powerpc/kprobes: Do not suppress instruction emulation if a single run failed

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>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
index c2a6ab38a67f092f09ef2d1e52f6729beff207b8..4c17024236766d12b2ceea8dd3cd3fc899f7c63f 100644 (file)
--- 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;
 }