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setuid(2)


NAME

   setuid - set user identity

SYNOPSIS

   #include <sys/types.h>
   #include <unistd.h>

   int setuid(uid_t uid);

DESCRIPTION

   setuid()  sets  the  effective  user ID of the calling process.  If the
   calling process is privileged (more precisely: if the process  has  the
   CAP_SETUID  capability  in  its user namespace), the real UID and saved
   set-user-ID are also set.

   Under Linux, setuid() is implemented like the POSIX  version  with  the
   _POSIX_SAVED_IDS  feature.  This allows a set-user-ID (other than root)
   program to drop all of its user privileges, do some un-privileged work,
   and then reengage the original effective user ID in a secure manner.

   If  the  user  is root or the program is set-user-ID-root, special care
   must be taken.  The setuid() function checks the effective user  ID  of
   the  caller  and  if it is the superuser, all process-related user ID's
   are set to uid.  After this has occurred,  it  is  impossible  for  the
   program to regain root privileges.

   Thus,  a  set-user-ID-root  program  wishing  to  temporarily drop root
   privileges, assume the identity  of  an  unprivileged  user,  and  then
   regain   root  privileges  afterward  cannot  use  setuid().   You  can
   accomplish this with seteuid(2).

RETURN VALUE

   On success, zero is returned.  On error, -1 is returned, and  errno  is
   set appropriately.

   Note:  there  are cases where setuid() can fail even when the caller is
   UID 0; it is a grave security error to  omit  checking  for  a  failure
   return from setuid().

ERRORS

   EAGAIN The  call would change the caller's real UID (i.e., uid does not
          match the caller's real UID), but there was a temporary  failure
          allocating the necessary kernel data structures.

   EAGAIN uid  does not match the real user ID of the caller and this call
          would bring the number of processes belonging to the  real  user
          ID  uid  over  the  caller's RLIMIT_NPROC resource limit.  Since
          Linux  3.1,  this  error  case  no  longer  occurs  (but  robust
          applications  should  check for this error); see the description
          of EAGAIN in execve(2).

   EINVAL The user  ID  specified  in  uid  is  not  valid  in  this  user
          namespace.

   EPERM  The  user is not privileged (Linux: does not have the CAP_SETUID
          capability) and uid does not match the real UID  or  saved  set-
          user-ID of the calling process.

CONFORMING TO

   POSIX.1-2001, POSIX.1-2008, SVr4.  Not quite compatible with the 4.4BSD
   call, which sets all of the real, saved, and effective user IDs.

NOTES

   Linux has the concept of the filesystem user ID, normally equal to  the
   effective  user ID.  The setuid() call also sets the filesystem user ID
   of the calling process.  See setfsuid(2).

   If uid is different from the old effective UID,  the  process  will  be
   forbidden from leaving core dumps.

   The original Linux setuid() system call supported only 16-bit user IDs.
   Subsequently, Linux 2.4 added setuid32() supporting  32-bit  IDs.   The
   glibc  setuid() wrapper function transparently deals with the variation
   across kernel versions.

   C library/kernel differences
   At the kernel level, user IDs and group IDs are a per-thread attribute.
   However,  POSIX  requires  that all threads in a process share the same
   credentials.  The  NPTL  threading  implementation  handles  the  POSIX
   requirements  by  providing  wrapper  functions  for the various system
   calls that change process  UIDs  and  GIDs.   These  wrapper  functions
   (including  the  one  for  setuid()) employ a signal-based technique to
   ensure that when one thread  changes  credentials,  all  of  the  other
   threads in the process also change their credentials.  For details, see
   nptl(7).

SEE ALSO

   getuid(2),  seteuid(2),  setfsuid(2),   setreuid(2),   capabilities(7),
   credentials(7), user_namespaces(7)

COLOPHON

   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at
   https://www.kernel.org/doc/man-pages/.



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