Date: Sun, 23 Nov 2003 09:59:47 -0800 (PST)
From: Linus Torvalds <torvalds@osdl.org>
To: "H. Peter Anvin" <hpa@zytor.com>
cc: linux-kernel@vger.kernel.org
Subject: Re: x86: SIGTRAP handling differences from 2.4 to 2.6
In-Reply-To: <bppjkt$ik0$1@cesium.transmeta.com>
Message-ID: <Pine.LNX.4.44.0311230954460.17378-100000@home.osdl.org>
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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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