[PATCH] kprobes: fix single-stepping when probing a trap variant

[Ananth N Mavinakayanahalli]

Hi,

We currently single-step inline if the instruction on which a kprobe is
set is a trap variant.

- variants (such as tdnei, used by BUG()) typically evaluate a condition
  and cause a trap only if the condition is satisfied
- kprobes uses the unconditional trap instruction (0x7fe00008) and
  single-stepping again on this instruction, resulting in another trap
  without evaluating the condition is obviously incorrect.

Signed-off-by: Ananth N Mavinakayanahalli <ananth at in.ibm.com>
---

 arch/powerpc/kernel/kprobes.c |   12 +++++++-----
 1 files changed, 7 insertions(+), 5 deletions(-)

Index: linux-2.6.16-rc6/arch/powerpc/kernel/kprobes.c
===================================================================
--- linux-2.6.16-rc6.orig/arch/powerpc/kernel/kprobes.c
+++ linux-2.6.16-rc6/arch/powerpc/kernel/kprobes.c
@@ -92,11 +92,13 @@ static inline void prepare_singlestep(st
 
 	regs->msr |= MSR_SE;
 
-	/* single step inline if it is a trap variant */
-	if (is_trap(insn))
-		regs->nip = (unsigned long)p->addr;
-	else
-		regs->nip = (unsigned long)p->ainsn.insn;
+	/*
+	 * On powerpc we should single step on the original
+	 * instruction even if the probed insn is a trap
+	 * variant as values in regs could play a part in
+	 * if the trap is taken or not
+	 */
+	regs->nip = (unsigned long)p->ainsn.insn;
 }
 
 static inline void save_previous_kprobe(struct kprobe_ctlblk *kcb)