Re: increase the number of system call parameters

[Andi Kleen <ak@muc.de>]

In article <Pine.LNX.4.33.0110221334200.1121-100000@hagbart.nvg.ntnu.no>,
=?iso-8859-1?Q?Roar_Thron=E6s?= <roart@nvg.ntnu.no> writes:
> Hi
> (I am sorry if this question has been asked and answered before)

> How do you increase the number of system call parameters, and how many
> can you at most have?

You can have upto 6 argument on i386. Each argument needs an register 
to pass and the i386 has only 8 and two are used for the stack pointer
and the syscall number. This leaves you 6.
Other architectures may not have that limitation.

> Would up to 12 parameters be possible, and how?

Yes. Just pass a pointer to an auxillary structure as the first argument
and do a copy_from_user on that structure at the entry point. Put the
arguments in that structure. In user space you can hide the structure in a 
stub.

Some system calls (mmap, old_select, socketcall) are in fact implemented 
like this because they were designed before the entry point supported
6 arguments.

A note on design: if you have a function call that needs 12 arguments you
probably forgot some[1] (in short it is a strong cue for a broken design,
you should probably split it in smaller calls) 

-Andi

[1] unknown author