Re: 32-bit syscalls from 64-bit process on x86-64?

[Andi Kleen <ak@suse.de>]

On Tue, Dec 14, 2004 at 11:01:12PM +0100, Petr Vandrovec wrote:
> On 14 Dec 04 at 8:45, Andi Kleen wrote:
> > > #define TOLM                            \
> > >                 "pushl %%cs\n"          \
> > >                 "pushl $91f\n"          \
> > >                 "ljmpl $0x33,$90f\n"    \
> > 
> > It's useless, there is nothing in the kernel code that checks the 
> > 32bit segment.
> 
> ???  Processor checks for 32bit/64bit segment.  It is impossible to load
> upper 32bit of all registers with non-zero value or call 64bit
> syscall entry point from 32bit mode.  As x86-64 kernel offers 64bit 
> interface through syscall only, only way how to issue 64bit system call
> is using syscall instruction in 64bit code.

Ah sorry. I misread the intention of your code. I thought you wanted
to do it the other way round - 32bit syscall from 64bit code.
I just wanted to point out that you can do it directly without
changing the code segment, as long as you use int $0x80.

>From 64bit-from-32bit the lcall is needed agreed. However as a 
warning it will not work for all calls since a few check a bit
in task_struct that says if the process is 32bit or 64bit
(rather rare though, most prominent is signal handling) 

> 
> Or are you trying to say that these samples do not work and you cannot
> call 64bit entry point from 32bit app, or vice versa?  Then I'm afraid
> that you are not completely right, as these samples do work...

I haven't ever tried it, but I see no reason it cannot work.

-Andi