Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task

[Linus Torvalds <torvalds@linux-foundation.org>]

Chuck seems to have caught a bug, although the wrong one:

On Thu, 29 Nov 2007, Chuck Ebbert wrote:
>
> On 11/28/2007 07:42 PM, Roland McGrath wrote:
> > --- a/arch/x86/ia32/ptrace32.c
> > +++ b/arch/x86/ia32/ptrace32.c
> > ...
> > +		if (child == current)
> > +			load_gs_index(child->thread.gsindex);

This is correct.

But the ones that do the same thing for fs/es/ds are *not*. Those three 
registers are kernel mode registers (ds/es are the regular kernel data 
segment, fs is the per-cpu data segment), and restored on return to user 
space from the stack.

For similar reasons, this is wrong:

> > @@ -129,15 +137,23 @@ static int getreg32(struct task_struct *child, unsigned regno, u32 *val)
> >  	switch (regno) {
> >  	case offsetof(struct user32, regs.fs):
> >  		*val = child->thread.fsindex;
> > +		if (child == current)
> > +			asm("movl %%fs,%0" : "=r" (*val));
> >  		break;

That %fs is the kernel per-cpu thing, not the user %fs.

But this one is correct:

> >  	case offsetof(struct user32, regs.gs):
> >  		*val = child->thread.gsindex;
> > +		if (child == current)
> > +			asm("movl %%gs,%0" : "=r" (*val));
> 
> Won't this return the kernel's GS instead of the user's?

No, %gs is untouched by the kernel, so it contains user space version, and 
getting the value directly from %gs looks correct.

		Linus
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