help me, please, help me

[Ken Arromdee]

I posted this before on jhunix, but I don't know if it got out; an apology if
you got 2 copies of this.

I have a program which is run setuid.  I want to set the uid back to the
original before exec'ing a shell.  It doesn't work.  The following illustrates
the problem (it's run on an AT&T 3B4000 with SYSV).  (uids is just a small
program to print out one's real and effective uids).
--------------------------------------------------------------
% uids
Real: user 7943 (ins_akaa), group 2048
Effective: user 7943 (ins_akaa), group 2048
% cd /s/adev/src/h/src
% cat test.c
extern unsigned short getuid();
extern unsigned short geteuid();

main()
{
        printf("Before setuid(getuid())\n");
        printf("Real UID=%d; effective UID=%d\n",
                (int)getuid(), (int)geteuid());
        setuid(getuid());
        printf("After setuid(getuid())\n");
        printf("Real UID=%d; effective UID=%d\n",
                (int)getuid(), (int)geteuid());
        execl("/bin/sh", "sh", 0);
}
% ls -l a.out
-rwsr-xr-x   1 adev     210        19905 Apr 10 23:10 a.out*
% a.out
Before setuid(getuid())
Real UID=7943; effective UID=210
After setuid(getuid())
Real UID=7943; effective UID=7943
% uids
Real: user 7943 (ins_akaa), group 2048
Effective: user 210 (adev), group 2048
% % 
--------------------------------------------------------------
Note that setuid(getuid()) seems to work, but the effective uid somehow gets
set back when I do the execl().
--
               EARTH          |       --Kenneth Arromdee
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Element chart from "Science Made Stupid".  (The chart seems rather popular...)