man2/setuid.2

   1 .\" Copyright (C), 1994, Graeme W. Wilford (Wilf).
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  25 .\" Fri Jul 29th 12:56:44 BST 1994  Wilf. <G.Wilford@ee.surrey.ac.uk>
  26 .\" Changes inspired by patch from Richard Kettlewell
  27 .\"   <richard@greenend.org.uk>, aeb 970616.
  28 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  29 .\"     Added notes on capability requirements
  30 .TH SETUID 2 2010-11-22 "Linux" "Linux Programmer's Manual"
  31 .SH NAME
  32 setuid \- set user identity
  33 .SH SYNOPSIS
  34 .B #include <sys/types.h>
  35 .br
  36 .B #include <unistd.h>
  37 .sp
  38 .BI "int setuid(uid_t " uid );
  39 .SH DESCRIPTION
  40 .BR setuid ()
  41 sets the effective user ID of the calling process.
  42 If the effective UID of the caller is root,
  43 the real UID and saved set-user-ID are also set.
  44 .PP
  45 Under Linux,
  46 .BR setuid ()
  47 is implemented like the POSIX version with the
  48 .B _POSIX_SAVED_IDS
  49 feature.
  50 This allows a set-user-ID (other than root) program to drop all of its user
  51 privileges, do some un-privileged work, and then reengage the original
  52 effective user ID in a secure manner.
  53 .PP
  54 If the user is root or the program is set-user-ID-root, special care must be
  55 taken.
  56 The
  57 .BR setuid ()
  58 function checks the effective user ID of the caller and if it is
  59 the superuser, all process-related user ID's are set to
  60 .IR uid .
  61 After this has occurred, it is impossible for the program to regain root
  62 privileges.
  63 .PP
  64 Thus, a set-user-ID-root program wishing to temporarily drop root
  65 privileges, assume the identity of an unprivileged user, and then regain
  66 root privileges afterward cannot use
  67 .BR setuid ().
  68 You can accomplish this with
  69 .BR seteuid (2).
  70 .SH RETURN VALUE
  71 On success, zero is returned.
  72 On error, \-1 is returned, and
  73 .I errno
  74 is set appropriately.
  75 .SH ERRORS
  76 .TP
  77 .B EAGAIN
  78 The call would change the caller's real UID (i.e.,
  79 .I uid
  80 does not match the caller's real UID),
  81 but there was a temporary failure allocating the
  82 necessary kernel data structures.
  83 .TP
  84 .B EAGAIN
  85 .I uid
  86 does not match the real user ID of the caller and this call would
  87 bring the number of processes belonging to the real user ID
  88 .I uid
  89 over the caller's
  90 .B RLIMIT_NPROC
  91 resource limit.
  92 .TP
  93 .B EPERM
  94 The user is not privileged (Linux: does not have the
  95 .B CAP_SETUID
  96 capability) and
  97 .I uid
  98 does not match the real UID or saved set-user-ID of the calling process.
  99 .SH CONFORMING TO
 100 SVr4, POSIX.1-2001.
 101 Not quite compatible with the 4.4BSD call, which
 102 sets all of the real, saved, and effective user IDs.
 103 .\" SVr4 documents an additional EINVAL error condition.
 104 .SH NOTES
 105 Linux has the concept of the filesystem user ID, normally equal to the
 106 effective user ID.
 107 The
 108 .BR setuid ()
 109 call also sets the filesystem user ID of the calling process.
 110 See
 111 .BR setfsuid (2).
 112 .PP
 113 If
 114 .I uid
 115 is different from the old effective UID, the process will
 116 be forbidden from leaving core dumps.
 117
 118 The original Linux
 119 .BR setuid ()
 120 system call supported only 16-bit user IDs.
 121 Subsequently, Linux 2.4 added
 122 .BR setuid32 ()
 123 supporting 32-bit IDs.
 124 The glibc
 125 .BR setuid ()
 126 wrapper function transparently deals with the variation across kernel versions.
 127 .SH SEE ALSO
 128 .BR getuid (2),
 129 .BR seteuid (2),
 130 .BR setfsuid (2),
 131 .BR setreuid (2),
 132 .BR capabilities (7),
 133 .BR credentials (7)