Re: PATCH: Enable x86 XML target descriptions

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

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> Date: Mon, 22 Feb 2010 13:41:07 -0800
> From: "H.J. Lu" <hjl.tools@gmail.com>
> 
> On Mon, Feb 22, 2010 at 1:30 PM, Mark Kettenis <mark.kettenis@xs4all.nl> wrote:
> >> 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?
> >
> 
> That is how strace checks 32bit process on Linux/x86-64  I have
> discussed it with Peter and Suresh. It is very unlikely Linux kernel will
> break strace.  In any case,  we will add a new ptrace option to Linux
> 2.6.35 to get TIF_64BIT_ILP32 among other things. I will update gdb
> to try the new ptrace option first and then fail back to CS register.

OK, that makes it less fragile than I thought.  But please use
#defines with meaningful names instead of magic constants.