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[PATCH 4/4] MIPS Kprobes: Support branch instructions probing - v2

To: Ralf Baechle <ralf@linux-mips.org>
Subject: [PATCH 4/4] MIPS Kprobes: Support branch instructions probing - v2
From: Maneesh Soni <manesoni@cisco.com>
Date: Tue, 8 Nov 2011 22:38:26 +0530
Cc: David Daney <david.daney@cavium.com>, ananth@in.ibm.com, kamensky@cisco.com, linux-kernel@vger.kernel.org, linux-mips@linux-mips.org
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User-agent: Mutt/1.5.20 (2009-06-14)
From: Maneesh Soni <manesoni@cisco.com>

MIPS Kprobes: Support branch instructions probing - v2

This patch provides support for kprobes on branch instructions. The branch
instruction at the probed address is actually emulated and not executed
out-of-line like other normal instructions. Instead the delay-slot instruction
is copied and single stepped out of line.

At the time of probe hit, the original branch instruction is evaluated
and the target cp0_epc is computed similar to compute_retrun_epc(). It
is also checked if the delay slot instruction can be skipped, which is
true if there is a NOP in delay slot or branch is taken in case of
branch likely instructions. Once the delay slot instruction is single
stepped the normal execution resume with the cp0_epc updated the earlier
computed cp0_epc as per the branch instructions.

o Changes from v1
- added missing preempt_enable_no_reshced()
- using refactored __compute_return_epc() to avoid missing instructions

Signed-off-by: Maneesh Soni <manesoni@cisco.com>
Signed-off-by: Victor Kamensky <kamensky@cisco.com>
---
 arch/mips/include/asm/kprobes.h |    5 ++
 arch/mips/kernel/kprobes.c      |  145 ++++++++++++++++++++++++++++++---------
 2 files changed, 117 insertions(+), 33 deletions(-)

diff --git a/arch/mips/include/asm/kprobes.h b/arch/mips/include/asm/kprobes.h
index e6ea4d4..1fbbca0 100644
--- a/arch/mips/include/asm/kprobes.h
+++ b/arch/mips/include/asm/kprobes.h
@@ -74,6 +74,8 @@ struct prev_kprobe {
                : MAX_JPROBES_STACK_SIZE)
 
 
+#define SKIP_DELAYSLOT 0x0001
+
 /* per-cpu kprobe control block */
 struct kprobe_ctlblk {
        unsigned long kprobe_status;
@@ -82,6 +84,9 @@ struct kprobe_ctlblk {
        unsigned long kprobe_saved_epc;
        unsigned long jprobe_saved_sp;
        struct pt_regs jprobe_saved_regs;
+       /* Per-thread fields, used while emulating branches */
+       unsigned long flags;
+       unsigned long target_epc;
        u8 jprobes_stack[MAX_JPROBES_STACK_SIZE];
        struct prev_kprobe prev_kprobe;
 };
diff --git a/arch/mips/kernel/kprobes.c b/arch/mips/kernel/kprobes.c
index 0ab1a5f..158467d 100644
--- a/arch/mips/kernel/kprobes.c
+++ b/arch/mips/kernel/kprobes.c
@@ -30,6 +30,7 @@
 #include <linux/slab.h>
 
 #include <asm/ptrace.h>
+#include <asm/branch.h>
 #include <asm/break.h>
 #include <asm/inst.h>
 
@@ -152,13 +153,6 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
                goto out;
        }
 
-       if (insn_has_delayslot(insn)) {
-               pr_notice("Kprobes for branch and jump instructions are not"
-                         "supported\n");
-               ret = -EINVAL;
-               goto out;
-       }
-
        if ((probe_kernel_read(&prev_insn, p->addr - 1,
                                sizeof(mips_instruction)) == 0) &&
                                insn_has_delayslot(prev_insn)) {
@@ -178,9 +172,20 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
         * In the kprobe->ainsn.insn[] array we store the original
         * instruction at index zero and a break trap instruction at
         * index one.
+        *
+        * On MIPS arch if the instruction at probed address is a
+        * branch instruction, we need to execute the instruction at
+        * Branch Delayslot (BD) at the time of probe hit. As MIPS also
+        * doesn't have single stepping support, the BD instruction can
+        * not be executed in-line and it would be executed on SSOL slot
+        * using a normal breakpoint instruction in the next slot.
+        * So, read the instruction and save it for later execution.
         */
+       if (insn_has_delayslot(insn))
+               memcpy(&p->ainsn.insn[0], p->addr + 1, sizeof(kprobe_opcode_t));
+       else
+               memcpy(&p->ainsn.insn[0], p->addr, sizeof(kprobe_opcode_t));
 
-       memcpy(&p->ainsn.insn[0], p->addr, sizeof(kprobe_opcode_t));
        p->ainsn.insn[1] = breakpoint2_insn;
        p->opcode = *p->addr;
 
@@ -231,16 +236,96 @@ static void set_current_kprobe(struct kprobe *p, struct 
pt_regs *regs,
        kcb->kprobe_saved_epc = regs->cp0_epc;
 }
 
-static void prepare_singlestep(struct kprobe *p, struct pt_regs *regs)
+/**
+ * evaluate_branch_instrucion -
+ *
+ * Evaluate the branch instruction at probed address during probe hit. The
+ * result of evaluation would be the updated epc. The insturction in delayslot
+ * would actually be single stepped using a normal breakpoint) on SSOL slot.
+ *
+ * The result is also saved in the kprobe control block for later use,
+ * in case we need to execute the delayslot instruction. The latter will be
+ * false for NOP instruction in dealyslot and the branch-likely instructions
+ * when the branch is taken. And for those cases we set a flag as
+ * SKIP_DELAYSLOT in the kprobe control block
+ */
+static int evaluate_branch_instruction(struct kprobe *p, struct pt_regs *regs,
+                                       struct kprobe_ctlblk *kcb)
 {
+       union mips_instruction insn = p->opcode;
+       long epc;
+       int ret = 0;
+
+       epc = regs->cp0_epc;
+       if (epc & 3)
+               goto unaligned;
+
+       if (p->ainsn.insn->word == 0)
+               kcb->flags |= SKIP_DELAYSLOT;
+       else
+               kcb->flags &= ~SKIP_DELAYSLOT;
+
+       ret = __compute_return_epc_for_insn(regs, insn);
+       if (ret < 0)
+               return ret;
+
+       if (ret == BRANCH_LIKELY_TAKEN)
+               kcb->flags |= SKIP_DELAYSLOT;
+
+       kcb->target_epc = regs->cp0_epc;
+
+       return 0;
+
+unaligned:
+       pr_notice("%s: unaligned epc - sending SIGBUS.\n", current->comm);
+       force_sig(SIGBUS, current);
+       return -EFAULT;
+
+}
+
+static void prepare_singlestep(struct kprobe *p, struct pt_regs *regs,
+                                               struct kprobe_ctlblk *kcb)
+{
+       int ret = 0;
+
        regs->cp0_status &= ~ST0_IE;
 
        /* single step inline if the instruction is a break */
        if (p->opcode.word == breakpoint_insn.word ||
            p->opcode.word == breakpoint2_insn.word)
                regs->cp0_epc = (unsigned long)p->addr;
-       else
-               regs->cp0_epc = (unsigned long)&p->ainsn.insn[0];
+       else if (insn_has_delayslot(p->opcode)) {
+               ret = evaluate_branch_instruction(p, regs, kcb);
+               if (ret < 0) {
+                       pr_notice("Kprobes: Error in evaluating branch\n");
+                       return;
+               }
+       }
+       regs->cp0_epc = (unsigned long)&p->ainsn.insn[0];
+}
+
+/*
+ * Called after single-stepping.  p->addr is the address of the
+ * instruction whose first byte has been replaced by the "break 0"
+ * instruction.  To avoid the SMP problems that can occur when we
+ * temporarily put back the original opcode to single-step, we
+ * single-stepped a copy of the instruction.  The address of this
+ * copy is p->ainsn.insn.
+ *
+ * This function prepares to return from the post-single-step
+ * breakpoint trap. In case of branch instructions, the target
+ * epc to be restored.
+ */
+static void __kprobes resume_execution(struct kprobe *p,
+                                      struct pt_regs *regs,
+                                      struct kprobe_ctlblk *kcb)
+{
+       if (insn_has_delayslot(p->opcode))
+               regs->cp0_epc = kcb->target_epc;
+       else {
+               unsigned long orig_epc = kcb->kprobe_saved_epc;
+               regs->cp0_epc = orig_epc + 4;
+       }
 }
 
 static int __kprobes kprobe_handler(struct pt_regs *regs)
@@ -279,8 +364,13 @@ static int __kprobes kprobe_handler(struct pt_regs *regs)
                        save_previous_kprobe(kcb);
                        set_current_kprobe(p, regs, kcb);
                        kprobes_inc_nmissed_count(p);
-                       prepare_singlestep(p, regs);
+                       prepare_singlestep(p, regs, kcb);
                        kcb->kprobe_status = KPROBE_REENTER;
+                       if (kcb->flags & SKIP_DELAYSLOT) {
+                               resume_execution(p, regs, kcb);
+                               restore_previous_kprobe(kcb);
+                               preempt_enable_no_resched();
+                       }
                        return 1;
                } else {
                        if (addr->word != breakpoint_insn.word) {
@@ -324,8 +414,16 @@ static int __kprobes kprobe_handler(struct pt_regs *regs)
        }
 
 ss_probe:
-       prepare_singlestep(p, regs);
-       kcb->kprobe_status = KPROBE_HIT_SS;
+       prepare_singlestep(p, regs, kcb);
+       if (kcb->flags & SKIP_DELAYSLOT) {
+               kcb->kprobe_status = KPROBE_HIT_SSDONE;
+               if (p->post_handler)
+                       p->post_handler(p, regs, 0);
+               resume_execution(p, regs, kcb);
+               preempt_enable_no_resched();
+       } else
+               kcb->kprobe_status = KPROBE_HIT_SS;
+
        return 1;
 
 no_kprobe:
@@ -334,25 +432,6 @@ no_kprobe:
 
 }
 
-/*
- * Called after single-stepping.  p->addr is the address of the
- * instruction whose first byte has been replaced by the "break 0"
- * instruction.  To avoid the SMP problems that can occur when we
- * temporarily put back the original opcode to single-step, we
- * single-stepped a copy of the instruction.  The address of this
- * copy is p->ainsn.insn.
- *
- * This function prepares to return from the post-single-step
- * breakpoint trap.
- */
-static void __kprobes resume_execution(struct kprobe *p,
-                                      struct pt_regs *regs,
-                                      struct kprobe_ctlblk *kcb)
-{
-       unsigned long orig_epc = kcb->kprobe_saved_epc;
-       regs->cp0_epc = orig_epc + 4;
-}
-
 static inline int post_kprobe_handler(struct pt_regs *regs)
 {
        struct kprobe *cur = kprobe_running();
-- 
1.7.1


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