Re: extending callbacks?

["H. Peter Anvin" <hpa@zytor.com>]

Followup to:  <Pine.GSO.4.44.0203191111320.20995-100000@speedy>
By author:    Matthias Scheidegger <mscheid@iam.unibe.ch>
In newsgroup: linux.dev.kernel
> 
> I've got the following problem: I want to register a callback in a kernel
> structure, but I need to supply an additional argument to my own code. I.e. I
> need a callback
> 
> int (*cb)(int u)
> 
> to really call
> 
> int (*real_cb)(int u, void* my_arg)
> 
> At the moment, I'm only focussing on the i386 architecture.
> In user space, I'd do this by generating some machine code, which takes the
> original args, pushes my_fixed_arg and calls real_cb (using mprotect to make
> the generated code callable). That way I'd use a function
> 
> int (*)(int) create_callback(int (*real_cb)(int, void*), void *arg);
> 
> Is there a good way to do that in the kernel?
> Not necessarily using self modifying code, I'll only use it if I must.
> 

In general, it's impossible.  On a lot of architectures, it happens to
"just work" with the appropriate cast, but that's completely dependent
on the ABI.

The extra arguemnt, of course, contains garbage.

	-hpa
-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt	<amsp@zytor.com>