Re: [Patch 02/12] Introducing generic hardware breakpoint handler interfaces

David Gibson
2009-05-28T06:16:00Z

On Mon, May 11, 2009 at 05:22:40PM +0530, K.Prasad wrote:
> This patch introduces the generic Hardware Breakpoint interfaces for
> both user and kernel space requests.

[snip]
> Index: kernel/hw_breakpoint.c
> ===================================================================
> --- /dev/null
> +++ kernel/hw_breakpoint.c
> @@ -0,0 +1,367 @@
> +/*
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation; either version 2 of the License, or
> + * (at your option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program; if not, write to the Free Software
> + * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
> + *
> + * Copyright (C) 2007 Alan Stern
> + * Copyright (C) IBM Corporation, 2009
> + */
> +
> +/*
> + * HW_breakpoint: a unified kernel/user-space hardware breakpoint facility,
> + * using the CPU's debug registers.
> + *
> + * This file contains the arch-independent routines. It is not meant
> + * to be compiled as a standalone source file; rather it should be
> + * #include'd by the arch-specific implementation.
> + */
> +
> +#include <linux/irqflags.h>
> +#include <linux/kallsyms.h>
> +#include <linux/notifier.h>
> +#include <linux/kprobes.h>
> +#include <linux/kdebug.h>
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/percpu.h>
> +#include <linux/mutex.h>
> +#include <linux/sched.h>
> +#include <linux/init.h>
> +#include <linux/smp.h>
> +
> +#include <asm/hw_breakpoint.h>
> +#include <asm/processor.h>
> +
> +#ifdef CONFIG_X86

That's a nasty #ifdef. Surely this include should instead go in the
x86 version of hw_breakpoint.h.

> +#include <asm/debugreg.h>
> +#endif
> +/*
> + * Spinlock that protects all (un)register operations over kernel/user-space
> + * breakpoint requests
> + */
> +DEFINE_SPINLOCK(hw_breakpoint_lock);
> +
> +/* Array of kernel-space breakpoint structures */
> +struct hw_breakpoint *hbp_kernel[HB_NUM];
> +
> +/*
> + * Per-processor copy of hbp_kernel[]. Used only when hbp_kernel is being
> + * modified but we need the older copy to handle any hbp exceptions. It will
> + * sync with hbp_kernel[] value after updation is done through IPIs.
> + */
> +DEFINE_PER_CPU(struct hw_breakpoint*, this_hbp_kernel[HB_NUM]);

Ok.. this seems to assume that all breakpoints are equal as far as
allocation goes - i.e. that the total number of breakpoints is the
only allocation limit. That's apparently true for x86, but on
embedded powerpc (4xx), for example, we can have up to 2 data
breakpoints and up to 4 instruction breakpoints independent of each
other.

> +/*
> + * Kernel breakpoints grow downwards, starting from HB_NUM
> + * 'hbp_kernel_pos' denotes lowest numbered breakpoint register occupied for
> + * kernel-space request. We will initialise it here and not in an __init
> + * routine because load_debug_registers(), which uses this variable can be
> + * called very early during CPU initialisation.
> + */
> +unsigned int hbp_kernel_pos = HB_NUM;

Likewise here. For 44x we would need separate tables and separate
pointers for data and instruction breakpoints.

> +/*
> + * An array containing refcount of threads using a given bkpt register
> + * Accesses are synchronised by acquiring hw_breakpoint_lock
> + */
> +unsigned int hbp_user_refcount[HB_NUM];
> +
> +/*
> + * Install the debug register values for a new thread.
> + */
> +void switch_to_thread_hw_breakpoint(struct task_struct *tsk)
> +{
> + /* Set the debug register */
> + arch_install_thread_hw_breakpoint(tsk);

Um.. and the point of this wrapper which does nothing extra around the
arch-specific function would be...?

> +}
> +
> +/*
> + * Install the debug register values for just the kernel, no thread.
> + */
> +void switch_to_none_hw_breakpoint(void)
> +{
> + arch_uninstall_thread_hw_breakpoint();

And again.

> +}
> +
> +/*
> + * Load the debug registers during startup of a CPU.
> + */
> +void load_debug_registers(void)

This name does not suggest a boot and/or cpu hotplug only function.

> +{
> + unsigned long flags;
> + struct task_struct *tsk = current;
> +
> + spin_lock_bh(&hw_breakpoint_lock);
> +
> + /* Prevent IPIs for new kernel breakpoint updates */
> + local_irq_save(flags);
> + arch_update_kernel_hw_breakpoint(NULL);
> + local_irq_restore(flags);
> +
> + if (test_tsk_thread_flag(tsk, TIF_DEBUG))
> + switch_to_thread_hw_breakpoint(tsk);
> +
> + spin_unlock_bh(&hw_breakpoint_lock);
> +}
> +
> +/*
> + * Erase all the hardware breakpoint info associated with a thread.
> + *
> + * If tsk != current then tsk must not be usable (for example, a
> + * child being cleaned up from a failed fork).
> + */
> +void flush_thread_hw_breakpoint(struct task_struct *tsk)
> +{
> + int i;
> + struct thread_struct *thread = &(tsk->thread);
> +
> + spin_lock_bh(&hw_breakpoint_lock);
> +
> + /* The thread no longer has any breakpoints associated with it */
> + clear_tsk_thread_flag(tsk, TIF_DEBUG);
> + for (i = 0; i < HB_NUM; i++) {
> + if (thread->hbp[i]) {
> + hbp_user_refcount[i]--;
> + kfree(thread->hbp[i]);
> + thread->hbp[i] = NULL;
> + }
> + }
> +
> + arch_flush_thread_hw_breakpoint(tsk);
> +
> + /* Actually uninstall the breakpoints if necessary */
> + if (tsk == current)
> + switch_to_none_hw_breakpoint();
> + spin_unlock_bh(&hw_breakpoint_lock);
> +}
> +
> +/*
> + * Copy the hardware breakpoint info from a thread to its cloned
> child.

This comment contradicts what the code actually does.

> + */
> +int copy_thread_hw_breakpoint(struct task_struct *tsk,
> + struct task_struct *child, unsigned long clone_flags)
> +{
> + /*
> + * We will assume that breakpoint settings are not inherited
> + * and the child starts out with no debug registers set.
> + * But what about CLONE_PTRACE?
> + */
> + clear_tsk_thread_flag(child, TIF_DEBUG);
> +
> + /* We will call flush routine since the debugregs are not inherited */
> + arch_flush_thread_hw_breakpoint(child);

> +
> + return 0;
> +}
> +
> +int __register_user_hw_breakpoint(int pos, struct task_struct *tsk,
> + struct hw_breakpoint *bp)
> +{
> + struct thread_struct *thread = &(tsk->thread);
> + int rc;
> +
> + /* Do not overcommit. Fail if kernel has used the hbp registers */
> + if (pos >= hbp_kernel_pos)
> + return -ENOSPC;

Again, this is generic code making a non-portable assumption about how
breakpoints must be allocated.

> + rc = arch_validate_hwbkpt_settings(bp, tsk);
> + if (rc)
> + return rc;
> +
> + thread->hbp[pos] = bp;
> + hbp_user_refcount[pos]++;
> +
> + arch_update_user_hw_breakpoint(pos, tsk);
> + /*
> + * Does it need to be installed right now?
> + * Otherwise it will get installed the next time tsk runs
> + */
> + if (tsk == current)
> + switch_to_thread_hw_breakpoint(tsk);
> +
> + return rc;
> +}
> +
> +/*
> + * Modify the address of a hbp register already in use by the task
> + * Do not invoke this in-lieu of a __unregister_user_hw_breakpoint()
> + */
> +int __modify_user_hw_breakpoint(int pos, struct task_struct *tsk,
> + struct hw_breakpoint *bp)
> +{
> + struct thread_struct *thread = &(tsk->thread);
> +
> + if ((pos >= hbp_kernel_pos) || (arch_validate_hwbkpt_settings(bp, tsk)))
> + return -EINVAL;
> +
> + if (thread->hbp[pos] == NULL)
> + return -EINVAL;
> +
> + thread->hbp[pos] = bp;
> + /*
> + * 'pos' must be that of a hbp register already used by 'tsk'
> + * Otherwise arch_modify_user_hw_breakpoint() will fail
> + */
> + arch_update_user_hw_breakpoint(pos, tsk);
> +
> + if (tsk == current)
> + switch_to_thread_hw_breakpoint(tsk);
> +
> + return 0;
> +}
> +
> +void __unregister_user_hw_breakpoint(int pos, struct task_struct *tsk)
> +{
> + hbp_user_refcount[pos]--;
> + tsk->thread.hbp[pos] = NULL;
> +
> + arch_update_user_hw_breakpoint(pos, tsk);
> +
> + if (tsk == current)
> + switch_to_thread_hw_breakpoint(tsk);
> +}
> +
> +/**
> + * register_user_hw_breakpoint - register a hardware breakpoint for user space
> + * @tsk: pointer to 'task_struct' of the process to which the address belongs
> + * @bp: the breakpoint structure to register
> + *
> + * @bp.info->name or @bp.info->address, @bp.info->len, @bp.info->type and
> + * @bp->triggered must be set properly before invocation
> + *
> + */
> +int register_user_hw_breakpoint(struct task_struct *tsk,
> + struct hw_breakpoint *bp)
> +{
> + struct thread_struct *thread = &(tsk->thread);
> + int i, rc = -ENOSPC;
> +
> + spin_lock_bh(&hw_breakpoint_lock);
> +
> + for (i = 0; i < hbp_kernel_pos; i++) {
> + if (!thread->hbp[i]) {
> + rc = __register_user_hw_breakpoint(i, tsk, bp);
> + break;
> + }
> + }
> +
> + spin_unlock_bh(&hw_breakpoint_lock);
> + return rc;
> +}
> +EXPORT_SYMBOL_GPL(register_user_hw_breakpoint);
> +
> +/**
> + * unregister_user_hw_breakpoint - unregister a user-space hardware breakpoint
> + * @tsk: pointer to 'task_struct' of the process to which the address belongs
> + * @bp: the breakpoint structure to unregister
> + *
> + */
> +void unregister_user_hw_breakpoint(struct task_struct *tsk,
> + struct hw_breakpoint *bp)
> +{
> + struct thread_struct *thread = &(tsk->thread);
> + int i;
> +
> + spin_lock_bh(&hw_breakpoint_lock);
> + for (i = 0; i < hbp_kernel_pos; i++) {
> + if (bp == thread->hbp[i]) {
> + __unregister_user_hw_breakpoint(i, tsk);
> + break;
> + }
> + }
> + spin_unlock_bh(&hw_breakpoint_lock);
> +}
> +EXPORT_SYMBOL_GPL(unregister_user_hw_breakpoint);
> +
> +/**
> + * register_kernel_hw_breakpoint - register a hardware breakpoint for kernel space
> + * @bp: the breakpoint structure to register
> + *
> + * @bp.info->name or @bp.info->address, @bp.info->len, @bp.info->type and
> + * @bp->triggered must be set properly before invocation
> + *
> + */
> +int register_kernel_hw_breakpoint(struct hw_breakpoint *bp)
> +{
> + int rc;
> +
> + rc = arch_validate_hwbkpt_settings(bp, NULL);
> + if (rc)
> + return rc;
> +
> + spin_lock_bh(&hw_breakpoint_lock);
> +
> + rc = -EINVAL;
> + /* Check if we are over-committing */
> + if ((hbp_kernel_pos > 0) && (!hbp_user_refcount[hbp_kernel_pos-1])) {
> + hbp_kernel_pos--;
> + hbp_kernel[hbp_kernel_pos] = bp;
> + on_each_cpu(arch_update_kernel_hw_breakpoint, NULL, 1);
> + rc = 0;
> + }
> +
> + spin_unlock_bh(&hw_breakpoint_lock);
> + return rc;
> +}
> +EXPORT_SYMBOL_GPL(register_kernel_hw_breakpoint);
> +
> +/**
> + * unregister_kernel_hw_breakpoint - unregister a HW breakpoint for kernel space
> + * @bp: the breakpoint structure to unregister
> + *
> + * Uninstalls and unregisters @bp.
> + */
> +void unregister_kernel_hw_breakpoint(struct hw_breakpoint *bp)
> +{
> + int i, j;
> +
> + spin_lock_bh(&hw_breakpoint_lock);
> +
> + /* Find the 'bp' in our list of breakpoints for kernel */
> + for (i = hbp_kernel_pos; i < HB_NUM; i++)
> + if (bp == hbp_kernel[i])
> + break;
> +
> + /* Check if we did not find a match for 'bp'. If so return early */
> + if (i == HB_NUM) {
> + spin_unlock_bh(&hw_breakpoint_lock);
> + return;
> + }
> +
> + /*
> + * We'll shift the breakpoints one-level above to compact if
> + * unregistration creates a hole
> + */
> + for (j = i; j > hbp_kernel_pos; j--)
> + hbp_kernel[j] = hbp_kernel[j-1];
> +
> + hbp_kernel[hbp_kernel_pos] = NULL;
> + on_each_cpu(arch_update_kernel_hw_breakpoint, NULL, 1);
> + hbp_kernel_pos++;
> +
> + spin_unlock_bh(&hw_breakpoint_lock);
> +}
> +EXPORT_SYMBOL_GPL(unregister_kernel_hw_breakpoint);
> +
> +static struct notifier_block hw_breakpoint_exceptions_nb = {
> + .notifier_call = hw_breakpoint_exceptions_notify,
> + /* we need to be notified first */
> + .priority = 0x7fffffff
> +};
> +
> +static int __init init_hw_breakpoint(void)
> +{
> + return register_die_notifier(&hw_breakpoint_exceptions_nb);
> +}
> +
> +core_initcall(init_hw_breakpoint);
>

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!