Many a times, the requested breakpoint length can be less than the fixed
breakpoint length i.e. 8 bytes supported by PowerPC BookIII S. This could lead
to extraneous interrupts resulting in false breakpoint notifications. The patch
below detects and discards such interrupts for non-ptrace requests (we don't
want to change ptrace behaviour for fear of breaking compatability).
[Suggestions from Paul Mackerras <pau...@samba.org> to add a new flag in
'struct arch_hw_breakpoint' to identify extraneous interrupts]
Signed-off-by: K.Prasad <pra...@linux.vnet.ibm.com>
---
arch/powerpc/include/asm/hw_breakpoint.h | 1 +
arch/powerpc/kernel/hw_breakpoint.c | 23 +++++++++++++++++++++--
2 files changed, 22 insertions(+), 2 deletions(-)
Index: linux-2.6.ppc64_test/arch/powerpc/kernel/hw_breakpoint.c
===================================================================
--- linux-2.6.ppc64_test.orig/arch/powerpc/kernel/hw_breakpoint.c
+++ linux-2.6.ppc64_test/arch/powerpc/kernel/hw_breakpoint.c
@@ -202,6 +202,7 @@ int __kprobes hw_breakpoint_handler(stru
struct pt_regs *regs = args->regs;
int stepped = 1;
struct arch_hw_breakpoint *info;
+ unsigned long dar = regs->dar;
/* Disable breakpoints during exception handling */
set_dabr(0);
@@ -232,6 +233,22 @@ int __kprobes hw_breakpoint_handler(stru
goto out;
}
+ /*
+ * Verify if dar lies within the address range occupied by the symbol
+ * being watched to filter extraneous exceptions.
+ */
+ if (!((bp->attr.bp_addr <= dar) &&
+ (dar <= (bp->attr.bp_addr + bp->attr.bp_len)))) {
+ /*
+ * This exception is triggered not because of a memory access
+ * on the monitored variable but in the double-word address
+ * range in which it is contained. We will consume this
+ * exception, considering it as 'noise'.
+ */
+ info->extraneous_interrupt = true;
+ } else
+ info->extraneous_interrupt = false;
+
/* Do not emulate user-space instructions, instead single-step them */
if (user_mode(regs)) {
bp->ctx->task->thread.last_hit_ubp = bp;
@@ -255,7 +272,8 @@ int __kprobes hw_breakpoint_handler(stru
* As a policy, the callback is invoked in a 'trigger-after-execute'
* fashion
*/
- perf_bp_event(bp, regs);
+ if (!info->extraneous_interrupt)
+ perf_bp_event(bp, regs);
set_dabr(info->address | info->type | DABR_TRANSLATION);
out:
@@ -286,7 +304,8 @@ int __kprobes single_step_dabr_instructi
* We shall invoke the user-defined callback function in the single
* stepping handler to confirm to 'trigger-after-execute' semantics
*/
- perf_bp_event(bp, regs);
+ if (!bp_info->extraneous_interrupt)
+ perf_bp_event(bp, regs);
/*
* Do not disable MSR_SE if the process was already in
Index: linux-2.6.ppc64_test/arch/powerpc/include/asm/hw_breakpoint.h
===================================================================
--- linux-2.6.ppc64_test.orig/arch/powerpc/include/asm/hw_breakpoint.h
+++ linux-2.6.ppc64_test/arch/powerpc/include/asm/hw_breakpoint.h
@@ -27,6 +27,7 @@
#ifdef CONFIG_HAVE_HW_BREAKPOINT
struct arch_hw_breakpoint {
+ bool extraneous_interrupt;
u8 len; /* length of the target data symbol */
int type;
unsigned long address;
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