Re: [PATCH 08/15] x86/alternatives: Teach text_poke_bp() to emulate instructions

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jun 07, 2019 at 05:41:42AM +0000, Nadav Amit wrote:

> > int poke_int3_handler(struct pt_regs *regs)
> > {
> > +	long ip = regs->ip - INT3_INSN_SIZE + CALL_INSN_SIZE;
> > +	struct opcode {
> > +		u8 insn;
> > +		s32 rel;
> > +	} __packed opcode;
> > +
> > 	/*
> > 	 * Having observed our INT3 instruction, we now must observe
> > 	 * bp_patching_in_progress.
> > 	 *
> > -	 * 	in_progress = TRUE		INT3
> > -	 * 	WMB				RMB
> > -	 * 	write INT3			if (in_progress)
> > +	 *	in_progress = TRUE		INT3
> > +	 *	WMB				RMB
> > +	 *	write INT3			if (in_progress)
> 
> I don’t see what has changed in this chunk… Whitespaces?

Yep, my editor kept marking that stuff red (space before tab), which
annoyed me enough to fix it.

> > 	 *
> > -	 * Idem for bp_int3_handler.
> > +	 * Idem for bp_int3_opcode.
> > 	 */
> > 	smp_rmb();
> > 
> > @@ -943,8 +949,21 @@ int poke_int3_handler(struct pt_regs *re
> > 	if (user_mode(regs) || regs->ip != (unsigned long)bp_int3_addr)
> > 		return 0;
> > 
> > -	/* set up the specified breakpoint handler */
> > -	regs->ip = (unsigned long) bp_int3_handler;
> > +	opcode = *(struct opcode *)bp_int3_opcode;
> > +
> > +	switch (opcode.insn) {
> > +	case 0xE8: /* CALL */
> > +		int3_emulate_call(regs, ip + opcode.rel);
> > +		break;
> > +
> > +	case 0xE9: /* JMP */
> > +		int3_emulate_jmp(regs, ip + opcode.rel);
> > +		break;
> 
> Consider using RELATIVECALL_OPCODE and RELATIVEJUMP_OPCODE instead of the
> constants (0xE8, 0xE9), just as you do later in the patch.

Those are private to kprobes..

but I can do something like so:

--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h
@@ -48,8 +48,14 @@ static inline void int3_emulate_jmp(stru
 	regs->ip = ip;
 }
 
-#define INT3_INSN_SIZE 1
-#define CALL_INSN_SIZE 5
+#define INT3_INSN_SIZE		1
+#define INT3_INSN_OPCODE	0xCC
+
+#define CALL_INSN_SIZE		5
+#define CALL_INSN_OPCODE	0xE8
+
+#define JMP_INSN_SIZE		5
+#define JMP_INSN_OPCODE		0xE9
 
 static inline void int3_emulate_push(struct pt_regs *regs, unsigned long val)
 {
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -952,11 +952,11 @@ int poke_int3_handler(struct pt_regs *re
 	opcode = *(struct opcode *)bp_int3_opcode;
 
 	switch (opcode.insn) {
-	case 0xE8: /* CALL */
+	case CALL_INSN_OPCODE:
 		int3_emulate_call(regs, ip + opcode.rel);
 		break;
 
-	case 0xE9: /* JMP */
+	case JMP_INSN_OPCODE:
 		int3_emulate_jmp(regs, ip + opcode.rel);
 		break;