Re: x86: SIGTRAP handling differences from 2.4 to 2.6

From: Linus Torvalds
Date: Sun Nov 23 2003 - 13:08:47 EST



On 22 Nov 2003, H. Peter Anvin wrote:
> >
> > Hmm.. Looking at the signal sending code, we actually do special-case
> > "init" there already - but only for the "kill -1" case. If the test for
> > "pid > 1" was moved into "group_send_sig_info()" instead, that would
> > pretty much do it, I think.
> >
>
> Okay... I'm going to ask the obvious dumb question:
>
> Why do we bother special-casing init at all?

Because the kernel depends on it existing. "init" literally _is_ special
from a kernel standpoint, because its' the "reaper of zombies" (and, may I
add, that would be a great name for a rock band).

So without init, the kernel wouldn't have anybody to fall back on when a
parent process dies, and would become very very unhappy. Historically it
actually oopsed the kernel.

UNIX semantics literally _require_ that "getppid()" should return 1 if
your parent dies, and that's "current->p_parent->tgid". So we have to have
a parent with pid 1, and thus init really _is_ special.

Yeah, we could have _other_ special cases (we could create another process
that is invisible and has pid 1), but the fact is, _some_ special case is
required. It might as well be "you can't kill init".

Linus

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