%%db6 register still not saved 2.2.16 (x86)

From: James Cownie (jcownie@etnus.com)
Date: Mon Jun 12 2000 - 07:33:07 EST

Guys,

I see you all discussing changes in the ptrace interface for x86
extended FP register support, so I hope you're the right people for
this...

There's still an outstanding bug in the 2.2. series (all of them)
which is that debug register 6 (the debug status register) is never
saved back into the tss as it should be.

This means that a debugger can never detect from the debug status
register that a data watch point has been hit.

This was reported (with a patch) back in December 1999 (see below).

Any chance that the (one line) fix will ever make it into the kernel ?
(Or any reason why it shouldn't ? It's one store instruction on a
debug trap, and it's _needed_).

2.0 series kernels got this right :-)

Keep up the good work.

-- Jim

James Cownie <jcownie@etnus.com>
Etnus, Inc. +44 117 9071438
http://www.etnus.com

PATCH: saving i386 debug register on debug traps

Eric Pouech (Eric.Pouech@wanadoo.fr)
Thu, 09 Dec 1999 20:57:11 +0000

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since 2.1.0 (!), the DR6 value was not saved on debug traps, so
not allowing to know which DR0-3 did trigger a trap (if any)

this patch (against 2.2.13) should solve this issue

A+ 

-- 
---------------
Eric Pouech (http://perso.wanadoo.fr/eric.pouech/)
"The future will be better tomorrow", Vice President Dan Quayle
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--- traps.c.orig        Thu Dec  9 21:39:40 1999
+++ traps.c     Thu Dec  9 21:49:13 1999
@@ -354,10 +354,11 @@
        unsigned int condition;
        struct task_struct *tsk = current;
 
+       __asm__ __volatile__("movl %%db6,%0" : "=r" (condition));
+       tsk->tss.debugreg[6] = condition;
+
        if (regs->eflags & VM_MASK)
                goto debug_vm86;
-
-       __asm__ __volatile__("movl %%db6,%0" : "=r" (condition));
 
        /* Mask out spurious TF errors due to lazy TF clearing */
        if (condition & DR_STEP) {

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