man2/setuid.2

   1 .\" Copyright (C), 1994, Graeme W. Wilford (Wilf).
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  23 .\" Fri Jul 29th 12:56:44 BST 1994  Wilf. <G.Wilford@ee.surrey.ac.uk>
  24 .\" Changes inspired by patch from Richard Kettlewell
  25 .\"   <richard@greenend.org.uk>, aeb 970616.
  26 .\" Modified, 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
  27 .\"     Added notes on capability requirements
  28 .TH SETUID 2 2004-05-27 "Linux 2.6.6" "Linux Programmer's Manual"
  29 .SH NAME
  30 setuid \- set user identity
  31 .SH SYNOPSIS
  32 .B #include <sys/types.h>
  33 .br
  34 .B #include <unistd.h>
  35 .sp
  36 .BI "int setuid(uid_t " uid );
  37 .SH DESCRIPTION
  38 .BR setuid ()
  39 sets the effective user ID of the current process.
  40 If the effective UID of the caller is root,
  41 the real UID and saved set-user-ID are also set.
  42 .PP
  43 Under Linux,
  44 .BR setuid ()
  45 is implemented like the POSIX version with the _POSIX_SAVED_IDS feature.
  46 This allows a set-user-ID (other than root) program to drop all of its user
  47 privileges, do some un-privileged work, and then re-engage the original
  48 effective user ID in a secure manner.
  49 .PP
  50 If the user is root or the program is set-user-ID-root, special care must be
  51 taken. The
  52 .BR setuid ()
  53 function checks the effective user ID of the caller and if it is
  54 the superuser, all process related user ID's are set to
  55 .IR uid .
  56 After this has occurred, it is impossible for the program to regain root
  57 privileges.
  58 .PP
  59 Thus, a set-user-ID-root program wishing to temporarily drop root
  60 privileges, assume the identity of a non-root user, and then regain
  61 root privileges afterwards cannot use
  62 .BR setuid ().
  63 You can accomplish this with the (non-POSIX, BSD) call
  64 .BR seteuid ().
  65 .SH "RETURN VALUE"
  66 On success, zero is returned.  On error, \-1 is returned, and
  67 .I errno
  68 is set appropriately.
  69 .SH ERRORS
  70 .TP
  71 .B EAGAIN
  72 The
  73 .I uid
  74 does not match the current uid and
  75 .I uid
  76 brings process over it's NPROC rlimit.
  77 .TP
  78 .B EPERM
  79 The user is not privileged (Linux: does not have the
  80 .B CAP_SETUID
  81 capability) and
  82 .I uid
  83 does not match the real UID or saved set-user-ID of the calling process.
  84 .SH "CONFORMING TO"
  85 SVr4, POSIX.1-2001.
  86 Not quite compatible with the 4.4BSD call, which
  87 sets all of the real, saved, and effective user IDs.
  88 .\" SVr4 documents an additional EINVAL error condition.
  89 .SH "LINUX-SPECIFIC REMARKS"
  90 Linux has the concept of filesystem user ID, normally equal to the
  91 effective user ID.  The
  92 .BR setuid ()
  93 call also sets the filesystem user ID of the current process.
  94 See
  95 .BR setfsuid (2).
  96 .PP
  97 If
  98 .I uid
  99 is different from the old effective uid, the process will
 100 be forbidden from leaving core dumps.
 101 .SH "SEE ALSO"
 102 .BR getuid (2),
 103 .BR seteuid (2),
 104 .BR setfsuid (2),
 105 .BR setreuid (2),
 106 .BR capabilities (7)