[Toybox] fun with vfork

[Josh Gao]

Do you have vfork tagged with __attribute__((returns_twice))? AFAIK, that's
the incantation to get gcc to do the right thing for setjmpy functions.

On Oct 11, 2016 4:23 AM, "Rob Landley" <rob at landley.net> wrote:

> While doing the rest of nommu support in netcat -L, I had some variant of:
>
>   function()
>   {
>     int child, blah = 3;
>
>     for (;;) {
>       crunch(blah);
>       child = vfork();
>       if (child<1) break;
>     }
>     thingy();
>
>     execlp(stuff);
>   }
>
> And gcc's optimizer went "blah isn't used anymore after the for loop,
> I'll trim the stack frame down so the return address in the call to
> thingy() in the child overwrites it, and then when vfork returns it's
> corrupted in the parent and the next call to crunch() goes bye-bye".
> Because gcc's optimizer does not understand vfork()'s impact on
> "liveness analysis". (You can think of vfork() as a setjmp that will
> fork() when it hits the next exec or exit, and then the parent process
> longjmp()s back to the stack until the child. But gcc's optimizer doesn't.)
>
> The fix is to add an unnecessary use of blah to the end of the function
> to let it know it's still %*#(%&& used, but then I need a GREAT BIG
> COMMENT to explain why so it isn't removed in future cleanup passes. And
> every other variable potentially has that same problem.
>
> As usual, I want to punch gcc's optimizer in the face and go "DO WHAT I
> TOLD YOU TO DO, DON'T MAKE STUFF UP!" but it never listens. (Do I have
> to start building everything with -O0? What optimization level gives me
> dead code elimination and nothing else?)
>
> Rob
>
> P.S. I'm always amused by the go/rust/swift developers who haven't hit
> their language with anything like the range of use cases you get in C,
> confidently stating that they have yet to see such strange corner cases
> in _their_ language yet. Uh-huh. There's a reason for that and it's
> probably not the one you think.
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>
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