powerpc, hw_breakpoint: Discard extraneous interrupt due to accesses outside symbol length

Many a times, the requested breakpoint length can be less than the
fixed breakpoint length i.e. 8 bytes supported by PowerPC 64-bit
server (Book III S) processors.  This could lead to extraneous
interrupts resulting in false breakpoint notifications.  This
detects and discards such interrupts for non-ptrace requests.
We don't change ptrace behaviour to avoid breaking compatability.

[Suggestion from Paul Mackerras <paulus@samba.org> to add a new flag in
'struct arch_hw_breakpoint' to identify extraneous interrupts]

Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

2 files changed, 22 insertions, 2 deletions

diff --git a/arch/powerpc/include/asm/hw_breakpoint.h b/arch/powerpc/include/asm/hw_breakpoint.h
index 6576bad1069c..ea87f8ae7bdb 100644
--- a/arch/powerpc/include/asm/hw_breakpoint.h
+++ b/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;
diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
index 7bd01a56d194..ed39805a3b84 100644
--- a/arch/powerpc/kernel/hw_breakpoint.c
+++ b/arch/powerpc/kernel/hw_breakpoint.c
@@ -204,6 +204,7 @@ int __kprobes hw_breakpoint_handler(struct die_args *args)
         int stepped = 1;
         struct arch_hw_breakpoint *info;
         unsigned int instr;
+        unsigned long dar = regs->dar;
 
         /* Disable breakpoints during exception handling */
         set_dabr(0);
@@ -234,6 +235,22 @@ int __kprobes hw_breakpoint_handler(struct die_args *args)
                 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;
@@ -261,7 +278,8 @@ int __kprobes hw_breakpoint_handler(struct die_args *args)
          * 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:
@@ -292,7 +310,8 @@ int __kprobes single_step_dabr_instruction(struct die_args *args)
          * 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