Why my SETUID does not work as expected? Post: 302960625

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LEARN ABOUT LINUX

setuid32

SETUID(2)						     Linux Programmer's Manual							 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 effective UID of the caller is root, 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 man-
       ner.

       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 afterwards 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.

ERRORS

       EAGAIN The uid does not match the current uid and uid brings process over its RLIMIT_NPROC resource limit.

       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

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

NOTES

   Linux Notes
       Linux has the concept of file system user ID, normally equal to the effective user ID.  The setuid() call also sets the file system 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.

SEE ALSO

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

COLOPHON

       This  page is part of release 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2010-02-21								 SETUID(2)