Process Management – How to Drop Process Privileges

Yes, it is. First, create a pipe: mkfifo /tmp/fifo. Use gdb to attach to the process: gdb -p PID

Then close stdin: call close (0); and open it again: call open ("/tmp/fifo", 0600)

Finally, write away (from a different terminal, as gdb will probably hang):

echo blah > /tmp/fifo

1. As you correctly state a SUID bit, sets the effective user-id of the new process to the owner of the program file being run. There's no concept of raising or lowering privileges. Often the user to which one "suids" is root, but it can just as legitimately be any other, and doesn't really change the concept of SUID, when you choose another, apart from some users have access to certain resources. However the root user is a little bit special, and the man page for setuid, explains the subtle differences. I don't think there is a special name for changing to a non-root user, and I have never associated suid meaning going up in privilege, just changing user.

2. Since you are developing the master yourself which runs as root it can setuid to any user after forking but before exec-ing the new process, I dont see that suid bit has to be used on executable at all, and incidently if you don't, then if any-old user ran the player, since executable is not suid, they wouldn't be able to run it correctly since setuid will fail if they were not root. For me this seems like better security, than using suid bits on executable files.

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So your manager (running as real and effective uid=0) starts player and you code/control player (not manager). If you do nothing special, then player will run with real+effective uid=0 and you want to prevent this.

You could just switch uid in player, no SUID bits on player, no need.