x86/kprobes: Use int3 instead of debug trap for single-step

Use int3 instead of debug trap exception for single-stepping the
probed instructions. Some instructions which change the ip
registers or modify IF flags are emulated because those are not
able to be single-stepped by int3 or may allow the interrupt
while single-stepping.

This actually changes the kprobes behavior.

- kprobes can not probe following instructions; int3, iret,
  far jmp/call which get absolute address as immediate,
  indirect far jmp/call, indirect near jmp/call with addressing
  by memory (register-based indirect jmp/call are OK), and
  vmcall/vmlaunch/vmresume/vmxoff.

- If the kprobe post_handler doesn't set before registering,
  it may not be called in some case even if you set it afterwards.
  (IOW, kprobe booster is enabled at registration, user can not
   change it)

But both are rare issue, unsupported instructions will not be
used in the kernel (or rarely used), and post_handlers are
rarely used (I don't see it except for the test code).
Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/161469874601.49483.11985325887166921076.stgit@devnote2

arch/x86/include/asm/kprobes.h

@@ -65,10 +65,22 @@ struct arch_specific_insn {
 	 * a post_handler).
 	 */
 	unsigned boostable:1;
-	unsigned if_modifier:1;
-	unsigned is_call:1;
-	unsigned is_pushf:1;
-	unsigned is_abs_ip:1;
+	unsigned char size;	/* The size of insn */
+	union {
+		unsigned char opcode;
+		struct {
+			unsigned char type;
+		} jcc;
+		struct {
+			unsigned char type;
+			unsigned char asize;
+		} loop;
+		struct {
+			unsigned char reg;
+		} indirect;
+	};
+	s32 rel32;	/* relative offset must be s32, s16, or s8 */
+	void (*emulate_op)(struct kprobe *p, struct pt_regs *regs);
 	/* Number of bytes of text poked */
 	int tp_len;
 };
@@ -107,7 +119,6 @@ extern int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
 extern int kprobe_exceptions_notify(struct notifier_block *self,
 				    unsigned long val, void *data);
 extern int kprobe_int3_handler(struct pt_regs *regs);
-extern int kprobe_debug_handler(struct pt_regs *regs);
 
 #else
 

arch/x86/kernel/traps.c

@@ -890,9 +890,6 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs,
 	if ((dr6 & DR_STEP) && is_sysenter_singlestep(regs))
 		dr6 &= ~DR_STEP;
 
-	if (kprobe_debug_handler(regs))
-		goto out;
-
 	/*
 	 * The kernel doesn't use INT1
 	 */