[PATCH 3/5] powerpc/kprobes: Check instruction validity during kprobe registration

[Naveen N. Rao]

In trap-based (classic) kprobes, we try to emulate the probed
instruction so as to avoid having to single step it. We use a flag to
determine if the probed instruction was successfully emulated, so that we
can speed up subsequent probe hits.

However, emulate_step() doesn't differentiate between unknown
instructions and an emulation attempt that failed. As such, the current
heuristic is not of much use. Instead, use analyse_instr() during kprobe
registration to determine if the probed instruction can be decoded by
our instruction emulation infrastructure. For unknown instructions, we
can then directly single-step while for other instructions, we can
attempt to emulate and fall back to single stepping if that fails.

Signed-off-by: Naveen N. Rao <naveen.n....@linux.vnet.ibm.com>
---
 arch/powerpc/kernel/kprobes.c | 62 +++++++++--------------------------
 1 file changed, 16 insertions(+), 46 deletions(-)

diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
index b7b20875d34d91..bbef9e918ecb39 100644
--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c
@@ -107,6 +107,8 @@ int arch_prepare_kprobe(struct kprobe *p)
 {
        int ret = 0;
        struct kprobe *prev;
+       struct pt_regs regs;
+       struct instruction_op op;
        struct ppc_inst insn = ppc_inst_read((struct ppc_inst *)p->addr);
 
        if ((unsigned long)p->addr & 0x03) {
@@ -140,9 +142,18 @@ int arch_prepare_kprobe(struct kprobe *p)
        if (!ret) {
                patch_instruction((struct ppc_inst *)p->ainsn.insn, insn);
                p->opcode = ppc_inst_val(insn);
+
+               /* Check if this is an instruction we recognise */
+               p->ainsn.boostable = 0;
+               memset(&regs, 0, sizeof(struct pt_regs));
+               regs.nip = (unsigned long)p->addr;
+               regs.msr = MSR_KERNEL;
+               ret = analyse_instr(&op, &regs, insn);
+               if (ret == 1 || (ret == 0 && GETTYPE(op.type) != UNKNOWN))
+                       p->ainsn.boostable = 1;
+               ret = 0;
        }
 
-       p->ainsn.boostable = 0;
        return ret;
 }
 NOKPROBE_SYMBOL(arch_prepare_kprobe);
@@ -225,47 +236,6 @@ void arch_prepare_kretprobe(struct kretprobe_instance *ri, 
struct pt_regs *regs)
 }
 NOKPROBE_SYMBOL(arch_prepare_kretprobe);
 
-static int try_to_emulate(struct kprobe *p, struct pt_regs *regs)
-{
-       int ret;
-       struct ppc_inst insn = ppc_inst_read((struct ppc_inst *)p->ainsn.insn);
-
-       /* regs->nip is also adjusted if emulate_step returns 1 */
-       ret = emulate_step(regs, insn);
-       if (ret > 0) {
-               /*
-                * Once this instruction has been boosted
-                * successfully, set the boostable flag
-                */
-               if (unlikely(p->ainsn.boostable == 0))
-                       p->ainsn.boostable = 1;
-       } else if (ret < 0) {
-               /*
-                * We don't allow kprobes on mtmsr(d)/rfi(d), etc.
-                * So, we should never get here... but, its still
-                * good to catch them, just in case...
-                */
-               printk("Can't step on instruction %s\n", ppc_inst_as_str(insn));
-               BUG();
-       } 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;
-}
-NOKPROBE_SYMBOL(try_to_emulate);
-
 int kprobe_handler(struct pt_regs *regs)
 {
        struct kprobe *p;
@@ -334,8 +304,8 @@ int kprobe_handler(struct pt_regs *regs)
                set_current_kprobe(p, regs, kcb);
                kprobes_inc_nmissed_count(p);
                kcb->kprobe_status = KPROBE_REENTER;
-               if (p->ainsn.boostable >= 0) {
-                       ret = try_to_emulate(p, regs);
+               if (p->ainsn.boostable) {
+                       ret = emulate_step(regs, ppc_inst_read((struct ppc_inst 
*)p->ainsn.insn));
 
                        if (ret > 0) {
                                restore_previous_kprobe(kcb);
@@ -356,8 +326,8 @@ int kprobe_handler(struct pt_regs *regs)
                return 1;
        }
 
-       if (p->ainsn.boostable >= 0) {
-               ret = try_to_emulate(p, regs);
+       if (p->ainsn.boostable) {
+               ret = emulate_step(regs, ppc_inst_read((struct ppc_inst 
*)p->ainsn.insn));
 
                if (ret > 0) {
                        if (p->post_handler)
-- 
2.30.2