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       setuid - set user identity


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

       int setuid(uid_t uid);


       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:
       setuid()  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).


       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().


       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

       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  in  its
              user  namespace)  and  uid  does not match the real UID or saved set-user-ID of the
              calling process.


       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.


       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

       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).


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


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