/* Support for MMIO probes.
   * Benfit many code from kprobes
   * (C) 2002 Louis Zhuang <louis.zhuang@intel.com>.
   *     2007 Alexander Eichner
   *     2008 Pekka Paalanen <pq@iki.fi>
   */
  
  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  
  #include <linux/list.h>
  #include <linux/rculist.h>
  #include <linux/spinlock.h>
  #include <linux/hash.h>
  #include <linux/module.h>
  #include <linux/kernel.h>
  #include <linux/uaccess.h>
  #include <linux/ptrace.h>
  #include <linux/preempt.h>
  #include <linux/percpu.h>
  #include <linux/kdebug.h>
  #include <linux/mutex.h>
  #include <linux/io.h>
  #include <linux/slab.h>
  #include <asm/cacheflush.h>
  #include <asm/tlbflush.h>
  #include <linux/errno.h>
  #include <asm/debugreg.h>
  #include <linux/mmiotrace.h>
  
  #define KMMIO_PAGE_HASH_BITS 4
  #define KMMIO_PAGE_TABLE_SIZE (1 << KMMIO_PAGE_HASH_BITS)
  
  struct kmmio_fault_page {
  	struct list_head list;
  	struct kmmio_fault_page *release_next;
  	unsigned long page; /* location of the fault page */
  	pteval_t old_presence; /* page presence prior to arming */
  	bool armed;
  
  	/*
  	 * Number of times this page has been registered as a part
  	 * of a probe. If zero, page is disarmed and this may be freed.
  	 * Used only by writers (RCU) and post_kmmio_handler().
  	 * Protected by kmmio_lock, when linked into kmmio_page_table.
  	 */
  	int count;
  
  	bool scheduled_for_release;
  };
  
  struct kmmio_delayed_release {
  	struct rcu_head rcu;
  	struct kmmio_fault_page *release_list;
  };
  
  struct kmmio_context {
  	struct kmmio_fault_page *fpage;
  	struct kmmio_probe *probe;
  	unsigned long saved_flags;
  	unsigned long addr;
  	int active;
  };
  
  static DEFINE_SPINLOCK(kmmio_lock);
  
  /* Protected by kmmio_lock */
  unsigned int kmmio_count;
  
  /* Read-protected by RCU, write-protected by kmmio_lock. */
  static struct list_head kmmio_page_table[KMMIO_PAGE_TABLE_SIZE];
  static LIST_HEAD(kmmio_probes);
  
  static struct list_head *kmmio_page_list(unsigned long page)
  {
  	return &kmmio_page_table[hash_long(page, KMMIO_PAGE_HASH_BITS)];
  }
  
  /* Accessed per-cpu */
  static DEFINE_PER_CPU(struct kmmio_context, kmmio_ctx);
  
  /*
   * this is basically a dynamic stabbing problem:
   * Could use the existing prio tree code or
   * Possible better implementations:
   * The Interval Skip List: A Data Structure for Finding All Intervals That
   * Overlap a Point (might be simple)
   * Space Efficient Dynamic Stabbing with Fast Queries - Mikkel Thorup
   */
  /* Get the kmmio at this addr (if any). You must be holding RCU read lock. */
  static struct kmmio_probe *get_kmmio_probe(unsigned long addr)
  {
  	struct kmmio_probe *p;
  	list_for_each_entry_rcu(p, &kmmio_probes, list) {
  		if (addr >= p->addr && addr < (p->addr + p->len))
  			return p;
  	}
  	return NULL;
  }
  
  /* You must be holding RCU read lock. */
  static struct kmmio_fault_page *get_kmmio_fault_page(unsigned long page)
  {
  	struct list_head *head;
  	struct kmmio_fault_page *f;
  
  	page &= PAGE_MASK;
  	head = kmmio_page_list(page);
  	list_for_each_entry_rcu(f, head, list) {
  		if (f->page == page)
  			return f;
  	}
  	return NULL;
  }
  
  static void clear_pmd_presence(pmd_t *pmd, bool clear, pmdval_t *old)
  {
  	pmdval_t v = pmd_val(*pmd);
  	if (clear) {
  		*old = v & _PAGE_PRESENT;
  		v &= ~_PAGE_PRESENT;
  	} else	/* presume this has been called with clear==true previously */
  		v |= *old;
  	set_pmd(pmd, __pmd(v));
  }
  
  static void clear_pte_presence(pte_t *pte, bool clear, pteval_t *old)
  {
  	pteval_t v = pte_val(*pte);
  	if (clear) {
  		*old = v & _PAGE_PRESENT;
  		v &= ~_PAGE_PRESENT;
  	} else	/* presume this has been called with clear==true previously */
  		v |= *old;
  	set_pte_atomic(pte, __pte(v));
  }
  
  static int clear_page_presence(struct kmmio_fault_page *f, bool clear)
  {
  	unsigned int level;
  	pte_t *pte = lookup_address(f->page, &level);
  
  	if (!pte) {
  		pr_err("no pte for page 0x%08lx
  ", f->page);
  		return -1;
  	}
  
  	switch (level) {
  	case PG_LEVEL_2M:
  		clear_pmd_presence((pmd_t *)pte, clear, &f->old_presence);
  		break;
  	case PG_LEVEL_4K:
  		clear_pte_presence(pte, clear, &f->old_presence);
  		break;
  	default:
  		pr_err("unexpected page level 0x%x.
  ", level);
  		return -1;
  	}
  
  	__flush_tlb_one(f->page);
  	return 0;
  }
  
  /*
   * Mark the given page as not present. Access to it will trigger a fault.
   *
   * Struct kmmio_fault_page is protected by RCU and kmmio_lock, but the
   * protection is ignored here. RCU read lock is assumed held, so the struct
   * will not disappear unexpectedly. Furthermore, the caller must guarantee,
   * that double arming the same virtual address (page) cannot occur.
   *
   * Double disarming on the other hand is allowed, and may occur when a fault
   * and mmiotrace shutdown happen simultaneously.
   */
  static int arm_kmmio_fault_page(struct kmmio_fault_page *f)
  {
  	int ret;
  	WARN_ONCE(f->armed, KERN_ERR pr_fmt("kmmio page already armed.
  "));
  	if (f->armed) {
  		pr_warning("double-arm: page 0x%08lx, ref %d, old %d
  ",
  			   f->page, f->count, !!f->old_presence);
  	}
  	ret = clear_page_presence(f, true);
  	WARN_ONCE(ret < 0, KERN_ERR pr_fmt("arming 0x%08lx failed.
  "),
  		  f->page);
  	f->armed = true;
  	return ret;
  }
  
  /** Restore the given page to saved presence state. */
  static void disarm_kmmio_fault_page(struct kmmio_fault_page *f)
  {
  	int ret = clear_page_presence(f, false);
  	WARN_ONCE(ret < 0,
  			KERN_ERR "kmmio disarming 0x%08lx failed.
  ", f->page);
  	f->armed = false;
  }
  
  /*
   * This is being called from do_page_fault().
   *
   * We may be in an interrupt or a critical section. Also prefecthing may
   * trigger a page fault. We may be in the middle of process switch.
   * We cannot take any locks, because we could be executing especially
   * within a kmmio critical section.
   *
   * Local interrupts are disabled, so preemption cannot happen.
   * Do not enable interrupts, do not sleep, and watch out for other CPUs.
   */
  /*
   * Interrupts are disabled on entry as trap3 is an interrupt gate
   * and they remain disabled throughout this function.
   */
  int kmmio_handler(struct pt_regs *regs, unsigned long addr)
  {
  	struct kmmio_context *ctx;
  	struct kmmio_fault_page *faultpage;
  	int ret = 0; /* default to fault not handled */
  
  	/*
  	 * Preemption is now disabled to prevent process switch during
  	 * single stepping. We can only handle one active kmmio trace
  	 * per cpu, so ensure that we finish it before something else
  	 * gets to run. We also hold the RCU read lock over single
  	 * stepping to avoid looking up the probe and kmmio_fault_page
  	 * again.
  	 */
  	preempt_disable();
  	rcu_read_lock();
  
  	faultpage = get_kmmio_fault_page(addr);
  	if (!faultpage) {
  		/*
  		 * Either this page fault is not caused by kmmio, or
  		 * another CPU just pulled the kmmio probe from under
  		 * our feet. The latter case should not be possible.
  		 */
  		goto no_kmmio;
  	}
  
  	ctx = &get_cpu_var(kmmio_ctx);
  	if (ctx->active) {
  		if (addr == ctx->addr) {
  			/*
  			 * A second fault on the same page means some other
  			 * condition needs handling by do_page_fault(), the
  			 * page really not being present is the most common.
  			 */
  			pr_debug("secondary hit for 0x%08lx CPU %d.
  ",
  				 addr, smp_processor_id());
  
  			if (!faultpage->old_presence)
  				pr_info("unexpected secondary hit for address 0x%08lx on CPU %d.
  ",
  					addr, smp_processor_id());
  		} else {
  			/*
  			 * Prevent overwriting already in-flight context.
  			 * This should not happen, let's hope disarming at
  			 * least prevents a panic.
  			 */
  			pr_emerg("recursive probe hit on CPU %d, for address 0x%08lx. Ignoring.
  ",
  				 smp_processor_id(), addr);
  			pr_emerg("previous hit was at 0x%08lx.
  ", ctx->addr);
  			disarm_kmmio_fault_page(faultpage);
  		}
  		goto no_kmmio_ctx;
  	}
  	ctx->active++;
  
  	ctx->fpage = faultpage;
  	ctx->probe = get_kmmio_probe(addr);
  	ctx->saved_flags = (regs->flags & (X86_EFLAGS_TF | X86_EFLAGS_IF));
  	ctx->addr = addr;
  
  	if (ctx->probe && ctx->probe->pre_handler)
  		ctx->probe->pre_handler(ctx->probe, regs, addr);
  
  	/*
  	 * Enable single-stepping and disable interrupts for the faulting
  	 * context. Local interrupts must not get enabled during stepping.
  	 */
  	regs->flags |= X86_EFLAGS_TF;
  	regs->flags &= ~X86_EFLAGS_IF;
  
  	/* Now we set present bit in PTE and single step. */
  	disarm_kmmio_fault_page(ctx->fpage);
  
  	/*
  	 * If another cpu accesses the same page while we are stepping,
  	 * the access will not be caught. It will simply succeed and the
  	 * only downside is we lose the event. If this becomes a problem,
  	 * the user should drop to single cpu before tracing.
  	 */
  
  	put_cpu_var(kmmio_ctx);
  	return 1; /* fault handled */
  
  no_kmmio_ctx:
  	put_cpu_var(kmmio_ctx);
  no_kmmio:
  	rcu_read_unlock();
  	preempt_enable_no_resched();
  	return ret;
  }
  
  /*
   * Interrupts are disabled on entry as trap1 is an interrupt gate
   * and they remain disabled throughout this function.
   * This must always get called as the pair to kmmio_handler().
   */
  static int post_kmmio_handler(unsigned long condition, struct pt_regs *regs)
  {
  	int ret = 0;
  	struct kmmio_context *ctx = &get_cpu_var(kmmio_ctx);
  
  	if (!ctx->active) {
  		/*
  		 * debug traps without an active context are due to either
  		 * something external causing them (f.e. using a debugger while
  		 * mmio tracing enabled), or erroneous behaviour
  		 */
  		pr_warning("unexpected debug trap on CPU %d.
  ",
  			   smp_processor_id());
  		goto out;
  	}
  
  	if (ctx->probe && ctx->probe->post_handler)
  		ctx->probe->post_handler(ctx->probe, condition, regs);
  
  	/* Prevent racing against release_kmmio_fault_page(). */
  	spin_lock(&kmmio_lock);
  	if (ctx->fpage->count)
  		arm_kmmio_fault_page(ctx->fpage);
  	spin_unlock(&kmmio_lock);
  
  	regs->flags &= ~X86_EFLAGS_TF;
  	regs->flags |= ctx->saved_flags;
  
  	/* These were acquired in kmmio_handler(). */
  	ctx->active--;
  	BUG_ON(ctx->active);
  	rcu_read_unlock();
  	preempt_enable_no_resched();
  
  	/*
  	 * if somebody else is singlestepping across a probe point, flags
  	 * will have TF set, in which case, continue the remaining processing
  	 * of do_debug, as if this is not a probe hit.
  	 */
  	if (!(regs->flags & X86_EFLAGS_TF))
  		ret = 1;
  out:
  	put_cpu_var(kmmio_ctx);
  	return ret;
  }
  
  /* You must be holding kmmio_lock. */
  static int add_kmmio_fault_page(unsigned long page)
  {
  	struct kmmio_fault_page *f;
  
  	page &= PAGE_MASK;
  	f = get_kmmio_fault_page(page);
  	if (f) {
  		if (!f->count)
  			arm_kmmio_fault_page(f);
  		f->count++;
  		return 0;
  	}
  
  	f = kzalloc(sizeof(*f), GFP_ATOMIC);
  	if (!f)
  		return -1;
  
  	f->count = 1;
  	f->page = page;
  
  	if (arm_kmmio_fault_page(f)) {
  		kfree(f);
  		return -1;
  	}
  
  	list_add_rcu(&f->list, kmmio_page_list(f->page));
  
  	return 0;
  }
  
  /* You must be holding kmmio_lock. */
  static void release_kmmio_fault_page(unsigned long page,
  				struct kmmio_fault_page **release_list)
  {
  	struct kmmio_fault_page *f;
  
  	page &= PAGE_MASK;
  	f = get_kmmio_fault_page(page);
  	if (!f)
  		return;
  
  	f->count--;
  	BUG_ON(f->count < 0);
  	if (!f->count) {
  		disarm_kmmio_fault_page(f);
  		if (!f->scheduled_for_release) {
  			f->release_next = *release_list;
  			*release_list = f;
  			f->scheduled_for_release = true;
  		}
  	}
  }
  
  /*
   * With page-unaligned ioremaps, one or two armed pages may contain
   * addresses from outside the intended mapping. Events for these addresses
   * are currently silently dropped. The events may result only from programming
   * mistakes by accessing addresses before the beginning or past the end of a
   * mapping.
   */
  int register_kmmio_probe(struct kmmio_probe *p)
  {
  	unsigned long flags;
  	int ret = 0;
  	unsigned long size = 0;
  	const unsigned long size_lim = p->len + (p->addr & ~PAGE_MASK);
  
  	spin_lock_irqsave(&kmmio_lock, flags);
  	if (get_kmmio_probe(p->addr)) {
  		ret = -EEXIST;
  		goto out;
  	}
  	kmmio_count++;
  	list_add_rcu(&p->list, &kmmio_probes);
  	while (size < size_lim) {
  		if (add_kmmio_fault_page(p->addr + size))
  			pr_err("Unable to set page fault.
  ");
  		size += PAGE_SIZE;
  	}
  out:
  	spin_unlock_irqrestore(&kmmio_lock, flags);
  	/*
  	 * XXX: What should I do here?
  	 * Here was a call to global_flush_tlb(), but it does not exist
  	 * anymore. It seems it's not needed after all.
  	 */
  	return ret;
  }
  EXPORT_SYMBOL(register_kmmio_probe);
  
  static void rcu_free_kmmio_fault_pages(struct rcu_head *head)
  {
  	struct kmmio_delayed_release *dr = container_of(
  						head,
  						struct kmmio_delayed_release,
  						rcu);
  	struct kmmio_fault_page *f = dr->release_list;
  	while (f) {
  		struct kmmio_fault_page *next = f->release_next;
  		BUG_ON(f->count);
  		kfree(f);
  		f = next;
  	}
  	kfree(dr);
  }
  
  static void remove_kmmio_fault_pages(struct rcu_head *head)
  {
  	struct kmmio_delayed_release *dr =
  		container_of(head, struct kmmio_delayed_release, rcu);
  	struct kmmio_fault_page *f = dr->release_list;
  	struct kmmio_fault_page **prevp = &dr->release_list;
  	unsigned long flags;
  
  	spin_lock_irqsave(&kmmio_lock, flags);
  	while (f) {
  		if (!f->count) {
  			list_del_rcu(&f->list);
  			prevp = &f->release_next;
  		} else {
  			*prevp = f->release_next;
  			f->release_next = NULL;
  			f->scheduled_for_release = false;
  		}
  		f = *prevp;
  	}
  	spin_unlock_irqrestore(&kmmio_lock, flags);
  
  	/* This is the real RCU destroy call. */
  	call_rcu(&dr->rcu, rcu_free_kmmio_fault_pages);
  }
  
  /*
   * Remove a kmmio probe. You have to synchronize_rcu() before you can be
   * sure that the callbacks will not be called anymore. Only after that
   * you may actually release your struct kmmio_probe.
   *
   * Unregistering a kmmio fault page has three steps:
   * 1. release_kmmio_fault_page()
   *    Disarm the page, wait a grace period to let all faults finish.
   * 2. remove_kmmio_fault_pages()
   *    Remove the pages from kmmio_page_table.
   * 3. rcu_free_kmmio_fault_pages()
   *    Actually free the kmmio_fault_page structs as with RCU.
   */
  void unregister_kmmio_probe(struct kmmio_probe *p)
  {
  	unsigned long flags;
  	unsigned long size = 0;
  	const unsigned long size_lim = p->len + (p->addr & ~PAGE_MASK);
  	struct kmmio_fault_page *release_list = NULL;
  	struct kmmio_delayed_release *drelease;
  
  	spin_lock_irqsave(&kmmio_lock, flags);
  	while (size < size_lim) {
  		release_kmmio_fault_page(p->addr + size, &release_list);
  		size += PAGE_SIZE;
  	}
  	list_del_rcu(&p->list);
  	kmmio_count--;
  	spin_unlock_irqrestore(&kmmio_lock, flags);
  
  	if (!release_list)
  		return;
  
  	drelease = kmalloc(sizeof(*drelease), GFP_ATOMIC);
  	if (!drelease) {
  		pr_crit("leaking kmmio_fault_page objects.
  ");
  		return;
  	}
  	drelease->release_list = release_list;
  
  	/*
  	 * This is not really RCU here. We have just disarmed a set of
  	 * pages so that they cannot trigger page faults anymore. However,
  	 * we cannot remove the pages from kmmio_page_table,
  	 * because a probe hit might be in flight on another CPU. The
  	 * pages are collected into a list, and they will be removed from
  	 * kmmio_page_table when it is certain that no probe hit related to
  	 * these pages can be in flight. RCU grace period sounds like a
  	 * good choice.
  	 *
  	 * If we removed the pages too early, kmmio page fault handler might
  	 * not find the respective kmmio_fault_page and determine it's not
  	 * a kmmio fault, when it actually is. This would lead to madness.
  	 */
  	call_rcu(&drelease->rcu, remove_kmmio_fault_pages);
  }
  EXPORT_SYMBOL(unregister_kmmio_probe);
  
  static int
  kmmio_die_notifier(struct notifier_block *nb, unsigned long val, void *args)
  {
  	struct die_args *arg = args;
  	unsigned long* dr6_p = (unsigned long *)ERR_PTR(arg->err);
  
  	if (val == DIE_DEBUG && (*dr6_p & DR_STEP))
  		if (post_kmmio_handler(*dr6_p, arg->regs) == 1) {
  			/*
  			 * Reset the BS bit in dr6 (pointed by args->err) to
  			 * denote completion of processing
  			 */
  			*dr6_p &= ~DR_STEP;
  			return NOTIFY_STOP;
  		}
  
  	return NOTIFY_DONE;
  }
  
  static struct notifier_block nb_die = {
  	.notifier_call = kmmio_die_notifier
  };
  
  int kmmio_init(void)
  {
  	int i;
  
  	for (i = 0; i < KMMIO_PAGE_TABLE_SIZE; i++)
  		INIT_LIST_HEAD(&kmmio_page_table[i]);
  
  	return register_die_notifier(&nb_die);
  }
  
  void kmmio_cleanup(void)
  {
  	int i;
  
  	unregister_die_notifier(&nb_die);
  	for (i = 0; i < KMMIO_PAGE_TABLE_SIZE; i++) {
  		WARN_ONCE(!list_empty(&kmmio_page_table[i]),
  			KERN_ERR "kmmio_page_table not empty at cleanup, any further tracing will leak memory.
  ");
  	}
  }