man2/seteuid.2

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  25 .\" [should really be seteuid.3]
  26 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  27 .\"     Added notes on capability requirements
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  29 .TH SETEUID 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  30 .SH NAME
  31 seteuid, setegid \- set effective user or group ID
  32 .SH SYNOPSIS
  33 .B #include <sys/types.h>
  34 .br
  35 .B #include <unistd.h>
  36 .PP
  37 .BI "int seteuid(uid_t " euid );
  38 .br
  39 .BI "int setegid(gid_t " egid );
  40 .PP
  41 .in -4n
  42 Feature Test Macro Requirements for glibc (see
  43 .BR feature_test_macros (7)):
  44 .in
  45 .PP
  46 .ad l
  47 .BR seteuid (),
  48 .BR setegid ():
  49 .RS 4
  50 _POSIX_C_SOURCE\ >=\ 200112L
  51     || /* Glibc versions <= 2.19: */ _BSD_SOURCE
  52 .RE
  53 .ad
  54 .SH DESCRIPTION
  55 .BR seteuid ()
  56 sets the effective user ID of the calling process.
  57 Unprivileged processes may only set the effective user ID to the
  58 real user ID, the effective user ID or the saved set-user-ID.
  59 .PP
  60 Precisely the same holds for
  61 .BR setegid ()
  62 with "group" instead of "user".
  63 .\" When
  64 .\" .I euid
  65 .\" equals \-1, nothing is changed.
  66 .\" (This is an artifact of the implementation in glibc of seteuid()
  67 .\" using setresuid(2).)
  68 .SH RETURN VALUE
  69 On success, zero is returned.
  70 On error, \-1 is returned, and
  71 .I errno
  72 is set appropriately.
  73 .PP
  74 .IR Note :
  75 there are cases where
  76 .BR seteuid ()
  77 can fail even when the caller is UID 0;
  78 it is a grave security error to omit checking for a failure return from
  79 .BR seteuid ().
  80 .SH ERRORS
  81 .TP
  82 .B EINVAL
  83 The target user or group ID is not valid in this user namespace.
  84 .TP
  85 .B EPERM
  86 In the case of
  87 .BR seteuid ():
  88 the calling process is not privileged (does not have the
  89 .BR CAP_SETUID
  90 capability in its user namespace) and
  91 .I euid
  92 does not match the current real user ID, current effective user ID,
  93 or current saved set-user-ID.
  94 .IP
  95 In the case of
  96 .BR setegid ():
  97 the calling process is not privileged (does not have the
  98 .BR CAP_SETGID
  99 capability in its user namespace) and
 100 .I egid
 101 does not match the current real group ID, current effective group ID,
 102 or current saved set-group-ID.
 103 .SH CONFORMING TO
 104 POSIX.1-2001, POSIX.1-2008, 4.3BSD.
 105 .SH NOTES
 106 Setting the effective user (group) ID to the
 107 saved set-user-ID (saved set-group-ID) is
 108 possible since Linux 1.1.37 (1.1.38).
 109 On an arbitrary system one should check
 110 .BR _POSIX_SAVED_IDS .
 111 .PP
 112 Under glibc 2.0,
 113 .BI seteuid( euid )
 114 is equivalent to
 115 .BI setreuid(\-1, " euid" )
 116 and hence may change the saved set-user-ID.
 117 Under glibc 2.1 and later, it is equivalent to
 118 .BI setresuid(\-1, " euid" ", \-1)"
 119 and hence does not change the saved set-user-ID.
 120 Analogous remarks hold for
 121 .BR setegid (),
 122 with the difference that the change in implementation from
 123 .BI setregid(\-1, " egid" )
 124 to
 125 .BI setresgid(\-1, " egid" ", \-1)"
 126 occurred in glibc 2.2 or 2.3 (depending on the hardware architecture).
 127 .PP
 128 According to POSIX.1,
 129 .BR seteuid ()
 130 .RB ( setegid ())
 131 need not permit
 132 .I euid
 133 .RI ( egid )
 134 to be the same value as the current effective user (group) ID,
 135 and some implementations do not permit this.
 136 .SS C library/kernel differences
 137 On Linux,
 138 .BR seteuid ()
 139 and
 140 .BR setegid ()
 141 are implemented as library functions that call, respectively,
 142 .BR setreuid (2)
 143 and
 144 .BR setregid (2).
 145 .SH SEE ALSO
 146 .BR geteuid (2),
 147 .BR setresuid (2),
 148 .BR setreuid (2),
 149 .BR setuid (2),
 150 .BR capabilities (7),
 151 .BR credentials (7),
 152 .BR user_namespaces (7)