Use case for ftag register modification on X86-64

[sujoy saraswati]

Hi all,
 On X86-64 architectures, (reference Intel 64 and IA-32 architectures software 
developer manual, volume 1, section 8.1.7 
 http://www.intel.com/Assets/PDF/manual/253665.pdf ), the X87 FPU tag word 
description has the folowing comment -
"Software cannot directly load or modify the tags in the tag register. The 
FLDENV and
FRSTOR instructions load an image of the tag register into the x87 FPU; 
however, 
the
x87 FPU uses those tag values only to determine if the data registers are empty
(11B) or non-empty (00B, 01B, or 10B)."
 However, using a gdb, I can change the ftag register value. I used gdb 6.8, 
the 
session output is given below.
 
  I have few questions -
 
1) Is it right for gdb users to be allowed to change ftag, given the 
restriction 
stated in the Intel document ?
 
2) What would be a use case for someone to change the ftag value in a debugging 
session ? I mean, when do gdb users see a need to change ftag value ?
 
3) When I change the register value from gdb, the ftag value change is not 
always what I give (see example output below). Is the kernel ptrace code 
validating the value before seting the ftag value ?
 
address@hidden gdb gdb/mnbn/vla_linux
GNU gdb Fedora (6.8-37.el5)
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu"...
(gdb) shell uname -a
Linux abacus 2.6.18-160.el5 #1 SMP Mon Jul 27 17:28:29 EDT 2009 x86_64 x86_64 
x86_64 GNU/Linux
(gdb) b main
Breakpoint 1 at 0x4004ec: file vla.c, line 16.
(gdb) r
Starting program: /home/sujoys/gdb/mnbn/vla_linux
Breakpoint 1, main () at vla.c:16
16         fl1 = 78.99;
(gdb) i reg $ftag
ftag           0xffff   65535
(gdb) p $ftag=0x7755
$1 = 30549
(gdb) i reg $ftag
ftag           0x7755   30549
(gdb) p $ftag=0x500
$2 = 1280
(gdb) i reg $ftag
ftag           0x5555   21845

 Do let me know if I need to give any more information / clarification 
regardign 
my query.
 
Regards,
Sujoy