powerpc/kernel/hw_breakpoint.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * HW_breakpoint: a unified kernel/user-space hardware breakpoint facility,
 * using the CPU's debug registers. Derived from
 * "arch/x86/kernel/hw_breakpoint.c"
 *
 * Copyright 2010 IBM Corporation
 * Author: K.Prasad <prasad@linux.vnet.ibm.com>
 */

#include <linux/hw_breakpoint.h>
#include <linux/notifier.h>
#include <linux/kprobes.h>
#include <linux/percpu.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/smp.h>
#include <linux/debugfs.h>
#include <linux/init.h>

#include <asm/hw_breakpoint.h>
#include <asm/processor.h>
#include <asm/sstep.h>
#include <asm/debug.h>
#include <asm/debugfs.h>
#include <asm/hvcall.h>
#include <linux/uaccess.h>

/*
 * Stores the breakpoints currently in use on each breakpoint address
 * register for every cpu
 */
static DEFINE_PER_CPU(struct perf_event *, bp_per_reg);

/*
 * Returns total number of data or instruction breakpoints available.
 */
int hw_breakpoint_slots(int type)
{
	if (type == TYPE_DATA)
		return HBP_NUM;
	return 0;		/* no instruction breakpoints available */
}

/*
 * Install a perf counter breakpoint.
 *
 * We seek a free debug address register and use it for this
 * breakpoint.
 *
 * Atomic: we hold the counter->ctx->lock and we only handle variables
 * and registers local to this cpu.
 */
int arch_install_hw_breakpoint(struct perf_event *bp)
{
	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
	struct perf_event **slot = this_cpu_ptr(&bp_per_reg);

	*slot = bp;

	/*
	 * Do not install DABR values if the instruction must be single-stepped.
	 * If so, DABR will be populated in single_step_dabr_instruction().
	 */
	if (current->thread.last_hit_ubp != bp)
		__set_breakpoint(info);

	return 0;
}

/*
 * Uninstall the breakpoint contained in the given counter.
 *
 * First we search the debug address register it uses and then we disable
 * it.
 *
 * Atomic: we hold the counter->ctx->lock and we only handle variables
 * and registers local to this cpu.
 */
void arch_uninstall_hw_breakpoint(struct perf_event *bp)
{
	struct perf_event **slot = this_cpu_ptr(&bp_per_reg);

	if (*slot != bp) {
		WARN_ONCE(1, "Can't find the breakpoint");
		return;
	}

	*slot = NULL;
	hw_breakpoint_disable();
}

/*
 * Perform cleanup of arch-specific counters during unregistration
 * of the perf-event
 */
void arch_unregister_hw_breakpoint(struct perf_event *bp)
{
	/*
	 * If the breakpoint is unregistered between a hw_breakpoint_handler()
	 * and the single_step_dabr_instruction(), then cleanup the breakpoint
	 * restoration variables to prevent dangling pointers.
	 * FIXME, this should not be using bp->ctx at all! Sayeth peterz.
	 */
	if (bp->ctx && bp->ctx->task && bp->ctx->task != ((void *)-1L))
		bp->ctx->task->thread.last_hit_ubp = NULL;
}

/*
 * Check for virtual address in kernel space.
 */
int arch_check_bp_in_kernelspace(struct arch_hw_breakpoint *hw)
{
	return is_kernel_addr(hw->address);
}

int arch_bp_generic_fields(int type, int *gen_bp_type)
{
	*gen_bp_type = 0;
	if (type & HW_BRK_TYPE_READ)
		*gen_bp_type |= HW_BREAKPOINT_R;
	if (type & HW_BRK_TYPE_WRITE)
		*gen_bp_type |= HW_BREAKPOINT_W;
	if (*gen_bp_type == 0)
		return -EINVAL;
	return 0;
}

/*
 * Validate the arch-specific HW Breakpoint register settings
 */
int hw_breakpoint_arch_parse(struct perf_event *bp,
			     const struct perf_event_attr *attr,
			     struct arch_hw_breakpoint *hw)
{
	int ret = -EINVAL, length_max;

	if (!bp)
		return ret;

	hw->type = HW_BRK_TYPE_TRANSLATE;
	if (attr->bp_type & HW_BREAKPOINT_R)
		hw->type |= HW_BRK_TYPE_READ;
	if (attr->bp_type & HW_BREAKPOINT_W)
		hw->type |= HW_BRK_TYPE_WRITE;
	if (hw->type == HW_BRK_TYPE_TRANSLATE)
		/* must set alteast read or write */
		return ret;
	if (!attr->exclude_user)
		hw->type |= HW_BRK_TYPE_USER;
	if (!attr->exclude_kernel)
		hw->type |= HW_BRK_TYPE_KERNEL;
	if (!attr->exclude_hv)
		hw->type |= HW_BRK_TYPE_HYP;
	hw->address = attr->bp_addr;
	hw->len = attr->bp_len;

	/*
	 * Since breakpoint length can be a maximum of HW_BREAKPOINT_LEN(8)
	 * and breakpoint addresses are aligned to nearest double-word
	 * HW_BREAKPOINT_ALIGN by rounding off to the lower address, the
	 * 'symbolsize' should satisfy the check below.
	 */
	if (!ppc_breakpoint_available())
		return -ENODEV;
	length_max = 8; /* DABR */
	if (dawr_enabled()) {
		length_max = 512 ; /* 64 doublewords */
		/* DAWR region can't cross 512 boundary */
		if ((attr->bp_addr >> 9) !=
		    ((attr->bp_addr + attr->bp_len - 1) >> 9))
			return -EINVAL;
	}
	if (hw->len >
	    (length_max - (hw->address & HW_BREAKPOINT_ALIGN)))
		return -EINVAL;
	return 0;
}

/*
 * Restores the breakpoint on the debug registers.
 * Invoke this function if it is known that the execution context is
 * about to change to cause loss of MSR_SE settings.
 */
void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs)
{
	struct arch_hw_breakpoint *info;

	if (likely(!tsk->thread.last_hit_ubp))
		return;

	info = counter_arch_bp(tsk->thread.last_hit_ubp);
	regs->msr &= ~MSR_SE;
	__set_breakpoint(info);
	tsk->thread.last_hit_ubp = NULL;
}

/*
 * Handle debug exception notifications.
 */
int hw_breakpoint_handler(struct die_args *args)
{
	int rc = NOTIFY_STOP;
	struct perf_event *bp;
	struct pt_regs *regs = args->regs;
#ifndef CONFIG_PPC_8xx
	int stepped = 1;
	unsigned int instr;
#endif
	struct arch_hw_breakpoint *info;
	unsigned long dar = regs->dar;

	/* Disable breakpoints during exception handling */
	hw_breakpoint_disable();

	/*
	 * The counter may be concurrently released but that can only
	 * occur from a call_rcu() path. We can then safely fetch
	 * the breakpoint, use its callback, touch its counter
	 * while we are in an rcu_read_lock() path.
	 */
	rcu_read_lock();

	bp = __this_cpu_read(bp_per_reg);
	if (!bp) {
		rc = NOTIFY_DONE;
		goto out;
	}
	info = counter_arch_bp(bp);

	/*
	 * Return early after invoking user-callback function without restoring
	 * DABR if the breakpoint is from ptrace which always operates in
	 * one-shot mode. The ptrace-ed process will receive the SIGTRAP signal
	 * generated in do_dabr().
	 */
	if (bp->overflow_handler == ptrace_triggered) {
		perf_bp_event(bp, regs);
		rc = NOTIFY_DONE;
		goto out;
	}

	/*
	 * Verify if dar lies within the address range occupied by the symbol
	 * being watched to filter extraneous exceptions.  If it doesn't,
	 * we still need to single-step the instruction, but we don't
	 * generate an event.
	 */
	info->type &= ~HW_BRK_TYPE_EXTRANEOUS_IRQ;
	if (!((bp->attr.bp_addr <= dar) &&
	      (dar - bp->attr.bp_addr < bp->attr.bp_len)))
		info->type |= HW_BRK_TYPE_EXTRANEOUS_IRQ;

#ifndef CONFIG_PPC_8xx
	/* Do not emulate user-space instructions, instead single-step them */
	if (user_mode(regs)) {
		current->thread.last_hit_ubp = bp;
		regs->msr |= MSR_SE;
		goto out;
	}

	stepped = 0;
	instr = 0;
	if (!__get_user_inatomic(instr, (unsigned int *) regs->nip))
		stepped = emulate_step(regs, instr);

	/*
	 * emulate_step() could not execute it. We've failed in reliably
	 * handling the hw-breakpoint. Unregister it and throw a warning
	 * message to let the user know about it.
	 */
	if (!stepped) {
		WARN(1, "Unable to handle hardware breakpoint. Breakpoint at "
			"0x%lx will be disabled.", info->address);
		perf_event_disable_inatomic(bp);
		goto out;
	}
#endif
	/*
	 * As a policy, the callback is invoked in a 'trigger-after-execute'
	 * fashion
	 */
	if (!(info->type & HW_BRK_TYPE_EXTRANEOUS_IRQ))
		perf_bp_event(bp, regs);

	__set_breakpoint(info);
out:
	rcu_read_unlock();
	return rc;
}
NOKPROBE_SYMBOL(hw_breakpoint_handler);

/*
 * Handle single-step exceptions following a DABR hit.
 */
static int single_step_dabr_instruction(struct die_args *args)
{
	struct pt_regs *regs = args->regs;
	struct perf_event *bp = NULL;
	struct arch_hw_breakpoint *info;

	bp = current->thread.last_hit_ubp;
	/*
	 * Check if we are single-stepping as a result of a
	 * previous HW Breakpoint exception
	 */
	if (!bp)
		return NOTIFY_DONE;

	info = counter_arch_bp(bp);

	/*
	 * We shall invoke the user-defined callback function in the single
	 * stepping handler to confirm to 'trigger-after-execute' semantics
	 */
	if (!(info->type & HW_BRK_TYPE_EXTRANEOUS_IRQ))
		perf_bp_event(bp, regs);

	__set_breakpoint(info);
	current->thread.last_hit_ubp = NULL;

	/*
	 * If the process was being single-stepped by ptrace, let the
	 * other single-step actions occur (e.g. generate SIGTRAP).
	 */
	if (test_thread_flag(TIF_SINGLESTEP))
		return NOTIFY_DONE;

	return NOTIFY_STOP;
}
NOKPROBE_SYMBOL(single_step_dabr_instruction);

/*
 * Handle debug exception notifications.
 */
int hw_breakpoint_exceptions_notify(
		struct notifier_block *unused, unsigned long val, void *data)
{
	int ret = NOTIFY_DONE;

	switch (val) {
	case DIE_DABR_MATCH:
		ret = hw_breakpoint_handler(data);
		break;
	case DIE_SSTEP:
		ret = single_step_dabr_instruction(data);
		break;
	}

	return ret;
}
NOKPROBE_SYMBOL(hw_breakpoint_exceptions_notify);

/*
 * Release the user breakpoints used by ptrace
 */
void flush_ptrace_hw_breakpoint(struct task_struct *tsk)
{
	struct thread_struct *t = &tsk->thread;

	unregister_hw_breakpoint(t->ptrace_bps[0]);
	t->ptrace_bps[0] = NULL;
}

void hw_breakpoint_pmu_read(struct perf_event *bp)
{
	/* TODO */
}

bool dawr_force_enable;
EXPORT_SYMBOL_GPL(dawr_force_enable);

static ssize_t dawr_write_file_bool(struct file *file,
				    const char __user *user_buf,
				    size_t count, loff_t *ppos)
{
	struct arch_hw_breakpoint null_brk = {0, 0, 0};
	size_t rc;

	/* Send error to user if they hypervisor won't allow us to write DAWR */
	if ((!dawr_force_enable) &&
	    (firmware_has_feature(FW_FEATURE_LPAR)) &&
	    (set_dawr(&null_brk) != H_SUCCESS))
		return -1;

	rc = debugfs_write_file_bool(file, user_buf, count, ppos);
	if (rc)
		return rc;

	/* If we are clearing, make sure all CPUs have the DAWR cleared */
	if (!dawr_force_enable)
		smp_call_function((smp_call_func_t)set_dawr, &null_brk, 0);

	return rc;
}

static const struct file_operations dawr_enable_fops = {
	.read =		debugfs_read_file_bool,
	.write =	dawr_write_file_bool,
	.open =		simple_open,
	.llseek =	default_llseek,
};

static int __init dawr_force_setup(void)
{
	dawr_force_enable = false;

	if (cpu_has_feature(CPU_FTR_DAWR)) {
		/* Don't setup sysfs file for user control on P8 */
		dawr_force_enable = true;
		return 0;
	}

	if (PVR_VER(mfspr(SPRN_PVR)) == PVR_POWER9) {
		/* Turn DAWR off by default, but allow admin to turn it on */
		dawr_force_enable = false;
		debugfs_create_file_unsafe("dawr_enable_dangerous", 0600,
					   powerpc_debugfs_root,
					   &dawr_force_enable,
					   &dawr_enable_fops);
	}
	return 0;
}
arch_initcall(dawr_force_setup);