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SubjectRe: [Patch 1/2] Improvements and minor fixes to HW Breakpoint interface
On Wed, May 20, 2009 at 10:04:09AM -0400, Alan Stern wrote:
> On Wed, 20 May 2009, K.Prasad wrote:
>
> > --- linux-2.6-tip.hbkpt.orig/arch/x86/kernel/hw_breakpoint.c
> > +++ linux-2.6-tip.hbkpt/arch/x86/kernel/hw_breakpoint.c
> > @@ -296,6 +296,19 @@ void arch_flush_thread_hw_breakpoint(str
> >
> > /*
> > * Handle debug exception notifications.
> > + *
> > + * Return value is either NOTIFY_STOP or NOTIFY_DONE as explained below.
> > + *
> > + * NOTIFY_DONE returned if one of the following conditions is true.
> > + * i) When the causative address is from user-space and the exception
> > + * is a valid one i.e. not triggered a result of lazy debug register
> --------------------^----^ missing ',' characters
> ---------------------------------------^ missing "as"
>
> > + * switching
> > + * ii) When there are more bits than trap<n> set in DR6 register (such
> > + * as BD, BS or BT) indicating that more than one debug condition is
> > + * met and requires some more action in do_debug().
> > + *
> > + * NOTIFY_STOP returned for all other cases
> > + *
> > */
> > int __kprobes hw_breakpoint_handler(struct die_args *args)
> > {
> > @@ -346,8 +359,10 @@ int __kprobes hw_breakpoint_handler(stru
> > * or due to the delay between updates of hbp_kernel_pos
> > * and this_hbp_kernel.
> > */
> > - if (!bp)
> > + if (!bp) {
> > + rc = NOTIFY_STOP;
> > continue;
> > + }
> >
> > (bp->triggered)(bp, args->regs);
>
> This is the same mistake as before. If rc had previously been set to
> NOTIFY_DONE then you will unconditionally change it back to
> NOTIFY_STOP. You should make the code do what the comment says, set rc
> to NOTIFY_DONE when a non-NULL user breakpoint occurs:
>
> if (i >= hbp_kernel_pos) {
> bp = per_cpu(this_hbp_kernel[i], cpu);
> } else {
> bp = current->thread.hbp[i];
> if (bp)
> rc = NOTIFY_DONE;
> }
> /*
> * bp can be NULL due to lazy debug register switching
> * or due to the delay between updates of hbp_kernel_pos
> * and this_hbp_kernel.
> */
> if (!bp)
> continue;
>
> (bp->triggered)(bp, args->regs);
>
>
> The rest of the changes are okay.
>
> Alan Stern
>

Thanks. I am pasting the hw_breakpoint_handler() function completely
below (as opposed to patch form which may be difficult to follow). Upon
your consent with the changes, I intend to send a consolidated patchset
to Ingo (merging the patches over hw_breakpoint.c) for inclusion in -tip
tree.

I have also moved the "(*dr6_p) &= ~(DR_TRAP0 << i)" before the "if (!bp)"
check.
/*
* Handle debug exception notifications.
*
* Return value is either NOTIFY_STOP or NOTIFY_DONE as explained below.
*
* NOTIFY_DONE returned if one of the following conditions is true.
* i) When the causative address is from user-space and the exception
* is a valid one, i.e. not triggered as a result of lazy debug register
* switching
* ii) When there are more bits than trap<n> set in DR6 register (such
* as BD, BS or BT) indicating that more than one debug condition is
* met and requires some more action in do_debug().
*
* NOTIFY_STOP returned for all other cases
*
*/
int __kprobes hw_breakpoint_handler(struct die_args *args)
{
int i, cpu, rc = NOTIFY_STOP;
struct hw_breakpoint *bp;
unsigned long dr7, dr6;
unsigned long *dr6_p;
/* The DR6 value is pointed by args->err */
dr6_p = (unsigned long *)ERR_PTR(args->err);
dr6 = *dr6_p;
/* Do an early return if no trap bits are set in DR6 */
if ((dr6 & DR_TRAP_BITS) == 0)
return NOTIFY_DONE;
/* Lazy debug register switching */
if (!test_tsk_thread_flag(current, TIF_DEBUG))
arch_uninstall_thread_hw_breakpoint();
get_debugreg(dr7, 7);
/* Disable breakpoints during exception handling */
set_debugreg(0UL, 7);
/*
* Assert that local interrupts are disabled
* Reset the DRn bits in the virtualized register value.
* The ptrace trigger routine will add in whatever is needed.
*/
current->thread.debugreg6 &= ~DR_TRAP_BITS;
cpu = get_cpu();
/* Handle all the breakpoints that were triggered */
for (i = 0; i < HBP_NUM; ++i) {
if (likely(!(dr6 & (DR_TRAP0 << i))))
continue;
/*
* Find the corresponding hw_breakpoint structure and
* invoke its triggered callback.
*/
if (i >= hbp_kernel_pos)
bp = per_cpu(this_hbp_kernel[i], cpu);
else {
bp = current->thread.hbp[i];
if (bp)
rc = NOTIFY_DONE;
}
/*
* Reset the 'i'th TRAP bit in dr6 to denote completion of
* exception handling
*/
(*dr6_p) &= ~(DR_TRAP0 << i);
/*
* bp can be NULL due to lazy debug register switching
* or due to the delay between updates of hbp_kernel_pos
* and this_hbp_kernel.
*/
if (!bp)
continue;
(bp->triggered)(bp, args->regs);
}
if (dr6 & (~DR_TRAP_BITS))
rc = NOTIFY_DONE;
set_debugreg(dr7, 7);
put_cpu_no_resched();
return rc;
}
Thanks,
K.Prasad


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