MIPS Kprobes: Support branch instructions probing
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.
Signed-off-by: Maneesh Soni <manesoni@cisco.com>
Signed-off-by: Victor Kamensky <kamensky@cisco.com>
Cc: David Daney <david.daney@cavium.com>
Cc: ananth@in.ibm.com
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2914/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
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 @@
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 @@
* 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 @@
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 @@
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 @@
}
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 @@
}
-/*
- * 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();
