Re: %gs:0 thread pseudoregister in oskit-mach

[Jeroen Dekkers]

On Tue, Apr 23, 2002 at 07:08:08PM +0200, Jeroen Dekkers wrote:
> On Sun, Apr 21, 2002 at 03:26:56PM -0400, Roland McGrath wrote:
> > > You forgot to make the page table entries also user accessible. Other
> > > than that, everything works perfect. Thank you.
> > 
> > Great.  Thanks for the fix, and thanks for testing.  Have you/can you also
> > test to be absolutely sure that users cannot access anything but the one
> > word from %gs, and that they cannot access >VM_MAX_ADDRESS using %ds
> > (i.e. normal accesses)?  I'm pretty darn sure that is all correct, but just
> > to be paranoid.
> 
> I tried to acces %gs:4 and this failed with an "illegal
> instruction". I also tried to access 0xc0000000 which got a
> segmentation fault. I don't know if I have to test more.

When hitting the send button I recalled that I had to remove an
assert, see the patch.

Index: i386/i386/trap.c
===================================================================
RCS file: /cvsroot/hurd/gnumach/i386/i386/trap.c,v
retrieving revision 1.4.2.3
diff -u -p -r1.4.2.3 trap.c
--- i386/i386/trap.c    5 Apr 2001 06:52:46 -0000       1.4.2.3
+++ i386/i386/trap.c    23 Apr 2002 17:11:00 -0000
@@ -468,7 +468,6 @@ int user_trap(regs)
                printf("user page fault at linear address %08x\n", subcode);
                trap_dump (regs);
 #endif
-               assert(subcode < LINEAR_MIN_KERNEL_ADDRESS);
                (void) vm_fault(thread->task->map,
                                trunc_page((vm_offset_t)subcode),
                                (regs->err & T_PF_WRITE)

Jeroen Dekkers
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