xref: /openbmc/linux/arch/mips/kernel/kprobes.c (revision f5e2d818)

// SPDX-License-Identifier: GPL-2.0-only
/*
 *  Kernel Probes (KProbes)
 *  arch/mips/kernel/kprobes.c
 *
 *  Copyright 2006 Sony Corp.
 *  Copyright 2010 Cavium Networks
 *
 *  Some portions copied from the powerpc version.
 *
 *   Copyright (C) IBM Corporation, 2002, 2004
 */

#define pr_fmt(fmt) "kprobes: " fmt

#include <linux/kprobes.h>
#include <linux/preempt.h>
#include <linux/uaccess.h>
#include <linux/kdebug.h>
#include <linux/slab.h>

#include <asm/ptrace.h>
#include <asm/branch.h>
#include <asm/break.h>

#include "probes-common.h"

static const union mips_instruction breakpoint_insn = {
	.b_format = {
		.opcode = spec_op,
		.code = BRK_KPROBE_BP,
		.func = break_op
	}
};

static const union mips_instruction breakpoint2_insn = {
	.b_format = {
		.opcode = spec_op,
		.code = BRK_KPROBE_SSTEPBP,
		.func = break_op
	}
};

DEFINE_PER_CPU(struct kprobe *, current_kprobe);
DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);

static int insn_has_delayslot(union mips_instruction insn)
{
	return __insn_has_delay_slot(insn);
}
NOKPROBE_SYMBOL(insn_has_delayslot);

/*
 * insn_has_ll_or_sc function checks whether instruction is ll or sc
 * one; putting breakpoint on top of atomic ll/sc pair is bad idea;
 * so we need to prevent it and refuse kprobes insertion for such
 * instructions; cannot do much about breakpoint in the middle of
 * ll/sc pair; it is upto user to avoid those places
 */
static int insn_has_ll_or_sc(union mips_instruction insn)
{
	int ret = 0;

	switch (insn.i_format.opcode) {
	case ll_op:
	case lld_op:
	case sc_op:
	case scd_op:
		ret = 1;
		break;
	default:
		break;
	}
	return ret;
}
NOKPROBE_SYMBOL(insn_has_ll_or_sc);

int arch_prepare_kprobe(struct kprobe *p)
{
	union mips_instruction insn;
	union mips_instruction prev_insn;
	int ret = 0;

	insn = p->addr[0];

	if (insn_has_ll_or_sc(insn)) {
		pr_notice("Kprobes for ll and sc instructions are not supported\n");
		ret = -EINVAL;
		goto out;
	}

	if (copy_from_kernel_nofault(&prev_insn, p->addr - 1,
			sizeof(mips_instruction)) == 0 &&
	    insn_has_delayslot(prev_insn)) {
		pr_notice("Kprobes for branch delayslot are not supported\n");
		ret = -EINVAL;
		goto out;
	}

	if (__insn_is_compact_branch(insn)) {
		pr_notice("Kprobes for compact branches are not supported\n");
		ret = -EINVAL;
		goto out;
	}

	/* insn: must be on special executable page on mips. */
	p->ainsn.insn = get_insn_slot();
	if (!p->ainsn.insn) {
		ret = -ENOMEM;
		goto out;
	}

	/*
	 * In the kprobe->ainsn.insn[] array we store the original
	 * instruction at index zero and a break trap instruction at
	 * index one.
	 *
	 * On MIPS arch if the instruction at probed address is a
	 * branch instruction, we need to execute the instruction at
	 * Branch Delayslot (BD) at the time of probe hit. As MIPS also
	 * doesn't have single stepping support, the BD instruction can
	 * not be executed in-line and it would be executed on SSOL slot
	 * using a normal breakpoint instruction in the next slot.
	 * So, read the instruction and save it for later execution.
	 */
	if (insn_has_delayslot(insn))
		memcpy(&p->ainsn.insn[0], p->addr + 1, sizeof(kprobe_opcode_t));
	else
		memcpy(&p->ainsn.insn[0], p->addr, sizeof(kprobe_opcode_t));

	p->ainsn.insn[1] = breakpoint2_insn;
	p->opcode = *p->addr;

out:
	return ret;
}
NOKPROBE_SYMBOL(arch_prepare_kprobe);

void arch_arm_kprobe(struct kprobe *p)
{
	*p->addr = breakpoint_insn;
	flush_insn_slot(p);
}
NOKPROBE_SYMBOL(arch_arm_kprobe);

void arch_disarm_kprobe(struct kprobe *p)
{
	*p->addr = p->opcode;
	flush_insn_slot(p);
}
NOKPROBE_SYMBOL(arch_disarm_kprobe);

void arch_remove_kprobe(struct kprobe *p)
{
	if (p->ainsn.insn) {
		free_insn_slot(p->ainsn.insn, 0);
		p->ainsn.insn = NULL;
	}
}
NOKPROBE_SYMBOL(arch_remove_kprobe);

static void save_previous_kprobe(struct kprobe_ctlblk *kcb)
{
	kcb->prev_kprobe.kp = kprobe_running();
	kcb->prev_kprobe.status = kcb->kprobe_status;
	kcb->prev_kprobe.old_SR = kcb->kprobe_old_SR;
	kcb->prev_kprobe.saved_SR = kcb->kprobe_saved_SR;
	kcb->prev_kprobe.saved_epc = kcb->kprobe_saved_epc;
}

static void restore_previous_kprobe(struct kprobe_ctlblk *kcb)
{
	__this_cpu_write(current_kprobe, kcb->prev_kprobe.kp);
	kcb->kprobe_status = kcb->prev_kprobe.status;
	kcb->kprobe_old_SR = kcb->prev_kprobe.old_SR;
	kcb->kprobe_saved_SR = kcb->prev_kprobe.saved_SR;
	kcb->kprobe_saved_epc = kcb->prev_kprobe.saved_epc;
}

static void set_current_kprobe(struct kprobe *p, struct pt_regs *regs,
			       struct kprobe_ctlblk *kcb)
{
	__this_cpu_write(current_kprobe, p);
	kcb->kprobe_saved_SR = kcb->kprobe_old_SR = (regs->cp0_status & ST0_IE);
	kcb->kprobe_saved_epc = regs->cp0_epc;
}

/**
 * evaluate_branch_instrucion -
 *
 * Evaluate the branch instruction at probed address during probe hit. The
 * result of evaluation would be the updated epc. The insturction in delayslot
 * would actually be single stepped using a normal breakpoint) on SSOL slot.
 *
 * The result is also saved in the kprobe control block for later use,
 * in case we need to execute the delayslot instruction. The latter will be
 * false for NOP instruction in dealyslot and the branch-likely instructions
 * when the branch is taken. And for those cases we set a flag as
 * SKIP_DELAYSLOT in the kprobe control block
 */
static int evaluate_branch_instruction(struct kprobe *p, struct pt_regs *regs,
					struct kprobe_ctlblk *kcb)
{
	union mips_instruction insn = p->opcode;
	long epc;
	int ret = 0;

	epc = regs->cp0_epc;
	if (epc & 3)
		goto unaligned;

	if (p->ainsn.insn->word == 0)
		kcb->flags |= SKIP_DELAYSLOT;
	else
		kcb->flags &= ~SKIP_DELAYSLOT;

	ret = __compute_return_epc_for_insn(regs, insn);
	if (ret < 0)
		return ret;

	if (ret == BRANCH_LIKELY_TAKEN)
		kcb->flags |= SKIP_DELAYSLOT;

	kcb->target_epc = regs->cp0_epc;

	return 0;

unaligned:
	pr_notice("Failed to emulate branch instruction because of unaligned epc - sending SIGBUS to %s.\n", current->comm);
	force_sig(SIGBUS);
	return -EFAULT;

}

static void prepare_singlestep(struct kprobe *p, struct pt_regs *regs,
						struct kprobe_ctlblk *kcb)
{
	int ret = 0;

	regs->cp0_status &= ~ST0_IE;

	/* single step inline if the instruction is a break */
	if (p->opcode.word == breakpoint_insn.word ||
	    p->opcode.word == breakpoint2_insn.word)
		regs->cp0_epc = (unsigned long)p->addr;
	else if (insn_has_delayslot(p->opcode)) {
		ret = evaluate_branch_instruction(p, regs, kcb);
		if (ret < 0)
			return;
	}
	regs->cp0_epc = (unsigned long)&p->ainsn.insn[0];
}

/*
 * Called after single-stepping.  p->addr is the address of the
 * instruction whose first byte has been replaced by the "break 0"
 * instruction.	 To avoid the SMP problems that can occur when we
 * temporarily put back the original opcode to single-step, we
 * single-stepped a copy of the instruction.  The address of this
 * copy is p->ainsn.insn.
 *
 * This function prepares to return from the post-single-step
 * breakpoint trap. In case of branch instructions, the target
 * epc to be restored.
 */
static void resume_execution(struct kprobe *p,
				       struct pt_regs *regs,
				       struct kprobe_ctlblk *kcb)
{
	if (insn_has_delayslot(p->opcode))
		regs->cp0_epc = kcb->target_epc;
	else {
		unsigned long orig_epc = kcb->kprobe_saved_epc;
		regs->cp0_epc = orig_epc + 4;
	}
}
NOKPROBE_SYMBOL(resume_execution);

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;
	}

	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;
	}

	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;
	}

	prepare_singlestep(p, regs, kcb);
	if (kcb->flags & SKIP_DELAYSLOT) {
		kcb->kprobe_status = KPROBE_HIT_SSDONE;
		if (p->post_handler)
			p->post_handler(p, regs, 0);
		resume_execution(p, regs, kcb);
		preempt_enable_no_resched();
	} else
		kcb->kprobe_status = KPROBE_HIT_SS;

	return 1;

no_kprobe:
	preempt_enable_no_resched();
	return ret;

}
NOKPROBE_SYMBOL(kprobe_handler);

static inline int post_kprobe_handler(struct pt_regs *regs)
{
	struct kprobe *cur = kprobe_running();
	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();

	if (!cur)
		return 0;

	if ((kcb->kprobe_status != KPROBE_REENTER) && cur->post_handler) {
		kcb->kprobe_status = KPROBE_HIT_SSDONE;
		cur->post_handler(cur, regs, 0);
	}

	resume_execution(cur, regs, kcb);

	regs->cp0_status |= kcb->kprobe_saved_SR;

	/* Restore back the original saved kprobes variables and continue. */
	if (kcb->kprobe_status == KPROBE_REENTER) {
		restore_previous_kprobe(kcb);
		goto out;
	}
	reset_current_kprobe();
out:
	preempt_enable_no_resched();

	return 1;
}

int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
{
	struct kprobe *cur = kprobe_running();
	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();

	if (kcb->kprobe_status & KPROBE_HIT_SS) {
		resume_execution(cur, regs, kcb);
		regs->cp0_status |= kcb->kprobe_old_SR;

		reset_current_kprobe();
		preempt_enable_no_resched();
	}
	return 0;
}

/*
 * Wrapper routine for handling exceptions.
 */
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;
	case DIE_SSTEPBP:
		if (post_kprobe_handler(args->regs))
			ret = NOTIFY_STOP;
		break;

	case DIE_PAGE_FAULT:
		/* kprobe_running() needs smp_processor_id() */
		preempt_disable();

		if (kprobe_running()
		    && kprobe_fault_handler(args->regs, args->trapnr))
			ret = NOTIFY_STOP;
		preempt_enable();
		break;
	default:
		break;
	}
	return ret;
}
NOKPROBE_SYMBOL(kprobe_exceptions_notify);

/*
 * Function return probe trampoline:
 *	- init_kprobes() establishes a probepoint here
 *	- When the probed function returns, this probe causes the
 *	  handlers to fire
 */
static void __used kretprobe_trampoline_holder(void)
{
	asm volatile(
		".set push\n\t"
		/* Keep the assembler from reordering and placing JR here. */
		".set noreorder\n\t"
		"nop\n\t"
		".global __kretprobe_trampoline\n"
		"__kretprobe_trampoline:\n\t"
		"nop\n\t"
		".set pop"
		: : : "memory");
}

void __kretprobe_trampoline(void);

void arch_prepare_kretprobe(struct kretprobe_instance *ri,
				      struct pt_regs *regs)
{
	ri->ret_addr = (kprobe_opcode_t *) regs->regs[31];
	ri->fp = NULL;

	/* Replace the return addr with trampoline addr */
	regs->regs[31] = (unsigned long)__kretprobe_trampoline;
}
NOKPROBE_SYMBOL(arch_prepare_kretprobe);

/*
 * Called when the probe at kretprobe trampoline is hit
 */
static int trampoline_probe_handler(struct kprobe *p,
						struct pt_regs *regs)
{
	instruction_pointer(regs) = __kretprobe_trampoline_handler(regs, NULL);
	/*
	 * By returning a non-zero value, we are telling
	 * kprobe_handler() that we don't want the post_handler
	 * to run (and have re-enabled preemption)
	 */
	return 1;
}
NOKPROBE_SYMBOL(trampoline_probe_handler);

int arch_trampoline_kprobe(struct kprobe *p)
{
	if (p->addr == (kprobe_opcode_t *)__kretprobe_trampoline)
		return 1;

	return 0;
}
NOKPROBE_SYMBOL(arch_trampoline_kprobe);

static struct kprobe trampoline_p = {
	.addr = (kprobe_opcode_t *)__kretprobe_trampoline,
	.pre_handler = trampoline_probe_handler
};

int __init arch_init_kprobes(void)
{
	return register_kprobe(&trampoline_p);
}