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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 effective UID of the
       caller is root (more precisely: if the caller has the CAP_SETUID capability), 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).


       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) 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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