linux-kernel - Re: [RFC PATCH 1/1] x86/kprobes: Use int3 instead of debug trap for single-step

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Date:   Tue, 2 Mar 2021 09:54:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S . Miller" <davem@...emloft.net>, X86 ML <x86@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC PATCH 1/1] x86/kprobes: Use int3 instead of debug trap for
 single-step

On Tue, Mar 02, 2021 at 09:41:32AM +0100, Peter Zijlstra wrote:

> Aside of a few nits:

something like so..

--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -457,13 +457,20 @@ static void kprobe_emulate_call(struct k
 }
 NOKPROBE_SYMBOL(kprobe_emulate_call);
 
-static void kprobe_emulate_jmp(struct kprobe *p, struct pt_regs *regs)
+static __always_inline 
+void __kprobe_emulate_jmp(struct kprobe *p, struct pt_regs *regs, bool cond)
 {
 	unsigned long ip = regs->ip - INT3_INSN_SIZE + p->ainsn.size;
 
-	ip += p->ainsn.rel32;
+	if (cond)
+		ip += p->ainsn.rel32;
 	int3_emulate_jmp(regs, ip);
 }
+
+static void kprobe_emulate_jmp(struct kprobe *p, struct pt_regs *regs)
+{
+	__kprobe_emulate_jmp(p, regs, true);
+}
 NOKPROBE_SYMBOL(kprobe_emulate_jmp);
 
 static const unsigned long jcc_mask[6] = {
@@ -488,10 +495,7 @@ static void kprobe_emulate_jcc(struct kp
 		if (p->ainsn.jcc.type >= 0xe)
 			match = match && (regs->flags & X86_EFLAGS_ZF);
 	}
-	if ((match && !invert) || (!match && invert))
-		kprobe_emulate_jmp(p, regs);
-	else
-		regs->ip = regs->ip - INT3_INSN_SIZE + p->ainsn.size;
+	__kprobe_emulate_jmp(p, regs, (match && !invert) || (!match && invert));
 }
 NOKPROBE_SYMBOL(kprobe_emulate_jcc);
 
@@ -524,10 +528,7 @@ static void kprobe_emulate_loop(struct k
 	else if (p->ainsn.loop.type == 1)	/* LOOPE */
 		match = match && (regs->flags & X86_EFLAGS_ZF);
 
-	if (match)
-		kprobe_emulate_jmp(p, regs);
-	else
-		regs->ip = regs->ip - INT3_INSN_SIZE + p->ainsn.size;
+	__kprobe_emulate_jmp(p, regs, match);
 }
 NOKPROBE_SYMBOL(kprobe_emulate_loop);
 
@@ -613,6 +614,12 @@ static int prepare_emulation(struct kpro
 		else
 			p->ainsn.rel32 = *(s32 *)&insn->immediate.value;
 		break;
+	case 0x70 ... 0x7f:
+		/* 1 byte conditional jump */
+		p->ainsn.emulate_op = kprobe_emulate_jcc;
+		p->ainsn.jcc.type = opcode & 0xf;
+		p->ainsn.rel32 = *(char *)insn->immediate.bytes;
+		break;
 	case 0x0f:
 		opcode = insn->opcode.bytes[1];
 		if ((opcode & 0xf0) == 0x80) {
@@ -667,12 +674,7 @@ static int prepare_emulation(struct kpro
 #endif
 		break;
 	default:
-		if ((opcode & 0xf0) == 0x70) {
-			/* 1 byte conditional jump */
-			p->ainsn.emulate_op = kprobe_emulate_jcc;
-			p->ainsn.jcc.type = opcode & 0xf;
-			p->ainsn.rel32 = *(char *)insn->immediate.bytes;
-		}
+		break;
 	}
 	p->ainsn.size = insn->length;