man2/setuid.2

   1 .\" Copyright (C), 1994, Graeme W. Wilford (Wilf).
   2 .\" and Copyright (C) 2010, 2014, 2015, Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\"
   4 .\" %%%LICENSE_START(VERBATIM)
   5 .\" Permission is granted to make and distribute verbatim copies of this
   6 .\" manual provided the copyright notice and this permission notice are
   7 .\" preserved on all copies.
   8 .\"
   9 .\" Permission is granted to copy and distribute modified versions of this
  10 .\" manual under the conditions for verbatim copying, provided that the
  11 .\" entire resulting derived work is distributed under the terms of a
  12 .\" permission notice identical to this one.
  13 .\"
  14 .\" Since the Linux kernel and libraries are constantly changing, this
  15 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  16 .\" responsibility for errors or omissions, or for damages resulting from
  17 .\" the use of the information contained herein.  The author(s) may not
  18 .\" have taken the same level of care in the production of this manual,
  19 .\" which is licensed free of charge, as they might when working
  20 .\" professionally.
  21 .\"
  22 .\" Formatted or processed versions of this manual, if unaccompanied by
  23 .\" the source, must acknowledge the copyright and authors of this work.
  24 .\" %%%LICENSE_END
  25 .\"
  26 .\" Fri Jul 29th 12:56:44 BST 1994  Wilf. <G.Wilford@ee.surrey.ac.uk>
  27 .\" Changes inspired by patch from Richard Kettlewell
  28 .\"   <richard@greenend.org.uk>, aeb 970616.
  29 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  30 .\"     Added notes on capability requirements
  31 .TH SETUID 2 2015-07-23 "Linux" "Linux Programmer's Manual"
  32 .SH NAME
  33 setuid \- set user identity
  34 .SH SYNOPSIS
  35 .B #include <sys/types.h>
  36 .br
  37 .B #include <unistd.h>
  38 .sp
  39 .BI "int setuid(uid_t " uid );
  40 .SH DESCRIPTION
  41 .BR setuid ()
  42 sets the effective user ID of the calling process.
  43 If the calling process is prvileged
  44 (more precisely: if the process has the
  45 .BR CAP_SETUID
  46 capability),
  47 the real UID and saved set-user-ID are also set.
  48 .PP
  49 Under Linux,
  50 .BR setuid ()
  51 is implemented like the POSIX version with the
  52 .B _POSIX_SAVED_IDS
  53 feature.
  54 This allows a set-user-ID (other than root) program to drop all of its user
  55 privileges, do some un-privileged work, and then reengage the original
  56 effective user ID in a secure manner.
  57 .PP
  58 If the user is root or the program is set-user-ID-root, special care must be
  59 taken.
  60 The
  61 .BR setuid ()
  62 function checks the effective user ID of the caller and if it is
  63 the superuser, all process-related user ID's are set to
  64 .IR uid .
  65 After this has occurred, it is impossible for the program to regain root
  66 privileges.
  67 .PP
  68 Thus, a set-user-ID-root program wishing to temporarily drop root
  69 privileges, assume the identity of an unprivileged user, and then regain
  70 root privileges afterward cannot use
  71 .BR setuid ().
  72 You can accomplish this with
  73 .BR seteuid (2).
  74 .SH RETURN VALUE
  75 On success, zero is returned.
  76 On error, \-1 is returned, and
  77 .I errno
  78 is set appropriately.
  79
  80 .IR Note :
  81 there are cases where
  82 .BR setuid ()
  83 can fail even when the caller is UID 0;
  84 it is a grave security error to omit checking for a failure return from
  85 .BR setuid ().
  86 .SH ERRORS
  87 .TP
  88 .B EAGAIN
  89 The call would change the caller's real UID (i.e.,
  90 .I uid
  91 does not match the caller's real UID),
  92 but there was a temporary failure allocating the
  93 necessary kernel data structures.
  94 .TP
  95 .B EAGAIN
  96 .I uid
  97 does not match the real user ID of the caller and this call would
  98 bring the number of processes belonging to the real user ID
  99 .I uid
 100 over the caller's
 101 .B RLIMIT_NPROC
 102 resource limit.
 103 Since Linux 3.1, this error case no longer occurs
 104 (but robust applications should check for this error);
 105 see the description of
 106 .B EAGAIN
 107 in
 108 .BR execve (2).
 109 .TP
 110 .B EINVAL
 111 The user ID specified in
 112 .I uid
 113 is not valid in this user namespace.
 114 .TP
 115 .B EPERM
 116 The user is not privileged (Linux: does not have the
 117 .B CAP_SETUID
 118 capability) and
 119 .I uid
 120 does not match the real UID or saved set-user-ID of the calling process.
 121 .SH CONFORMING TO
 122 POSIX.1-2001, POSIX.1-2008, SVr4.
 123 Not quite compatible with the 4.4BSD call, which
 124 sets all of the real, saved, and effective user IDs.
 125 .\" SVr4 documents an additional EINVAL error condition.
 126 .SH NOTES
 127 Linux has the concept of the filesystem user ID, normally equal to the
 128 effective user ID.
 129 The
 130 .BR setuid ()
 131 call also sets the filesystem user ID of the calling process.
 132 See
 133 .BR setfsuid (2).
 134 .PP
 135 If
 136 .I uid
 137 is different from the old effective UID, the process will
 138 be forbidden from leaving core dumps.
 139
 140 The original Linux
 141 .BR setuid ()
 142 system call supported only 16-bit user IDs.
 143 Subsequently, Linux 2.4 added
 144 .BR setuid32 ()
 145 supporting 32-bit IDs.
 146 The glibc
 147 .BR setuid ()
 148 wrapper function transparently deals with the variation across kernel versions.
 149 .\"
 150 .SS C library/kernel differences
 151 At the kernel level, user IDs and group IDs are a per-thread attribute.
 152 However, POSIX requires that all threads in a process
 153 share the same credentials.
 154 The NPTL threading implementation handles the POSIX requirements by
 155 providing wrapper functions for
 156 the various system calls that change process UIDs and GIDs.
 157 These wrapper functions (including the one for
 158 .BR setuid ())
 159 employ a signal-based technique to ensure
 160 that when one thread changes credentials,
 161 all of the other threads in the process also change their credentials.
 162 For details, see
 163 .BR nptl (7).
 164 .SH SEE ALSO
 165 .BR getuid (2),
 166 .BR seteuid (2),
 167 .BR setfsuid (2),
 168 .BR setreuid (2),
 169 .BR capabilities (7),
 170 .BR credentials (7),
 171 .BR user_namespaces (7)