", gid_t " egid ", gid_t " sgid );
  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 appropriately.
  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)