Re: [PATCH 0/3] clone64() and unshare64() system calls

["H. Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>]

sukadev-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org wrote:
> | 
> | I thought that the consensus was that adding a new system call was
> | better than trying to force extensibility on to the existing
> | non-extensible system call.
> 
> There were couple of objections to extensible system calls like
> sys_indirect() and to Pavel's approach.
> 

This is a very different thing, though.  sys_indirect is pretty much a 
mechanism for having a sideband channel -- a second ABI -- into each and 
every system call, making it extremely hard to analyze what the full set 
of impact of a specific system call is.  Worse, as it was being proposed 
to have been used, it would have set state variables inside the kernel 
in a very opaque manner.

> | But if we are adding a new system call, why not make the new one
> | extensible to reduce the need for yet another new call in the future?
> 
> hypothetically, can we make a variant of clone() extensible to the point
> of requiring a copy_from_user() ?

The only issue is whether or not it's acceptable from a performance 
standpoint.  clone() is reasonably expensive, though.

	-hpa