Re: PATCH: Enable x86 XML target descriptions

H.J. Lu
2010-02-22T21:41:16Z

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.

Thanks.

--
H.J.