Re: Questions about kprobe handler

[Jinyang He]

在 2022/11/17 21:09, Masami Hiramatsu (Google) 写道:

> On Thu, 17 Nov 2022 09:07:37 +0800
> Tiezhu Yang <yangtiezhu@xxxxxxxxxxx> wrote:
>
> > Hi KPROBES maintainers,
> >
> > There are some differences of kprobe handler implementations on various
> > archs, the implementations are almost same on arm64, riscv, csky, the
> > code logic is clear and understandable. But on mips and loongarch (not
> > upstreamed yet), if get_kprobe() returns NULL, what is the purpose of
> > the check "if (addr->word != breakpoint_insn.word)", is it necessary?
> > Can we just return directly? Please take a look, thank you.
> Good question!
>
> This means that when the software breakpoint was hit on that CPU, but
> before calling kprobe handler function, the other CPU can remove that
> kprobe from hash table, becahse the hash table is not locked.
> In that case, the get_kprobe(addr) will return NULL, and the software
> breakpoint instruction is already removed (replaced with the original
> instruction). Thus it is safe to go back. But this is originally
> implemented for x86, which doesn't need stop_machine() to modify the
> code. On the other hand, if an architecture which needs stop_machine()
> to modify code, the above scenario never happen. In that case, you
> don't need this "if" case.
>
> Thank you,
>
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/mips/kernel/kprobes.c#n323
> > 		p = get_kprobe(addr);
> > 		if (p) {
> > 			...
> > 		} else if (addr->word != breakpoint_insn.word) {

Hi,


Sorry for the late reply, but I think there should be some public
comments so that I can get the authoritative response, as offline
communication with Tiezhu is always one-sided.

I think the branch you answered here is " if (p)... " rather than
"else if (addr->word != breakpoint_insn.word)". It is right if we
not use stop_machine here we need this branch. In fact, Tiezhu
and Huacai, the maintainer of LoongArch are more concerned
about the latter why we need compare with the breakpoint_insn.

The reason I gave as follows, and I show mips code here,

    p = get_kprobe(addr);
    if (!p) {
        if (addr->word != breakpoint_insn.word) {
            /*
             * The breakpoint instruction was removed right
             * after we hit it.  Another cpu has removed
             * either a probepoint or a debugger breakpoint
             * at this address.  In either case, no further
             * handling of this interrupt is appropriate.
             */
            ret = 1;
        }
        /* Not one of ours: let kernel handle it */
        goto no_kprobe;
    }

...
int kprobe_exceptions_notify(struct notifier_block *self,
                       unsigned long val, void *data)
{
    struct die_args *args = (struct die_args *)data;
    int ret = NOTIFY_DONE;

    switch (val) {
    case DIE_BREAK:
        if (kprobe_handler(args->regs))
            ret = NOTIFY_STOP;
        break;
...

The !p means this insn has been moved, and then in most cases the COND
"addr->word != breakpoint_insn.word" is true, we return 1 so that the return
value in kprobe_exceptions_notify will be changed to NOTIFY_STOP.
The mips use soft breakpoint like "break hint". How if the original insn
is same as breakpoint_insn? That is a few case when COND is false. In that
case, it means we should handle it by other handlers and doesn't change 
ret to
keep NOTIFY_DONE as kprobe_exceptions_notify return value.

Is this idea reasonable? Thanks!


Jinyang

> > 			/*
> > 			 * The breakpoint instruction was removed by
> > 			 * another cpu right after we hit, no further
> > 			 * handling of this interrupt is appropriate
> > 			 */
> > 			ret = 1;
> > 		}
> > https://github.com/loongson/linux/blob/loongarch-next-generic-stub/arch/loongarch/kernel/kprobes.c#L262
> > 	p = get_kprobe(addr);
> > 	if (p) {
> > 		...
> > 	} else {
> > 		if (addr->word != breakpoint_insn.word) {
> > 			/*
> > 			 * The breakpoint instruction was removed right
> > 			 * after we hit it.  Another cpu has removed
> > 			 * either a probepoint or a debugger breakpoint
> > 			 * at this address.  In either case, no further
> > 			 * handling of this interrupt is appropriate.
> > 			 */
> > 			preempt_enable_no_resched();
> > 			return 1;
> > 		}
> > 	}
> >
> > (1) arm64
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/kernel/probes/kprobes.c#n309
> > static void __kprobes kprobe_handler(struct pt_regs *regs)
> > {
> > 	struct kprobe *p, *cur_kprobe;
> > 	struct kprobe_ctlblk *kcb;
> > 	unsigned long addr = instruction_pointer(regs);
> >
> > 	kcb = get_kprobe_ctlblk();
> > 	cur_kprobe = kprobe_running();
> >
> > 	p = get_kprobe((kprobe_opcode_t *) addr);
> >
> > 	if (p) {
> > 		if (cur_kprobe) {
> > 			if (reenter_kprobe(p, regs, kcb))
> > 				return;
> > 		} else {
> > 			/* Probe hit */
> > 			set_current_kprobe(p);
> > 			kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> >
> > 			/*
> > 			 * If we have no pre-handler or it returned 0, we
> > 			 * continue with normal processing.  If we have a
> > 			 * pre-handler and it returned non-zero, it will
> > 			 * modify the execution path and no need to single
> > 			 * stepping. Let's just reset current kprobe and exit.
> > 			 */
> > 			if (!p->pre_handler || !p->pre_handler(p, regs)) {
> > 				setup_singlestep(p, regs, kcb, 0);
> > 			} else
> > 				reset_current_kprobe();
> > 		}
> > 	}
> > 	/*
> > 	 * The breakpoint instruction was removed right
> > 	 * after we hit it.  Another cpu has removed
> > 	 * either a probepoint or a debugger breakpoint
> > 	 * at this address.  In either case, no further
> > 	 * handling of this interrupt is appropriate.
> > 	 * Return back to original instruction, and continue.
> > 	 */
> > }
> >
> > (2) riscv
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/riscv/kernel/probes/kprobes.c#n269
> > bool __kprobes
> > kprobe_breakpoint_handler(struct pt_regs *regs)
> > {
> > 	struct kprobe *p, *cur_kprobe;
> > 	struct kprobe_ctlblk *kcb;
> > 	unsigned long addr = instruction_pointer(regs);
> >
> > 	kcb = get_kprobe_ctlblk();
> > 	cur_kprobe = kprobe_running();
> >
> > 	p = get_kprobe((kprobe_opcode_t *) addr);
> >
> > 	if (p) {
> > 		if (cur_kprobe) {
> > 			if (reenter_kprobe(p, regs, kcb))
> > 				return true;
> > 		} else {
> > 			/* Probe hit */
> > 			set_current_kprobe(p);
> > 			kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> >
> > 			/*
> > 			 * If we have no pre-handler or it returned 0, we
> > 			 * continue with normal processing.  If we have a
> > 			 * pre-handler and it returned non-zero, it will
> > 			 * modify the execution path and no need to single
> > 			 * stepping. Let's just reset current kprobe and exit.
> > 			 *
> > 			 * pre_handler can hit a breakpoint and can step thru
> > 			 * before return.
> > 			 */
> > 			if (!p->pre_handler || !p->pre_handler(p, regs))
> > 				setup_singlestep(p, regs, kcb, 0);
> > 			else
> > 				reset_current_kprobe();
> > 		}
> > 		return true;
> > 	}
> >
> > 	/*
> > 	 * The breakpoint instruction was removed right
> > 	 * after we hit it.  Another cpu has removed
> > 	 * either a probepoint or a debugger breakpoint
> > 	 * at this address.  In either case, no further
> > 	 * handling of this interrupt is appropriate.
> > 	 * Return back to original instruction, and continue.
> > 	 */
> > 	return false;
> > }
> >
> > (3) csky
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/csky/kernel/probes/kprobes.c#n311
> > int __kprobes
> > kprobe_breakpoint_handler(struct pt_regs *regs)
> > {
> > 	struct kprobe *p, *cur_kprobe;
> > 	struct kprobe_ctlblk *kcb;
> > 	unsigned long addr = instruction_pointer(regs);
> >
> > 	kcb = get_kprobe_ctlblk();
> > 	cur_kprobe = kprobe_running();
> >
> > 	p = get_kprobe((kprobe_opcode_t *) addr);
> >
> > 	if (p) {
> > 		if (cur_kprobe) {
> > 			if (reenter_kprobe(p, regs, kcb))
> > 				return 1;
> > 		} else {
> > 			/* Probe hit */
> > 			set_current_kprobe(p);
> > 			kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> >
> > 			/*
> > 			 * If we have no pre-handler or it returned 0, we
> > 			 * continue with normal processing.  If we have a
> > 			 * pre-handler and it returned non-zero, it will
> > 			 * modify the execution path and no need to single
> > 			 * stepping. Let's just reset current kprobe and exit.
> > 			 *
> > 			 * pre_handler can hit a breakpoint and can step thru
> > 			 * before return.
> > 			 */
> > 			if (!p->pre_handler || !p->pre_handler(p, regs))
> > 				setup_singlestep(p, regs, kcb, 0);
> > 			else
> > 				reset_current_kprobe();
> > 		}
> > 		return 1;
> > 	}
> >
> > 	/*
> > 	 * The breakpoint instruction was removed right
> > 	 * after we hit it.  Another cpu has removed
> > 	 * either a probepoint or a debugger breakpoint
> > 	 * at this address.  In either case, no further
> > 	 * handling of this interrupt is appropriate.
> > 	 * Return back to original instruction, and continue.
> > 	 */
> > 	return 0;
> > }
> >
> > (4) mips
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/mips/kernel/kprobes.c#n279
> > static int kprobe_handler(struct pt_regs *regs)
> > {
> > 	struct kprobe *p;
> > 	int ret = 0;
> > 	kprobe_opcode_t *addr;
> > 	struct kprobe_ctlblk *kcb;
> >
> > 	addr = (kprobe_opcode_t *) regs->cp0_epc;
> >
> > 	/*
> > 	 * We don't want to be preempted for the entire
> > 	 * duration of kprobe processing
> > 	 */
> > 	preempt_disable();
> > 	kcb = get_kprobe_ctlblk();
> >
> > 	/* Check we're not actually recursing */
> > 	if (kprobe_running()) {
> > 		p = get_kprobe(addr);
> > 		if (p) {
> > 			if (kcb->kprobe_status == KPROBE_HIT_SS &&
> > 			    p->ainsn.insn->word == breakpoint_insn.word) {
> > 				regs->cp0_status &= ~ST0_IE;
> > 				regs->cp0_status |= kcb->kprobe_saved_SR;
> > 				goto no_kprobe;
> > 			}
> > 			/*
> > 			 * We have reentered the kprobe_handler(), since
> > 			 * another probe was hit while within the handler.
> > 			 * We here save the original kprobes variables and
> > 			 * just single step on the instruction of the new probe
> > 			 * without calling any user handlers.
> > 			 */
> > 			save_previous_kprobe(kcb);
> > 			set_current_kprobe(p, regs, kcb);
> > 			kprobes_inc_nmissed_count(p);
> > 			prepare_singlestep(p, regs, kcb);
> > 			kcb->kprobe_status = KPROBE_REENTER;
> > 			if (kcb->flags & SKIP_DELAYSLOT) {
> > 				resume_execution(p, regs, kcb);
> > 				restore_previous_kprobe(kcb);
> > 				preempt_enable_no_resched();
> > 			}
> > 			return 1;
> > 		} else if (addr->word != breakpoint_insn.word) {
> > 			/*
> > 			 * The breakpoint instruction was removed by
> > 			 * another cpu right after we hit, no further
> > 			 * handling of this interrupt is appropriate
> > 			 */
> > 			ret = 1;
> > 		}
> > 		goto no_kprobe;
> > 	}
> > ...
> > }
> >
> > (5) loongarch
> > https://github.com/loongson/linux/blob/loongarch-next-generic-stub/arch/loongarch/kernel/kprobes.c#L228
> > static int __kprobes kprobe_handler(struct pt_regs *regs)
> > {
> > 	struct kprobe *p;
> > 	kprobe_opcode_t *addr;
> > 	struct kprobe_ctlblk *kcb;
> >
> > 	addr = (kprobe_opcode_t *) regs->csr_era;
> >
> > 	/*
> > 	 * We don't want to be preempted for the entire
> > 	 * duration of kprobe processing
> > 	 */
> > 	preempt_disable();
> > 	kcb = get_kprobe_ctlblk();
> >
> > 	p = get_kprobe(addr);
> > 	if (p) {
> > 		if (kprobe_running()) {
> > 			if (reenter_kprobe(p, regs, kcb))
> > 				return 1;
> > 		} else {
> > 			set_current_kprobe(p, regs, kcb);
> > 			kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> > 			if (p->pre_handler && p->pre_handler(p, regs)) {
> > 			/* handler has already set things up, so skip ss setup */
> > 				reset_current_kprobe();
> > 				preempt_enable_no_resched();
> > 				return 1;
> > 			} else {
> > 				setup_singlestep(p, regs, kcb, 0);
> > 				return 1;
> > 			}
> > 		}
> > 	} else {
> > 		if (addr->word != breakpoint_insn.word) {
> > 			/*
> > 			 * The breakpoint instruction was removed right
> > 			 * after we hit it.  Another cpu has removed
> > 			 * either a probepoint or a debugger breakpoint
> > 			 * at this address.  In either case, no further
> > 			 * handling of this interrupt is appropriate.
> > 			 */
> > 			preempt_enable_no_resched();
> > 			return 1;
> > 		}
> > 	}
> >
> > 	preempt_enable_no_resched();
> >
> > 	return 0;
> > }
> >
> > Thanks,
> > Tiezhu