Re: [PATCH 2/3] x86: Pass in pt_regs pointer for syscalls that need it

[Brian Gerst <brgerst@gmail.com>]

On Wed, Feb 11, 2009 at 2:41 AM, Tejun Heo <tj@kernel.org> wrote:
> Hello, Brian.
>
> Brian Gerst wrote:
>> Some syscalls need to access the pt_regs structure, either to copy
>> user register state or to modifiy it.  This patch adds stubs to load
>> the address of the pt_regs struct into the %eax register, and changes
>> the syscalls to regparm(1) to receive the pt_regs pointer as the
>> first argument.
>
> Heh... neat.  Just one question.
>
>> -asmlinkage long sys_iopl(unsigned long regsp)
>> +ptregscall long sys_iopl(struct pt_regs *regs, unsigned int level)
>>  {
>> -     struct pt_regs *regs = (struct pt_regs *)&regsp;
>> -     unsigned int level = regs->bx;
>
> Here and at other places where the function takes more than one
> arguments, wouldn't it be better to just take *regs and use other
> parameters from regs?  That way we won't have to worry about gcc
> corrupting register frame at all and I think it's cleaner that way.

Expanding the parameters is good documentation.  If there is a risk of
tail-call optimization causing the register corruption, then
asmlinkage_protect() should be used.  The problem isn't limited to
just the syscalls that take pt_regs.  It's just getting the args out
of the pt_regs struct was an easy hack to get around it.  I checked
the disassembly of these functions and didn't see this happen on gcc
4.3.0.

--
Brian Gerst