ClabureDB: Classified Bug-Reports Database

  /* 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>
   */
  
  #include <linux/list.h>
  #include <linux/rculist.h>
 #include <linux/spinlock.h>
 #include <linux/hash.h>
 #include <linux/init.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 <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 */
 
         /*
          * 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).
          */
         int count;
 };
 
 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 *p;
 
         page &= PAGE_MASK;
        head = kmmio_page_list(page);
        list_for_each_entry_rcu(p, head, list) {
                if (p->page == page)
                        return p;
        }
        return NULL;
}

static void set_page_present(unsigned long addr, bool present,
                                                        unsigned int *pglevel)
{
        pteval_t pteval;
        pmdval_t pmdval;
        unsigned int level;
        pmd_t *pmd;
        pte_t *pte = lookup_address(addr, &level);

        if (!pte) {
                pr_err("kmmio: no pte for page 0x%08lx\n", addr);
                return;
        }

        if (pglevel)
                *pglevel = level;

        switch (level) {
        case PG_LEVEL_2M:
                pmd = (pmd_t *)pte;
                pmdval = pmd_val(*pmd) & ~_PAGE_PRESENT;
                if (present)
                        pmdval |= _PAGE_PRESENT;
                set_pmd(pmd, __pmd(pmdval));
                break;

        case PG_LEVEL_4K:
                pteval = pte_val(*pte) & ~_PAGE_PRESENT;
                if (present)
                        pteval |= _PAGE_PRESENT;
                set_pte_atomic(pte, __pte(pteval));
                break;

        default:
                pr_err("kmmio: unexpected page level 0x%x.\n", level);
                return;
        }

        __flush_tlb_one(addr);
}

/** Mark the given page as not present. Access to it will trigger a fault. */
static void arm_kmmio_fault_page(unsigned long page, unsigned int *pglevel)
{
        set_page_present(page & PAGE_MASK, false, pglevel);
}

/** Mark the given page as present. */
static void disarm_kmmio_fault_page(unsigned long page, unsigned int *pglevel)
{
        set_page_present(page & PAGE_MASK, true, pglevel);
}

/*
 * 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 thorough out 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) {
                disarm_kmmio_fault_page(faultpage->page, NULL);
                if (addr == ctx->addr) {
                        /*
                         * On SMP we sometimes get recursive probe hits on the
                         * same address. Context is already saved, fall out.
                         */
                        pr_debug("kmmio: duplicate probe hit on CPU %d, for "
                                                "address 0x%08lx.\n",
                                                smp_processor_id(), addr);
                        ret = 1;
                        goto no_kmmio_ctx;
                }
                /*
                 * Prevent overwriting already in-flight context.
                 * This should not happen, let's hope disarming at least
                 * prevents a panic.
                 */
                pr_emerg("kmmio: recursive probe hit on CPU %d, "
                                        "for address 0x%08lx. Ignoring.\n",
                                        smp_processor_id(), addr);
                pr_emerg("kmmio: previous hit was at 0x%08lx.\n",
                                        ctx->addr);
                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->page, NULL);

        /*
         * 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 thorough out 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) {
                pr_debug("kmmio: spurious debug trap on CPU %d.\n",
                                                        smp_processor_id());
                goto out;
        }

        if (ctx->probe && ctx->probe->post_handler)
                ctx->probe->post_handler(ctx->probe, condition, regs);

        arm_kmmio_fault_page(ctx->fpage->page, NULL);

        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->page, NULL);
                f->count++;
                return 0;
        }

        f = kmalloc(sizeof(*f), GFP_ATOMIC);
        if (!f)
                return -1;

        f->count = 1;
        f->page = page;
        list_add_rcu(&f->list, kmmio_page_list(f->page));

        arm_kmmio_fault_page(f->page, NULL);

        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->page, NULL);
                f->release_next = *release_list;
                *release_list = f;
        }
}

/*
 * 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("kmmio: Unable to set page fault.\n");
                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 *p = dr->release_list;
        while (p) {
                struct kmmio_fault_page *next = p->release_next;
                BUG_ON(p->count);
                kfree(p);
                p = 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 *p = dr->release_list;
        struct kmmio_fault_page **prevp = &dr->release_list;
        unsigned long flags;
        spin_lock_irqsave(&kmmio_lock, flags);
        while (p) {
                if (!p->count)
                        list_del_rcu(&p->list);
                else
                        *prevp = p->release_next;
                prevp = &p->release_next;
                p = p->release_next;
        }
        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()
 *    Actally 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);

        drelease = kmalloc(sizeof(*drelease), GFP_ATOMIC);
        if (!drelease) {
                pr_crit("kmmio: leaking kmmio_fault_page objects.\n");
                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;

        if (val == DIE_DEBUG && (arg->err & DR_STEP))
                if (post_kmmio_handler(arg->err, arg->regs) == 1)
                        return NOTIFY_STOP;

        return NOTIFY_DONE;
}

static struct notifier_block nb_die = {
        .notifier_call = kmmio_die_notifier
};

static int __init init_kmmio(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);
}
fs_initcall(init_kmmio); /* should be before device_initcall() */