Re: PATCH: Enable x86 XML target descriptions

[Mark Kettenis <mark.kettenis@xs4all.nl>]

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> Date: Mon, 22 Feb 2010 13:06:31 -0800
> From: "H.J. Lu" <hjl.tools@gmail.com>
> 
> On Mon, Feb 22, 2010 at 11:50 AM, Mark Kettenis <mark.kettenis@xs4all.nl> wrote:
> >> Date: Mon, 22 Feb 2010 12:03:03 -0500
> >> From: Daniel Jacobowitz <dan@codesourcery.com>
> >>
> >> On Mon, Feb 22, 2010 at 05:56:58PM +0100, Mark Kettenis wrote:
> >> > I've looked at the Linux kernel sources for the kernel on my
> >> > workstation (2.6.27 in its OpenSUSE incarnation), and the only way to
> >> > distinguish between a 32-bit and a 64-bit process seems to be to
> >> > attempt to write one of the debug address registers with a value
> >> > that's larger than 0xffffffff.  If that fails, you have a 32-bit
> >> > process, otherwise it's a 64-bit process.
> >>
> >> Yuck :-(  But I didn't see anything else either.
> >
> > Indeed.
> >
> >> Is there an eflags bit for this?  Even if so, IIRC, we may not want to
> >> use it; it's possible to run 32-bit code in a 64-bit process and some
> >> overly clever programs may do so.
> >
> > Nope, there is no %eflags/%rflags bit for this.  Not quite sure what
> > running 32-bit code in a 64-bit process actually means.  But I'd guess
> > you want the 64-bit view on the registers in that case.
> >
> > Anyway, I think it's probably best if HJ leaves this bit out of this
> > diff for now.  We can revisit the issue when AVX support is
> > introduced.
> >
> 
> Please see if my latest patch is OK:
> 
> ---
>  /* Get CS register.  */
>   errno = 0;
>   cs = ptrace (PTRACE_PEEKUSER, tid,
>                offsetof (struct user_regs_struct, cs), 0);
>   if (errno != 0)
>     perror_with_name (_("Couldn't get CS register"));
> 
>   /* Value of CS register:
>      1. 64bit: 0x33.
>      2. 32bit: 0x23.
>    */
>   if (cs == 0x33)
>     return tdesc_amd64_linux;
>   else
>     return tdesc_i386_linux;
> ---
> 
> In kernel, there is
> 
> 	regs->cs = test_thread_flag(TIF_64BIT_ILP32) ? __USER_CS : __USER32_CS;

I fear that's rather fragile.  I mean, the actual value of
__USER_CS/__USER32_CS is just an implementation detail isn't it?