kmmio.c

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/* Support for MMIO probes.
 * Benfit many code from kprobes
 * (C) 2002 Louis Zhuang <[email protected]>.
 *     2007 Alexander Eichner
 *     2008 Pekka Paalanen <[email protected]>
 */

#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/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 <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\n", 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.\n", 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.\n"));
    if (f->armed) {
        pr_warning("double-arm: page 0x%08lx, ref %d, old %d\n",
               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.\n"),
          f->page);
    f->armed = true;
    return ret;
}

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.\n", 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.\n",
                 addr, smp_processor_id());

            if (!faultpage->old_presence)
                pr_info("unexpected secondary hit for address 0x%08lx on CPU %d.\n",
                    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.\n",
                 smp_processor_id(), addr);
            pr_emerg("previous hit was at 0x%08lx.\n", 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.\n",
               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.\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 *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.\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;
    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.\n");
    }
}