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> Traps are neat but beware, they aren't completely reliable. You can't trap SIGKILL or SIGSTOP.

A long time ago, there actually was a sneaky way on some Unixes to trap SIGKILL. If a program was being run under ptrace then any signal would pause the program and alert the program that was doing the trace--even SIGKILL.

So I made a program that I named "sh" and carefully made to have the same memory size as /bin/sh, that just forked and exec'ed another program of mine under ptrace. The other program was named "superman". Whenever my fake sh received notification that "superman" had received a signal, it would write the number of the signal into a variable in superman's address space, and then make it so "superman" continued but with the signal changed to SIGINT. The SIGINT handler in "superman" would would check that variable to see the real signal, and print an appropriate smart remark.

I started this running, then went to the head system admin/system programmer and told him something was wrong and I couldn't kill my program. After seeing that ^C and ^\ did nothing useful he logged into another terminal, became root, found "superman" with ps, and did a kill -9.

The look on his face was priceless when "superman" just printed something like "SIGKILL is not strong enough to harm a Kryptonian!" and continued running.

I was a little sad when later Unixes made SIGKILL kill processes being traced.

I love this story. Thanks for sharing!

I suppose these days you could just write a kernel module, either bundling a shell language in there, or do something root-kit like with protecting the process...

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