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Date:   Fri, 14 Aug 2020 15:36:08 -0700 (PDT)
From:   Palmer Dabbelt <palmerdabbelt@...gle.com>
To:     guoren@...nel.org
CC:     Paul Walmsley <paul.walmsley@...ive.com>, mhiramat@...nel.org,
        oleg@...hat.com, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, anup@...infault.org,
        linux-csky@...r.kernel.org, greentime.hu@...ive.com,
        zong.li@...ive.com, guoren@...nel.org, me@...ki.ch,
        Bjorn Topel <bjorn.topel@...il.com>, guoren@...ux.alibaba.com
Subject:     Re: [PATCH v3 4/7] riscv: Add kprobes supported

On Mon, 13 Jul 2020 16:39:19 PDT (-0700), guoren@...nel.org wrote:
> From: Guo Ren <guoren@...ux.alibaba.com>
>
> This patch enables "kprobe & kretprobe" to work with ftrace
> interface. It utilized software breakpoint as single-step
> mechanism.
>
> Some instructions which can't be single-step executed must be
> simulated in kernel execution slot, such as: branch, jal, auipc,
> la ...
>
> Some instructions should be rejected for probing and we use a
> blacklist to filter, such as: ecall, ebreak, ...
>
> We use ebreak & c.ebreak to replace origin instruction and the
> kprobe handler prepares an executable memory slot for out-of-line
> execution with a copy of the original instruction being probed.
> In execution slot we add ebreak behind original instruction to
> simulate a single-setp mechanism.
>
> The patch is based on packi's work [1] and csky's work [2].
>  - The kprobes_trampoline.S is all from packi's patch
>  - The single-step mechanism is new designed for riscv without hw
>    single-step trap
>  - The simulation codes are from csky
>  - Frankly, all codes refer to other archs' implementation
>
>  [1] https://lore.kernel.org/linux-riscv/20181113195804.22825-1-me@packi.ch/
>  [2] https://lore.kernel.org/linux-csky/20200403044150.20562-9-guoren@kernel.org/
>
> Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> Co-Developed-by: Patrick Stählin <me@...ki.ch>
> Acked-by: Masami Hiramatsu <mhiramat@...nel.org>
> Tested-by: Zong Li <zong.li@...ive.com>
> Reviewed-by: Pekka Enberg <penberg@...nel.org>
> Cc: Patrick Stählin <me@...ki.ch>
> Cc: Palmer Dabbelt <palmerdabbelt@...gle.com>
> Cc: Björn Töpel <bjorn.topel@...il.com>
> ---
>  arch/riscv/Kconfig                            |   2 +
>  arch/riscv/include/asm/kprobes.h              |  40 +++
>  arch/riscv/include/asm/probes.h               |  24 ++
>  arch/riscv/kernel/Makefile                    |   1 +
>  arch/riscv/kernel/probes/Makefile             |   4 +
>  arch/riscv/kernel/probes/decode-insn.c        |  48 +++
>  arch/riscv/kernel/probes/decode-insn.h        |  18 +
>  arch/riscv/kernel/probes/kprobes.c            | 471 ++++++++++++++++++++++++++
>  arch/riscv/kernel/probes/kprobes_trampoline.S |  93 +++++
>  arch/riscv/kernel/probes/simulate-insn.c      |  85 +++++
>  arch/riscv/kernel/probes/simulate-insn.h      |  47 +++
>  arch/riscv/kernel/traps.c                     |   9 +
>  arch/riscv/mm/fault.c                         |   4 +
>  13 files changed, 846 insertions(+)
>  create mode 100644 arch/riscv/include/asm/probes.h
>  create mode 100644 arch/riscv/kernel/probes/Makefile
>  create mode 100644 arch/riscv/kernel/probes/decode-insn.c
>  create mode 100644 arch/riscv/kernel/probes/decode-insn.h
>  create mode 100644 arch/riscv/kernel/probes/kprobes.c
>  create mode 100644 arch/riscv/kernel/probes/kprobes_trampoline.S
>  create mode 100644 arch/riscv/kernel/probes/simulate-insn.c
>  create mode 100644 arch/riscv/kernel/probes/simulate-insn.h
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index e70449a..b86b2a2 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -59,6 +59,8 @@ config RISCV
>  	select HAVE_EBPF_JIT if MMU
>  	select HAVE_FUTEX_CMPXCHG if FUTEX
>  	select HAVE_GENERIC_VDSO if MMU && 64BIT
> +	select HAVE_KPROBES
> +	select HAVE_KRETPROBES
>  	select HAVE_PCI
>  	select HAVE_PERF_EVENTS
>  	select HAVE_PERF_REGS
> diff --git a/arch/riscv/include/asm/kprobes.h b/arch/riscv/include/asm/kprobes.h
> index 56a98ea3..4647d38 100644
> --- a/arch/riscv/include/asm/kprobes.h
> +++ b/arch/riscv/include/asm/kprobes.h
> @@ -11,4 +11,44 @@
>
>  #include <asm-generic/kprobes.h>
>
> +#ifdef CONFIG_KPROBES
> +#include <linux/types.h>
> +#include <linux/ptrace.h>
> +#include <linux/percpu.h>
> +
> +#define __ARCH_WANT_KPROBES_INSN_SLOT
> +#define MAX_INSN_SIZE			2
> +
> +#define flush_insn_slot(p)		do { } while (0)
> +#define kretprobe_blacklist_size	0
> +
> +#include <asm/probes.h>
> +
> +struct prev_kprobe {
> +	struct kprobe *kp;
> +	unsigned int status;
> +};
> +
> +/* Single step context for kprobe */
> +struct kprobe_step_ctx {
> +	unsigned long ss_pending;
> +	unsigned long match_addr;
> +};
> +
> +/* per-cpu kprobe control block */
> +struct kprobe_ctlblk {
> +	unsigned int kprobe_status;
> +	unsigned long saved_status;
> +	struct prev_kprobe prev_kprobe;
> +	struct kprobe_step_ctx ss_ctx;
> +};
> +
> +void arch_remove_kprobe(struct kprobe *p);
> +int kprobe_fault_handler(struct pt_regs *regs, unsigned int trapnr);
> +bool kprobe_breakpoint_handler(struct pt_regs *regs);
> +bool kprobe_single_step_handler(struct pt_regs *regs);
> +void kretprobe_trampoline(void);
> +void __kprobes *trampoline_probe_handler(struct pt_regs *regs);
> +
> +#endif /* CONFIG_KPROBES */
>  #endif /* _ASM_RISCV_KPROBES_H */
> diff --git a/arch/riscv/include/asm/probes.h b/arch/riscv/include/asm/probes.h
> new file mode 100644
> index 00000000..a787e6d
> --- /dev/null
> +++ b/arch/riscv/include/asm/probes.h
> @@ -0,0 +1,24 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _ASM_RISCV_PROBES_H
> +#define _ASM_RISCV_PROBES_H
> +
> +typedef u32 probe_opcode_t;
> +typedef bool (probes_handler_t) (u32 opcode, unsigned long addr, struct pt_regs *);
> +
> +/* architecture specific copy of original instruction */
> +struct arch_probe_insn {
> +	probe_opcode_t *insn;
> +	probes_handler_t *handler;
> +	/* restore address after simulation */
> +	unsigned long restore;
> +};
> +
> +#ifdef CONFIG_KPROBES
> +typedef u32 kprobe_opcode_t;
> +struct arch_specific_insn {
> +	struct arch_probe_insn api;
> +};
> +#endif
> +
> +#endif /* _ASM_RISCV_PROBES_H */
> diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> index b355cf4..c3fff3e 100644
> --- a/arch/riscv/kernel/Makefile
> +++ b/arch/riscv/kernel/Makefile
> @@ -29,6 +29,7 @@ obj-y	+= riscv_ksyms.o
>  obj-y	+= stacktrace.o
>  obj-y	+= cacheinfo.o
>  obj-y	+= patch.o
> +obj-y	+= probes/
>  obj-$(CONFIG_MMU) += vdso.o vdso/
>
>  obj-$(CONFIG_RISCV_M_MODE)	+= clint.o traps_misaligned.o
> diff --git a/arch/riscv/kernel/probes/Makefile b/arch/riscv/kernel/probes/Makefile
> new file mode 100644
> index 00000000..8a39507
> --- /dev/null
> +++ b/arch/riscv/kernel/probes/Makefile
> @@ -0,0 +1,4 @@
> +# SPDX-License-Identifier: GPL-2.0
> +obj-$(CONFIG_KPROBES)		+= kprobes.o decode-insn.o simulate-insn.o
> +obj-$(CONFIG_KPROBES)		+= kprobes_trampoline.o
> +CFLAGS_REMOVE_simulate-insn.o = $(CC_FLAGS_FTRACE)
> diff --git a/arch/riscv/kernel/probes/decode-insn.c b/arch/riscv/kernel/probes/decode-insn.c
> new file mode 100644
> index 00000000..0876c30
> --- /dev/null
> +++ b/arch/riscv/kernel/probes/decode-insn.c
> @@ -0,0 +1,48 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +
> +#include <linux/kernel.h>
> +#include <linux/kprobes.h>
> +#include <linux/module.h>
> +#include <linux/kallsyms.h>
> +#include <asm/sections.h>
> +
> +#include "decode-insn.h"
> +#include "simulate-insn.h"
> +
> +/* Return:
> + *   INSN_REJECTED     If instruction is one not allowed to kprobe,
> + *   INSN_GOOD_NO_SLOT If instruction is supported but doesn't use its slot.
> + */
> +enum probe_insn __kprobes
> +riscv_probe_decode_insn(probe_opcode_t *addr, struct arch_probe_insn *api)
> +{
> +	probe_opcode_t insn = le32_to_cpu(*addr);
> +
> +	/*
> +	 * Reject instructions list:
> +	 */
> +	RISCV_INSN_REJECTED(system,		insn);
> +	RISCV_INSN_REJECTED(fence,		insn);
> +
> +	/*
> +	 * Simulate instructions list:
> +	 * TODO: the REJECTED ones below need to be implemented
> +	 */
> +#ifdef CONFIG_RISCV_ISA_C
> +	RISCV_INSN_REJECTED(c_j,		insn);
> +	RISCV_INSN_REJECTED(c_jr,		insn);
> +	RISCV_INSN_REJECTED(c_jal,		insn);
> +	RISCV_INSN_REJECTED(c_jalr,		insn);
> +	RISCV_INSN_REJECTED(c_beqz,		insn);
> +	RISCV_INSN_REJECTED(c_bnez,		insn);
> +	RISCV_INSN_REJECTED(c_ebreak,		insn);
> +#endif
> +
> +	RISCV_INSN_REJECTED(auipc,		insn);
> +	RISCV_INSN_REJECTED(branch,		insn);
> +
> +	RISCV_INSN_SET_SIMULATE(jal,		insn);
> +	RISCV_INSN_SET_SIMULATE(jalr,		insn);
> +
> +	return INSN_GOOD;
> +}

IIRC I mentioned this in the original version, but I'd anticipate that we need
to at least prevent LR/SC sequences from being probed.

> diff --git a/arch/riscv/kernel/probes/decode-insn.h b/arch/riscv/kernel/probes/decode-insn.h
> new file mode 100644
> index 00000000..42269a7
> --- /dev/null
> +++ b/arch/riscv/kernel/probes/decode-insn.h
> @@ -0,0 +1,18 @@
> +/* SPDX-License-Identifier: GPL-2.0+ */
> +
> +#ifndef _RISCV_KERNEL_KPROBES_DECODE_INSN_H
> +#define _RISCV_KERNEL_KPROBES_DECODE_INSN_H
> +
> +#include <asm/sections.h>
> +#include <asm/kprobes.h>
> +
> +enum probe_insn {
> +	INSN_REJECTED,
> +	INSN_GOOD_NO_SLOT,
> +	INSN_GOOD,
> +};
> +
> +enum probe_insn __kprobes
> +riscv_probe_decode_insn(probe_opcode_t *addr, struct arch_probe_insn *asi);
> +
> +#endif /* _RISCV_KERNEL_KPROBES_DECODE_INSN_H */
> diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
> new file mode 100644
> index 00000000..31b6196
> --- /dev/null
> +++ b/arch/riscv/kernel/probes/kprobes.c
> @@ -0,0 +1,471 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +
> +#include <linux/kprobes.h>
> +#include <linux/extable.h>
> +#include <linux/slab.h>
> +#include <linux/stop_machine.h>
> +#include <asm/ptrace.h>
> +#include <linux/uaccess.h>
> +#include <asm/sections.h>
> +#include <asm/cacheflush.h>
> +#include <asm/bug.h>
> +#include <asm/patch.h>
> +
> +#include "decode-insn.h"
> +
> +DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
> +DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
> +
> +static void __kprobes
> +post_kprobe_handler(struct kprobe_ctlblk *, struct pt_regs *);
> +
> +static void __kprobes arch_prepare_ss_slot(struct kprobe *p)
> +{
> +	unsigned long offset = GET_INSN_LENGTH(p->opcode);
> +
> +	p->ainsn.api.restore = (unsigned long)p->addr + offset;
> +
> +	patch_text(p->ainsn.api.insn, p->opcode);
> +	patch_text((void *)((unsigned long)(p->ainsn.api.insn) + offset),
> +		   __BUG_INSN_32);
> +}
> +
> +static void __kprobes arch_prepare_simulate(struct kprobe *p)
> +{
> +	p->ainsn.api.restore = 0;
> +}
> +
> +static void __kprobes arch_simulate_insn(struct kprobe *p, struct pt_regs *regs)
> +{
> +	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
> +
> +	if (p->ainsn.api.handler)
> +		p->ainsn.api.handler((u32)p->opcode,
> +					(unsigned long)p->addr, regs);
> +
> +	post_kprobe_handler(kcb, regs);
> +}
> +
> +int __kprobes arch_prepare_kprobe(struct kprobe *p)
> +{
> +	unsigned long probe_addr = (unsigned long)p->addr;
> +
> +	if (probe_addr & 0x1) {
> +		pr_warn("Address not aligned.\n");
> +
> +		return -EINVAL;
> +	}
> +
> +	/* copy instruction */
> +	p->opcode = le32_to_cpu(*p->addr);
> +
> +	/* decode instruction */
> +	switch (riscv_probe_decode_insn(p->addr, &p->ainsn.api)) {
> +	case INSN_REJECTED:	/* insn not supported */
> +		return -EINVAL;
> +
> +	case INSN_GOOD_NO_SLOT:	/* insn need simulation */
> +		p->ainsn.api.insn = NULL;
> +		break;
> +
> +	case INSN_GOOD:	/* instruction uses slot */
> +		p->ainsn.api.insn = get_insn_slot();
> +		if (!p->ainsn.api.insn)
> +			return -ENOMEM;
> +		break;
> +	}
> +
> +	/* prepare the instruction */
> +	if (p->ainsn.api.insn)
> +		arch_prepare_ss_slot(p);
> +	else
> +		arch_prepare_simulate(p);
> +
> +	return 0;
> +}
> +
> +/* install breakpoint in text */
> +void __kprobes arch_arm_kprobe(struct kprobe *p)
> +{
> +	if ((p->opcode & __INSN_LENGTH_MASK) == __INSN_LENGTH_32)
> +		patch_text(p->addr, __BUG_INSN_32);
> +	else
> +		patch_text(p->addr, __BUG_INSN_16);
> +}
> +
> +/* remove breakpoint from text */
> +void __kprobes arch_disarm_kprobe(struct kprobe *p)
> +{
> +	patch_text(p->addr, p->opcode);
> +}
> +
> +void __kprobes arch_remove_kprobe(struct kprobe *p)
> +{
> +}
> +
> +static void __kprobes save_previous_kprobe(struct kprobe_ctlblk *kcb)
> +{
> +	kcb->prev_kprobe.kp = kprobe_running();
> +	kcb->prev_kprobe.status = kcb->kprobe_status;
> +}
> +
> +static void __kprobes restore_previous_kprobe(struct kprobe_ctlblk *kcb)
> +{
> +	__this_cpu_write(current_kprobe, kcb->prev_kprobe.kp);
> +	kcb->kprobe_status = kcb->prev_kprobe.status;
> +}
> +
> +static void __kprobes set_current_kprobe(struct kprobe *p)
> +{
> +	__this_cpu_write(current_kprobe, p);
> +}
> +
> +/*
> + * Interrupts need to be disabled before single-step mode is set, and not
> + * reenabled until after single-step mode ends.
> + * Without disabling interrupt on local CPU, there is a chance of
> + * interrupt occurrence in the period of exception return and  start of
> + * out-of-line single-step, that result in wrongly single stepping
> + * into the interrupt handler.
> + */
> +static void __kprobes kprobes_save_local_irqflag(struct kprobe_ctlblk *kcb,
> +						struct pt_regs *regs)
> +{
> +	kcb->saved_status = regs->status;
> +	regs->status &= ~SR_SPIE;
> +}
> +
> +static void __kprobes kprobes_restore_local_irqflag(struct kprobe_ctlblk *kcb,
> +						struct pt_regs *regs)
> +{
> +	regs->status = kcb->saved_status;
> +}
> +
> +static void __kprobes
> +set_ss_context(struct kprobe_ctlblk *kcb, unsigned long addr, struct kprobe *p)
> +{
> +	unsigned long offset = GET_INSN_LENGTH(p->opcode);
> +
> +	kcb->ss_ctx.ss_pending = true;
> +	kcb->ss_ctx.match_addr = addr + offset;
> +}
> +
> +static void __kprobes clear_ss_context(struct kprobe_ctlblk *kcb)
> +{
> +	kcb->ss_ctx.ss_pending = false;
> +	kcb->ss_ctx.match_addr = 0;
> +}
> +
> +static void __kprobes setup_singlestep(struct kprobe *p,
> +				       struct pt_regs *regs,
> +				       struct kprobe_ctlblk *kcb, int reenter)
> +{
> +	unsigned long slot;
> +
> +	if (reenter) {
> +		save_previous_kprobe(kcb);
> +		set_current_kprobe(p);
> +		kcb->kprobe_status = KPROBE_REENTER;
> +	} else {
> +		kcb->kprobe_status = KPROBE_HIT_SS;
> +	}
> +
> +	if (p->ainsn.api.insn) {
> +		/* prepare for single stepping */
> +		slot = (unsigned long)p->ainsn.api.insn;
> +
> +		set_ss_context(kcb, slot, p);	/* mark pending ss */
> +
> +		/* IRQs and single stepping do not mix well. */
> +		kprobes_save_local_irqflag(kcb, regs);
> +
> +		instruction_pointer_set(regs, slot);
> +	} else {
> +		/* insn simulation */
> +		arch_simulate_insn(p, regs);
> +	}
> +}
> +
> +static int __kprobes reenter_kprobe(struct kprobe *p,
> +				    struct pt_regs *regs,
> +				    struct kprobe_ctlblk *kcb)
> +{
> +	switch (kcb->kprobe_status) {
> +	case KPROBE_HIT_SSDONE:
> +	case KPROBE_HIT_ACTIVE:
> +		kprobes_inc_nmissed_count(p);
> +		setup_singlestep(p, regs, kcb, 1);
> +		break;
> +	case KPROBE_HIT_SS:
> +	case KPROBE_REENTER:
> +		pr_warn("Unrecoverable kprobe detected.\n");
> +		dump_kprobe(p);
> +		BUG();
> +		break;
> +	default:
> +		WARN_ON(1);
> +		return 0;
> +	}
> +
> +	return 1;
> +}
> +
> +static void __kprobes
> +post_kprobe_handler(struct kprobe_ctlblk *kcb, struct pt_regs *regs)
> +{
> +	struct kprobe *cur = kprobe_running();
> +
> +	if (!cur)
> +		return;
> +
> +	/* return addr restore if non-branching insn */
> +	if (cur->ainsn.api.restore != 0)
> +		regs->epc = cur->ainsn.api.restore;
> +
> +	/* restore back original saved kprobe variables and continue */
> +	if (kcb->kprobe_status == KPROBE_REENTER) {
> +		restore_previous_kprobe(kcb);
> +		return;
> +	}
> +
> +	/* call post handler */
> +	kcb->kprobe_status = KPROBE_HIT_SSDONE;
> +	if (cur->post_handler)	{
> +		/* post_handler can hit breakpoint and single step
> +		 * again, so we enable D-flag for recursive exception.
> +		 */
> +		cur->post_handler(cur, regs, 0);
> +	}
> +
> +	reset_current_kprobe();
> +}
> +
> +int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int trapnr)
> +{
> +	struct kprobe *cur = kprobe_running();
> +	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
> +
> +	switch (kcb->kprobe_status) {
> +	case KPROBE_HIT_SS:
> +	case KPROBE_REENTER:
> +		/*
> +		 * We are here because the instruction being single
> +		 * stepped caused a page fault. We reset the current
> +		 * kprobe and the ip points back to the probe address
> +		 * and allow the page fault handler to continue as a
> +		 * normal page fault.
> +		 */
> +		regs->epc = (unsigned long) cur->addr;
> +		if (!instruction_pointer(regs))
> +			BUG();
> +
> +		if (kcb->kprobe_status == KPROBE_REENTER)
> +			restore_previous_kprobe(kcb);
> +		else
> +			reset_current_kprobe();
> +
> +		break;
> +	case KPROBE_HIT_ACTIVE:
> +	case KPROBE_HIT_SSDONE:
> +		/*
> +		 * We increment the nmissed count for accounting,
> +		 * we can also use npre/npostfault count for accounting
> +		 * these specific fault cases.
> +		 */
> +		kprobes_inc_nmissed_count(cur);
> +
> +		/*
> +		 * We come here because instructions in the pre/post
> +		 * handler caused the page_fault, this could happen
> +		 * if handler tries to access user space by
> +		 * copy_from_user(), get_user() etc. Let the
> +		 * user-specified handler try to fix it first.
> +		 */
> +		if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
> +			return 1;
> +
> +		/*
> +		 * In case the user-specified fault handler returned
> +		 * zero, try to fix up.
> +		 */
> +		if (fixup_exception(regs))
> +			return 1;
> +	}
> +	return 0;
> +}
> +
> +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;
> +}
> +
> +bool __kprobes
> +kprobe_single_step_handler(struct pt_regs *regs)
> +{
> +	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
> +
> +	if ((kcb->ss_ctx.ss_pending)
> +	    && (kcb->ss_ctx.match_addr == instruction_pointer(regs))) {
> +		clear_ss_context(kcb);	/* clear pending ss */
> +
> +		kprobes_restore_local_irqflag(kcb, regs);
> +
> +		post_kprobe_handler(kcb, regs);
> +		return true;
> +	}
> +	return false;
> +}
> +
> +/*
> + * Provide a blacklist of symbols identifying ranges which cannot be kprobed.
> + * This blacklist is exposed to userspace via debugfs (kprobes/blacklist).
> + */
> +int __init arch_populate_kprobe_blacklist(void)
> +{
> +	int ret;
> +
> +	ret = kprobe_add_area_blacklist((unsigned long)__irqentry_text_start,
> +					(unsigned long)__irqentry_text_end);
> +	return ret;
> +}
> +
> +void __kprobes __used *trampoline_probe_handler(struct pt_regs *regs)
> +{
> +	struct kretprobe_instance *ri = NULL;
> +	struct hlist_head *head, empty_rp;
> +	struct hlist_node *tmp;
> +	unsigned long flags, orig_ret_address = 0;
> +	unsigned long trampoline_address =
> +		(unsigned long)&kretprobe_trampoline;
> +	kprobe_opcode_t *correct_ret_addr = NULL;
> +
> +	INIT_HLIST_HEAD(&empty_rp);
> +	kretprobe_hash_lock(current, &head, &flags);
> +
> +	/*
> +	 * It is possible to have multiple instances associated with a given
> +	 * task either because multiple functions in the call path have
> +	 * return probes installed on them, and/or more than one
> +	 * return probe was registered for a target function.
> +	 *
> +	 * We can handle this because:
> +	 *     - instances are always pushed into the head of the list
> +	 *     - when multiple return probes are registered for the same
> +	 *	 function, the (chronologically) first instance's ret_addr
> +	 *	 will be the real return address, and all the rest will
> +	 *	 point to kretprobe_trampoline.
> +	 */
> +	hlist_for_each_entry_safe(ri, tmp, head, hlist) {
> +		if (ri->task != current)
> +			/* another task is sharing our hash bucket */
> +			continue;
> +
> +		orig_ret_address = (unsigned long)ri->ret_addr;
> +
> +		if (orig_ret_address != trampoline_address)
> +			/*
> +			 * This is the real return address. Any other
> +			 * instances associated with this task are for
> +			 * other calls deeper on the call stack
> +			 */
> +			break;
> +	}
> +
> +	kretprobe_assert(ri, orig_ret_address, trampoline_address);
> +
> +	correct_ret_addr = ri->ret_addr;
> +	hlist_for_each_entry_safe(ri, tmp, head, hlist) {
> +		if (ri->task != current)
> +			/* another task is sharing our hash bucket */
> +			continue;
> +
> +		orig_ret_address = (unsigned long)ri->ret_addr;
> +		if (ri->rp && ri->rp->handler) {
> +			__this_cpu_write(current_kprobe, &ri->rp->kp);
> +			get_kprobe_ctlblk()->kprobe_status = KPROBE_HIT_ACTIVE;
> +			ri->ret_addr = correct_ret_addr;
> +			ri->rp->handler(ri, regs);
> +			__this_cpu_write(current_kprobe, NULL);
> +		}
> +
> +		recycle_rp_inst(ri, &empty_rp);
> +
> +		if (orig_ret_address != trampoline_address)
> +			/*
> +			 * This is the real return address. Any other
> +			 * instances associated with this task are for
> +			 * other calls deeper on the call stack
> +			 */
> +			break;
> +	}
> +
> +	kretprobe_hash_unlock(current, &flags);
> +
> +	hlist_for_each_entry_safe(ri, tmp, &empty_rp, hlist) {
> +		hlist_del(&ri->hlist);
> +		kfree(ri);
> +	}
> +	return (void *)orig_ret_address;
> +}
> +
> +void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
> +				      struct pt_regs *regs)
> +{
> +	ri->ret_addr = (kprobe_opcode_t *)regs->ra;
> +	regs->ra = (unsigned long) &kretprobe_trampoline;
> +}
> +
> +int __kprobes arch_trampoline_kprobe(struct kprobe *p)
> +{
> +	return 0;
> +}
> +
> +int __init arch_init_kprobes(void)
> +{
> +	return 0;
> +}
> diff --git a/arch/riscv/kernel/probes/kprobes_trampoline.S b/arch/riscv/kernel/probes/kprobes_trampoline.S
> new file mode 100644
> index 00000000..6e85d02
> --- /dev/null
> +++ b/arch/riscv/kernel/probes/kprobes_trampoline.S
> @@ -0,0 +1,93 @@
> +/* SPDX-License-Identifier: GPL-2.0+ */
> +/*
> + * Author: Patrick Stählin <me@...ki.ch>
> + */
> +#include <linux/linkage.h>
> +
> +#include <asm/asm.h>
> +#include <asm/asm-offsets.h>
> +
> +	.text
> +	.altmacro
> +
> +	.macro save_all_base_regs
> +	REG_S x1,  PT_RA(sp)
> +	REG_S x3,  PT_GP(sp)
> +	REG_S x4,  PT_TP(sp)
> +	REG_S x5,  PT_T0(sp)
> +	REG_S x6,  PT_T1(sp)
> +	REG_S x7,  PT_T2(sp)
> +	REG_S x8,  PT_S0(sp)
> +	REG_S x9,  PT_S1(sp)
> +	REG_S x10, PT_A0(sp)
> +	REG_S x11, PT_A1(sp)
> +	REG_S x12, PT_A2(sp)
> +	REG_S x13, PT_A3(sp)
> +	REG_S x14, PT_A4(sp)
> +	REG_S x15, PT_A5(sp)
> +	REG_S x16, PT_A6(sp)
> +	REG_S x17, PT_A7(sp)
> +	REG_S x18, PT_S2(sp)
> +	REG_S x19, PT_S3(sp)
> +	REG_S x20, PT_S4(sp)
> +	REG_S x21, PT_S5(sp)
> +	REG_S x22, PT_S6(sp)
> +	REG_S x23, PT_S7(sp)
> +	REG_S x24, PT_S8(sp)
> +	REG_S x25, PT_S9(sp)
> +	REG_S x26, PT_S10(sp)
> +	REG_S x27, PT_S11(sp)
> +	REG_S x28, PT_T3(sp)
> +	REG_S x29, PT_T4(sp)
> +	REG_S x30, PT_T5(sp)
> +	REG_S x31, PT_T6(sp)
> +	.endm
> +
> +	.macro restore_all_base_regs
> +	REG_L x3,  PT_GP(sp)
> +	REG_L x4,  PT_TP(sp)
> +	REG_L x5,  PT_T0(sp)
> +	REG_L x6,  PT_T1(sp)
> +	REG_L x7,  PT_T2(sp)
> +	REG_L x8,  PT_S0(sp)
> +	REG_L x9,  PT_S1(sp)
> +	REG_L x10, PT_A0(sp)
> +	REG_L x11, PT_A1(sp)
> +	REG_L x12, PT_A2(sp)
> +	REG_L x13, PT_A3(sp)
> +	REG_L x14, PT_A4(sp)
> +	REG_L x15, PT_A5(sp)
> +	REG_L x16, PT_A6(sp)
> +	REG_L x17, PT_A7(sp)
> +	REG_L x18, PT_S2(sp)
> +	REG_L x19, PT_S3(sp)
> +	REG_L x20, PT_S4(sp)
> +	REG_L x21, PT_S5(sp)
> +	REG_L x22, PT_S6(sp)
> +	REG_L x23, PT_S7(sp)
> +	REG_L x24, PT_S8(sp)
> +	REG_L x25, PT_S9(sp)
> +	REG_L x26, PT_S10(sp)
> +	REG_L x27, PT_S11(sp)
> +	REG_L x28, PT_T3(sp)
> +	REG_L x29, PT_T4(sp)
> +	REG_L x30, PT_T5(sp)
> +	REG_L x31, PT_T6(sp)
> +	.endm
> +
> +ENTRY(kretprobe_trampoline)
> +	addi sp, sp, -(PT_SIZE_ON_STACK)
> +	save_all_base_regs
> +
> +	move a0, sp /* pt_regs */
> +
> +	call trampoline_probe_handler
> +
> +	/* use the result as the return-address */
> +	move ra, a0
> +
> +	restore_all_base_regs
> +	addi sp, sp, PT_SIZE_ON_STACK
> +
> +	ret
> +ENDPROC(kretprobe_trampoline)
> diff --git a/arch/riscv/kernel/probes/simulate-insn.c b/arch/riscv/kernel/probes/simulate-insn.c
> new file mode 100644
> index 00000000..2519ce2
> --- /dev/null
> +++ b/arch/riscv/kernel/probes/simulate-insn.c
> @@ -0,0 +1,85 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +
> +#include <linux/bitops.h>
> +#include <linux/kernel.h>
> +#include <linux/kprobes.h>
> +
> +#include "decode-insn.h"
> +#include "simulate-insn.h"
> +
> +static inline bool rv_insn_reg_get_val(struct pt_regs *regs, u32 index,
> +				       unsigned long *ptr)
> +{
> +	if (index == 0)
> +		*ptr = 0;
> +	else if (index <= 31)
> +		*ptr = *((unsigned long *)regs + index);
> +	else
> +		return false;
> +
> +	return true;
> +}
> +
> +static inline bool rv_insn_reg_set_val(struct pt_regs *regs, u32 index,
> +				       unsigned long val)
> +{
> +	if (index == 0)
> +		return false;
> +	else if (index <= 31)
> +		*((unsigned long *)regs + index) = val;
> +	else
> +		return false;
> +
> +	return true;
> +}
> +
> +bool __kprobes simulate_jal(u32 opcode, unsigned long addr, struct pt_regs *regs)
> +{
> +	/*
> +	 *     31    30       21    20     19        12 11 7 6      0
> +	 * imm [20] | imm[10:1] | imm[11] | imm[19:12] | rd | opcode
> +	 *     1         10          1           8       5    JAL/J
> +	 */
> +	bool ret;
> +	u32 imm;
> +	u32 index = (opcode >> 7) & 0x1f;
> +
> +	ret = rv_insn_reg_set_val(regs, index, addr + 4);
> +	if (!ret)
> +		return ret;
> +
> +	imm  = ((opcode >> 21) & 0x3ff) << 1;
> +	imm |= ((opcode >> 20) & 0x1)   << 11;
> +	imm |= ((opcode >> 12) & 0xff)  << 12;
> +	imm |= ((opcode >> 31) & 0x1)   << 20;
> +
> +	instruction_pointer_set(regs, addr + sign_extend32((imm), 20));
> +
> +	return ret;
> +}
> +
> +bool __kprobes simulate_jalr(u32 opcode, unsigned long addr, struct pt_regs *regs)
> +{
> +	/*
> +	 * 31          20 19 15 14 12 11 7 6      0
> +	 *  offset[11:0] | rs1 | 010 | rd | opcode
> +	 *      12         5      3    5    JALR/JR
> +	 */
> +	bool ret;
> +	unsigned long base_addr;
> +	u32 imm = (opcode >> 20) & 0xfff;
> +	u32 rd_index = (opcode >> 7) & 0x1f;
> +	u32 rs1_index = (opcode >> 15) & 0x1f;
> +
> +	ret = rv_insn_reg_set_val(regs, rd_index, addr + 4);
> +	if (!ret)
> +		return ret;
> +
> +	ret = rv_insn_reg_get_val(regs, rs1_index, &base_addr);
> +	if (!ret)
> +		return ret;
> +
> +	instruction_pointer_set(regs, (base_addr + sign_extend32((imm), 11))&~1);
> +
> +	return ret;
> +}
> diff --git a/arch/riscv/kernel/probes/simulate-insn.h b/arch/riscv/kernel/probes/simulate-insn.h
> new file mode 100644
> index 00000000..a62d784
> --- /dev/null
> +++ b/arch/riscv/kernel/probes/simulate-insn.h
> @@ -0,0 +1,47 @@
> +/* SPDX-License-Identifier: GPL-2.0+ */
> +
> +#ifndef _RISCV_KERNEL_PROBES_SIMULATE_INSN_H
> +#define _RISCV_KERNEL_PROBES_SIMULATE_INSN_H
> +
> +#define __RISCV_INSN_FUNCS(name, mask, val)				\
> +static __always_inline bool riscv_insn_is_##name(probe_opcode_t code)	\
> +{									\
> +	BUILD_BUG_ON(~(mask) & (val));					\
> +	return (code & (mask)) == (val);				\
> +}									\
> +bool simulate_##name(u32 opcode, unsigned long addr,			\
> +		     struct pt_regs *regs);
> +
> +#define RISCV_INSN_REJECTED(name, code)					\
> +	do {								\
> +		if (riscv_insn_is_##name(code)) {			\
> +			return INSN_REJECTED;				\
> +		}							\
> +	} while (0)
> +
> +__RISCV_INSN_FUNCS(system,	0x7f, 0x73)
> +__RISCV_INSN_FUNCS(fence,	0x7f, 0x0f)
> +
> +#define RISCV_INSN_SET_SIMULATE(name, code)				\
> +	do {								\
> +		if (riscv_insn_is_##name(code)) {			\
> +			api->handler = simulate_##name;			\
> +			return INSN_GOOD_NO_SLOT;			\
> +		}							\
> +	} while (0)
> +
> +__RISCV_INSN_FUNCS(c_j,		0xe003, 0xa001)
> +__RISCV_INSN_FUNCS(c_jr,	0xf007, 0x8002)
> +__RISCV_INSN_FUNCS(c_jal,	0xe003, 0x2001)
> +__RISCV_INSN_FUNCS(c_jalr,	0xf007, 0x9002)
> +__RISCV_INSN_FUNCS(c_beqz,	0xe003, 0xc001)
> +__RISCV_INSN_FUNCS(c_bnez,	0xe003, 0xe001)
> +__RISCV_INSN_FUNCS(c_ebreak,	0xffff, 0x9002)
> +
> +__RISCV_INSN_FUNCS(auipc,	0x7f, 0x17)
> +__RISCV_INSN_FUNCS(branch,	0x7f, 0x63)
> +
> +__RISCV_INSN_FUNCS(jal,		0x7f, 0x6f)
> +__RISCV_INSN_FUNCS(jalr,	0x707f, 0x67)
> +
> +#endif /* _RISCV_KERNEL_PROBES_SIMULATE_INSN_H */
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index 7d95cce..c6846dd 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -12,6 +12,7 @@
>  #include <linux/signal.h>
>  #include <linux/kdebug.h>
>  #include <linux/uaccess.h>
> +#include <linux/kprobes.h>
>  #include <linux/mm.h>
>  #include <linux/module.h>
>  #include <linux/irq.h>
> @@ -145,6 +146,14 @@ static inline unsigned long get_break_insn_length(unsigned long pc)
>
>  asmlinkage __visible void do_trap_break(struct pt_regs *regs)
>  {
> +#ifdef CONFIG_KPROBES
> +	if (kprobe_single_step_handler(regs))
> +		return;
> +
> +	if (kprobe_breakpoint_handler(regs))
> +		return;
> +#endif
> +
>  	if (user_mode(regs))
>  		force_sig_fault(SIGTRAP, TRAP_BRKPT, (void __user *)regs->epc);
>  #ifdef CONFIG_KGDB
> diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
> index ae7b7fe..da0c08c 100644
> --- a/arch/riscv/mm/fault.c
> +++ b/arch/riscv/mm/fault.c
> @@ -13,6 +13,7 @@
>  #include <linux/perf_event.h>
>  #include <linux/signal.h>
>  #include <linux/uaccess.h>
> +#include <linux/kprobes.h>
>
>  #include <asm/pgalloc.h>
>  #include <asm/ptrace.h>
> @@ -40,6 +41,9 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
>  	tsk = current;
>  	mm = tsk->mm;
>
> +	if (kprobe_page_fault(regs, cause))
> +		return;
> +
>  	/*
>  	 * Fault-in kernel-space virtual memory on-demand.
>  	 * The 'reference' page table is init_mm.pgd.

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