Re: silent semantic changes with reiser4

[Linus Torvalds]

On Tue, 31 Aug 2004, Alan Cox wrote:
> 
> Several do TCP in user space. The only thing you need in kernel for
> TCP/IP is enough decode to decide who gets the packet.

Only thing? I don't think so. 

You also want to make sure that regular users cannot send "impossible" 
packets. Think about the old "ping of death" kind of thing, where a normal 
mis-behaving (and I'm not saying intentionally so: it might be a small bug 
that just overwrites some data) program should _not_ be able to cause 
problems on the network.

Admins absolutely _hate_ that. They will ban an OS if it sends out packets
that cause troublem. You should remember that - we used to do strange
things on the net (long long time ago), and we brought down servers by
mistake, and nobody ever considered it a server bug: it was a Linux bug
that it wouldn't do the right thing.

Things like not sending FIN-packets when a program suddenly goes away is 
NOT acceptable behaviour! Neither is it acceptable behaviour to allow user 
programs to make up their own packets.

> Even some non microkernel embedded OS's do this in order to keep kernel
> size down.

..and I'm not disagreeing that it doesn't happen. I explicitly mentioned 
PalmOS, I bet it happens in other cases too. But I'd strongly argue that 
it's a bug, not a feature.

It's a bug that people tend to accept in a "single-client" environment. 

NOTE! This is totally ignoring the fact that you can't be called "UNIX" 
any more. You _need_ to have sequence numbers etc be shared between 
multiple programs that all write to the stream. Again, that _does_ mean 
that you have another protection domain (aka "kernel" or "TCP deamon") 
that keeps track of the sequence number. 

		Linus
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