[PATCH v2 2/6] powerpc/kprobes: Do not suppress instruction emulation if a single run failed

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Currently, we disable instruction emulation if emulate_step() fails for
any reason. However, such failures could be transient and specific to a
particular run. Instead, only disable instruction emulation if we have
never been able to emulate this. If we had emulated this instruction
successfully at least once, then we single step only this probe hit and
continue to try emulating the instruction in subsequent probe hits.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
 arch/powerpc/kernel/kprobes.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
index c2a6ab38a67f..4c1702423676 100644
--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c
@@ -261,9 +261,20 @@ static int try_to_emulate(struct kprobe *p, struct pt_regs *regs)
 		 */
 		printk("Can't step on instruction %x\n", insn);
 		BUG();
-	} else
-		/* This instruction can't be boosted */
-		p->ainsn.boostable = -1;
+	} else {
+		/*
+		 * If we haven't previously emulated this instruction, then it
+		 * can't be boosted. Note it down so we don't try to do so again.
+		 *
+		 * If, however, we had emulated this instruction in the past,
+		 * then this is just an error with the current run (for
+		 * instance, exceptions due to a load/store). We return 0 so
+		 * that this is now single-stepped, but continue to try
+		 * emulating it in subsequent probe hits.
+		 */
+		if (unlikely(p->ainsn.boostable != 1))
+			p->ainsn.boostable = -1;
+	}
 
 	return ret;
 }
-- 
2.14.1