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 09:06:47 +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 Mon, Mar 01, 2021 at 11:08:15PM +0900, Masami Hiramatsu wrote:

> +static void kprobe_emulate_jmp(struct kprobe *p, struct pt_regs *regs)
> +{
> +	unsigned long ip = regs->ip - INT3_INSN_SIZE + p->ainsn.size;
> +
> +	ip += p->ainsn.rel32;
> +	int3_emulate_jmp(regs, ip);
> +}
> +NOKPROBE_SYMBOL(kprobe_emulate_jmp);

Would it make sense to have this be something like:

static void kprobe_emulate_jmp(struct kprobe *p, struct pt_regs *regs, bool cond)
{
	unsigned long ip = regs->ip - INT3_INSN_SIZE + p->ainsn.size;

	if (cond)
		ip += p->ainsn.rel32;
	int3_emulate_jmp(regs, ip);
}

> +static void kprobe_emulate_jcc(struct kprobe *p, struct pt_regs *regs)
> +{
> +	bool invert = p->ainsn.jcc.type & 1;
> +	bool match;
> +
> +	if (p->ainsn.jcc.type < 0xc) {
> +		match = regs->flags & jcc_mask[p->ainsn.jcc.type >> 1];
> +	} else {
> +		match = ((regs->flags & X86_EFLAGS_SF) >> X86_EFLAGS_SF_BIT) ^
> +			((regs->flags & X86_EFLAGS_OF) >> X86_EFLAGS_OF_BIT);
> +		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;

Then you can do:

	kprobe_emulate_jmp(p, regs, match);

> +}
> +NOKPROBE_SYMBOL(kprobe_emulate_jcc);
> +
> +static void kprobe_emulate_loop(struct kprobe *p, struct pt_regs *regs)
> +{
> +	bool match;
> +
> +	if (p->ainsn.loop.type != 3) {	/* LOOP* */
> +		if (p->ainsn.loop.asize == 32)
> +			match = ((*(u32 *)&regs->cx)--) != 0;
> +#ifdef CONFIG_X86_64
> +		else if (p->ainsn.loop.asize == 64)
> +			match = ((*(u64 *)&regs->cx)--) != 0;
> +#endif
> +		else
> +			match = ((*(u16 *)&regs->cx)--) != 0;
> +	} else {			/* JCXZ */
> +		if (p->ainsn.loop.asize == 32)
> +			match = *(u32 *)(&regs->cx) == 0;
> +#ifdef CONFIG_X86_64
> +		else if (p->ainsn.loop.asize == 64)
> +			match = *(u64 *)(&regs->cx) == 0;
> +#endif
> +		else
> +			match = *(u16 *)(&regs->cx) == 0;
> +	}
> +
> +	if (p->ainsn.loop.type == 0)	/* LOOPNE */
> +		match = match && !(regs->flags & X86_EFLAGS_ZF);
> +	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;

and here.

> +}
> +NOKPROBE_SYMBOL(kprobe_emulate_loop);