coregrind/x86-linux/ldt.c

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/*--- Simulation of Local Descriptor Tables        x86-linux/ldt.c ---*/
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/*
   This file is part of Valgrind, a dynamic binary instrumentation
   framework.

   Copyright (C) 2000-2004 Julian Seward 
      jseward@acm.org

   This program is free software; you can redistribute it and/or
   modify it under the terms of the GNU General Public License as
   published by the Free Software Foundation; either version 2 of the
   License, or (at your option) any later version.

   This program is distributed in the hope that it will be useful, but
   WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307, USA.

   The GNU General Public License is contained in the file COPYING.
*/

/* Details of the LDT simulation
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
   When a program runs natively, the linux kernel allows each *thread*
   in it to have its own LDT.  Almost all programs never do this --
   it's wildly unportable, after all -- and so the kernel never
   allocates the structure, which is just as well as an LDT occupies
   64k of memory (8192 entries of size 8 bytes).

   A thread may choose to modify its LDT entries, by doing the
   __NR_modify_ldt syscall.  In such a situation the kernel will then
   allocate an LDT structure for it.  Each LDT entry is basically a
   (base, limit) pair.  A virtual address in a specific segment is
   translated to a linear address by adding the segment's base value.
   In addition, the virtual address must not exceed the limit value.

   To use an LDT entry, a thread loads one of the segment registers
   (%cs, %ss, %ds, %es, %fs, %gs) with the index of the LDT entry (0
   .. 8191) it wants to use.  In fact, the required value is (index <<
   3) + 7, but that's not important right now.  Any normal instruction
   which includes an addressing mode can then be made relative to that
   LDT entry by prefixing the insn with a so-called segment-override
   prefix, a byte which indicates which of the 6 segment registers
   holds the LDT index.

   Now, a key constraint is that valgrind's address checks operate in
   terms of linear addresses.  So we have to explicitly translate
   virtual addrs into linear addrs, and that means doing a complete
   LDT simulation.

   Calls to modify_ldt are intercepted.  For each thread, we maintain
   an LDT (with the same normally-never-allocated optimisation that
   the kernel does).  This is updated as expected via calls to
   modify_ldt.

   When a thread does an amode calculation involving a segment
   override prefix, the relevant LDT entry for the thread is
   consulted.  It all works.

   There is a conceptual problem, which appears when switching back to
   native execution, either temporarily to pass syscalls to the
   kernel, or permanently, when debugging V.  Problem at such points
   is that it's pretty pointless to copy the simulated machine's
   segment registers to the real machine, because we'd also need to
   copy the simulated LDT into the real one, and that's prohibitively
   expensive.

   Fortunately it looks like no syscalls rely on the segment regs or
   LDT being correct, so we can get away with it.  Apart from that the
   simulation is pretty straightforward.  All 6 segment registers are
   tracked, although only %ds, %es, %fs and %gs are allowed as
   prefixes.  Perhaps it could be restricted even more than that -- I
   am not sure what is and isn't allowed in user-mode.
*/

/* 13 Dec 04: all this stuff has been moved into
   coregrind/x86/state.c. */

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