ARM: optprobes: execute instruction during restoring if possible.

This patch removes software emulation or simulation for most of probed
instructions. If the instruction doesn't use PC relative addressing,
it will be translated into following instructions in the restore code
in code template:

ldmia {r0 - r14} // restore all instruction except PC
<instruction> // direct execute the probed instruction
b next_insn // branch to next instruction.

Signed-off-by: Wang Nan <[e-mail]>
Reviewed-by: Masami Hiramatsu <[e-mail]>
Signed-off-by: Jon Medhurst <[e-mail]>

arch/arm/include/asm/kprobes.h

···
 extern __visible kprobe_opcode_t optprobe_template_end;
 extern __visible kprobe_opcode_t optprobe_template_sub_sp;
 extern __visible kprobe_opcode_t optprobe_template_add_sp;
+extern __visible kprobe_opcode_t optprobe_template_restore_begin;
+extern __visible kprobe_opcode_t optprobe_template_restore_orig_insn;
+extern __visible kprobe_opcode_t optprobe_template_restore_end;
 
 #define MAX_OPTIMIZED_LENGTH	4
 #define MAX_OPTINSN_SIZE				\

arch/arm/include/asm/probes.h

···
 	probes_insn_fn_t		*insn_fn;
 	int				stack_space;
 	unsigned long			register_usage_flags;
+	bool				kprobe_direct_exec;
 };
 
 #endif /* __ASSEMBLY__ */

arch/arm/probes/kprobes/opt-arm.c

···
 #include "core.h"
 
 /*
+ * See register_usage_flags. If the probed instruction doesn't use PC,
+ * we can copy it into template and have it executed directly without
+ * simulation or emulation.
+ */
+#define ARM_REG_PC	15
+#define can_kprobe_direct_exec(m)	(!test_bit(ARM_REG_PC, &(m)))
+
+/*
  * NOTE: the first sub and add instruction will be modified according
  * to the stack cost of the instruction.
  */
···
 			"	orrne	r2, #1\n"
 			"	strne	r2, [sp, #60] @ set bit0 of PC for thumb\n"
 			"	msr	cpsr_cxsf, r1\n"
+			".global optprobe_template_restore_begin\n"
+			"optprobe_template_restore_begin:\n"
 			"	ldmia	sp, {r0 - r15}\n"
+			".global optprobe_template_restore_orig_insn\n"
+			"optprobe_template_restore_orig_insn:\n"
+			"	nop\n"
+			".global optprobe_template_restore_end\n"
+			"optprobe_template_restore_end:\n"
+			"	nop\n"
 			".global optprobe_template_val\n"
 			"optprobe_template_val:\n"
 			"1:	.long 0\n"
···
 	((unsigned long *)&optprobe_template_add_sp - (unsigned long *)&optprobe_template_entry)
 #define TMPL_SUB_SP \
 	((unsigned long *)&optprobe_template_sub_sp - (unsigned long *)&optprobe_template_entry)
+#define TMPL_RESTORE_BEGIN \
+	((unsigned long *)&optprobe_template_restore_begin - (unsigned long *)&optprobe_template_entry)
+#define TMPL_RESTORE_ORIGN_INSN \
+	((unsigned long *)&optprobe_template_restore_orig_insn - (unsigned long *)&optprobe_template_entry)
+#define TMPL_RESTORE_END \
+	((unsigned long *)&optprobe_template_restore_end - (unsigned long *)&optprobe_template_entry)
 
 /*
  * ARM can always optimize an instruction when using ARM ISA, except
···
 		__this_cpu_write(current_kprobe, NULL);
 	}
 
-	/* In each case, we must singlestep the replaced instruction. */
-	op->kp.ainsn.insn_singlestep(p->opcode, &p->ainsn, regs);
+	/*
+	 * We singlestep the replaced instruction only when it can't be
+	 * executed directly during restore.
+	 */
+	if (!p->ainsn.kprobe_direct_exec)
+		op->kp.ainsn.insn_singlestep(p->opcode, &p->ainsn, regs);
 
 	local_irq_restore(flags);
 }
···
 	/* Set probe function call */
 	val = (unsigned long)optimized_callback;
 	code[TMPL_CALL_IDX] = val;
+
+	/* If possible, copy insn and have it executed during restore */
+	orig->ainsn.kprobe_direct_exec = false;
+	if (can_kprobe_direct_exec(orig->ainsn.register_usage_flags)) {
+		kprobe_opcode_t final_branch = arm_gen_branch(
+				(unsigned long)(&code[TMPL_RESTORE_END]),
+				(unsigned long)(op->kp.addr) + 4);
+		if (final_branch != 0) {
+			/*
+			 * Replace original 'ldmia sp, {r0 - r15}' with
+			 * 'ldmia {r0 - r14}', restore all registers except pc.
+			 */
+			code[TMPL_RESTORE_BEGIN] = __opcode_to_mem_arm(0xe89d7fff);
+
+			/* The original probed instruction */
+			code[TMPL_RESTORE_ORIGN_INSN] = __opcode_to_mem_arm(orig->opcode);
+
+			/* Jump back to next instruction */
+			code[TMPL_RESTORE_END] = __opcode_to_mem_arm(final_branch);
+			orig->ainsn.kprobe_direct_exec = true;
+		}
+	}
 
 	flush_icache_range((unsigned long)code,
 			   (unsigned long)(&code[TMPL_END_IDX]));