uprobes/core: Make background page replacement logic account for rss_stat counters

[cascardo/linux.git] / kernel / events / uprobes.c

/*
 * User-space Probes (UProbes)
 *
 * 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) IBM Corporation, 2008-2012
 * Authors:
 *      Srikar Dronamraju
 *      Jim Keniston
 * Copyright (C) 2011-2012 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
 */

#include <linux/kernel.h>
#include <linux/highmem.h>
#include <linux/pagemap.h>      /* read_mapping_page */
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/rmap.h>         /* anon_vma_prepare */
#include <linux/mmu_notifier.h> /* set_pte_at_notify */
#include <linux/swap.h>         /* try_to_free_swap */
#include <linux/ptrace.h>       /* user_enable_single_step */
#include <linux/kdebug.h>       /* notifier mechanism */

#include <linux/uprobes.h>

#define UINSNS_PER_PAGE                 (PAGE_SIZE/UPROBE_XOL_SLOT_BYTES)
#define MAX_UPROBE_XOL_SLOTS            UINSNS_PER_PAGE

static struct srcu_struct uprobes_srcu;
static struct rb_root uprobes_tree = RB_ROOT;

static DEFINE_SPINLOCK(uprobes_treelock);       /* serialize rbtree access */

#define UPROBES_HASH_SZ 13

/* serialize (un)register */
static struct mutex uprobes_mutex[UPROBES_HASH_SZ];

#define uprobes_hash(v)         (&uprobes_mutex[((unsigned long)(v)) % UPROBES_HASH_SZ])

/* serialize uprobe->pending_list */
static struct mutex uprobes_mmap_mutex[UPROBES_HASH_SZ];
#define uprobes_mmap_hash(v)    (&uprobes_mmap_mutex[((unsigned long)(v)) % UPROBES_HASH_SZ])

/*
 * uprobe_events allows us to skip the uprobe_mmap if there are no uprobe
 * events active at this time.  Probably a fine grained per inode count is
 * better?
 */
static atomic_t uprobe_events = ATOMIC_INIT(0);

/*
 * Maintain a temporary per vma info that can be used to search if a vma
 * has already been handled. This structure is introduced since extending
 * vm_area_struct wasnt recommended.
 */
struct vma_info {
        struct list_head        probe_list;
        struct mm_struct        *mm;
        loff_t                  vaddr;
};

struct uprobe {
        struct rb_node          rb_node;        /* node in the rb tree */
        atomic_t                ref;
        struct rw_semaphore     consumer_rwsem;
        struct list_head        pending_list;
        struct uprobe_consumer  *consumers;
        struct inode            *inode;         /* Also hold a ref to inode */
        loff_t                  offset;
        int                     flags;
        struct arch_uprobe      arch;
};

/*
 * valid_vma: Verify if the specified vma is an executable vma
 * Relax restrictions while unregistering: vm_flags might have
 * changed after breakpoint was inserted.
 *      - is_register: indicates if we are in register context.
 *      - Return 1 if the specified virtual address is in an
 *        executable vma.
 */
static bool valid_vma(struct vm_area_struct *vma, bool is_register)
{
        if (!vma->vm_file)
                return false;

        if (!is_register)
                return true;

        if ((vma->vm_flags & (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)) == (VM_READ|VM_EXEC))
                return true;

        return false;
}

static loff_t vma_address(struct vm_area_struct *vma, loff_t offset)
{
        loff_t vaddr;

        vaddr = vma->vm_start + offset;
        vaddr -= vma->vm_pgoff << PAGE_SHIFT;

        return vaddr;
}

/**
 * __replace_page - replace page in vma by new page.
 * based on replace_page in mm/ksm.c
 *
 * @vma:      vma that holds the pte pointing to page
 * @page:     the cowed page we are replacing by kpage
 * @kpage:    the modified page we replace page by
 *
 * Returns 0 on success, -EFAULT on failure.
 */
static int __replace_page(struct vm_area_struct *vma, struct page *page, struct page *kpage)
{
        struct mm_struct *mm = vma->vm_mm;
        pgd_t *pgd;
        pud_t *pud;
        pmd_t *pmd;
        pte_t *ptep;
        spinlock_t *ptl;
        unsigned long addr;
        int err = -EFAULT;

        addr = page_address_in_vma(page, vma);
        if (addr == -EFAULT)
                goto out;

        pgd = pgd_offset(mm, addr);
        if (!pgd_present(*pgd))
                goto out;

        pud = pud_offset(pgd, addr);
        if (!pud_present(*pud))
                goto out;

        pmd = pmd_offset(pud, addr);
        if (!pmd_present(*pmd))
                goto out;

        ptep = pte_offset_map_lock(mm, pmd, addr, &ptl);
        if (!ptep)
                goto out;

        get_page(kpage);
        page_add_new_anon_rmap(kpage, vma, addr);

        if (!PageAnon(page)) {
                dec_mm_counter(mm, MM_FILEPAGES);
                inc_mm_counter(mm, MM_ANONPAGES);
        }

        flush_cache_page(vma, addr, pte_pfn(*ptep));
        ptep_clear_flush(vma, addr, ptep);
        set_pte_at_notify(mm, addr, ptep, mk_pte(kpage, vma->vm_page_prot));

        page_remove_rmap(page);
        if (!page_mapped(page))
                try_to_free_swap(page);
        put_page(page);
        pte_unmap_unlock(ptep, ptl);
        err = 0;

out:
        return err;
}

/**
 * is_swbp_insn - check if instruction is breakpoint instruction.
 * @insn: instruction to be checked.
 * Default implementation of is_swbp_insn
 * Returns true if @insn is a breakpoint instruction.
 */
bool __weak is_swbp_insn(uprobe_opcode_t *insn)
{
        return *insn == UPROBE_SWBP_INSN;
}

/*
 * NOTE:
 * Expect the breakpoint instruction to be the smallest size instruction for
 * the architecture. If an arch has variable length instruction and the
 * breakpoint instruction is not of the smallest length instruction
 * supported by that architecture then we need to modify read_opcode /
 * write_opcode accordingly. This would never be a problem for archs that
 * have fixed length instructions.
 */

/*
 * write_opcode - write the opcode at a given virtual address.
 * @auprobe: arch breakpointing information.
 * @mm: the probed process address space.
 * @vaddr: the virtual address to store the opcode.
 * @opcode: opcode to be written at @vaddr.
 *
 * Called with mm->mmap_sem held (for read and with a reference to
 * mm).
 *
 * For mm @mm, write the opcode at @vaddr.
 * Return 0 (success) or a negative errno.
 */
static int write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
                        unsigned long vaddr, uprobe_opcode_t opcode)
{
        struct page *old_page, *new_page;
        struct address_space *mapping;
        void *vaddr_old, *vaddr_new;
        struct vm_area_struct *vma;
        struct uprobe *uprobe;
        loff_t addr;
        int ret;

        /* Read the page with vaddr into memory */
        ret = get_user_pages(NULL, mm, vaddr, 1, 0, 0, &old_page, &vma);
        if (ret <= 0)
                return ret;

        ret = -EINVAL;

        /*
         * We are interested in text pages only. Our pages of interest
         * should be mapped for read and execute only. We desist from
         * adding probes in write mapped pages since the breakpoints
         * might end up in the file copy.
         */
        if (!valid_vma(vma, is_swbp_insn(&opcode)))
                goto put_out;

        uprobe = container_of(auprobe, struct uprobe, arch);
        mapping = uprobe->inode->i_mapping;
        if (mapping != vma->vm_file->f_mapping)
                goto put_out;

        addr = vma_address(vma, uprobe->offset);
        if (vaddr != (unsigned long)addr)
                goto put_out;

        ret = -ENOMEM;
        new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, vaddr);
        if (!new_page)
                goto put_out;

        __SetPageUptodate(new_page);

        /*
         * lock page will serialize against do_wp_page()'s
         * PageAnon() handling
         */
        lock_page(old_page);
        /* copy the page now that we've got it stable */
        vaddr_old = kmap_atomic(old_page);
        vaddr_new = kmap_atomic(new_page);

        memcpy(vaddr_new, vaddr_old, PAGE_SIZE);

        /* poke the new insn in, ASSUMES we don't cross page boundary */
        vaddr &= ~PAGE_MASK;
        BUG_ON(vaddr + UPROBE_SWBP_INSN_SIZE > PAGE_SIZE);
        memcpy(vaddr_new + vaddr, &opcode, UPROBE_SWBP_INSN_SIZE);

        kunmap_atomic(vaddr_new);
        kunmap_atomic(vaddr_old);

        ret = anon_vma_prepare(vma);
        if (ret)
                goto unlock_out;

        lock_page(new_page);
        ret = __replace_page(vma, old_page, new_page);
        unlock_page(new_page);

unlock_out:
        unlock_page(old_page);
        page_cache_release(new_page);

put_out:
        put_page(old_page);

        return ret;
}

/**
 * read_opcode - read the opcode at a given virtual address.
 * @mm: the probed process address space.
 * @vaddr: the virtual address to read the opcode.
 * @opcode: location to store the read opcode.
 *
 * Called with mm->mmap_sem held (for read and with a reference to
 * mm.
 *
 * For mm @mm, read the opcode at @vaddr and store it in @opcode.
 * Return 0 (success) or a negative errno.
 */
static int read_opcode(struct mm_struct *mm, unsigned long vaddr, uprobe_opcode_t *opcode)
{
        struct page *page;
        void *vaddr_new;
        int ret;

        ret = get_user_pages(NULL, mm, vaddr, 1, 0, 0, &page, NULL);
        if (ret <= 0)
                return ret;

        lock_page(page);
        vaddr_new = kmap_atomic(page);
        vaddr &= ~PAGE_MASK;
        memcpy(opcode, vaddr_new + vaddr, UPROBE_SWBP_INSN_SIZE);
        kunmap_atomic(vaddr_new);
        unlock_page(page);

        put_page(page);

        return 0;
}

static int is_swbp_at_addr(struct mm_struct *mm, unsigned long vaddr)
{
        uprobe_opcode_t opcode;
        int result;

        result = read_opcode(mm, vaddr, &opcode);
        if (result)
                return result;

        if (is_swbp_insn(&opcode))
                return 1;

        return 0;
}

/**
 * set_swbp - store breakpoint at a given address.
 * @auprobe: arch specific probepoint information.
 * @mm: the probed process address space.
 * @vaddr: the virtual address to insert the opcode.
 *
 * For mm @mm, store the breakpoint instruction at @vaddr.
 * Return 0 (success) or a negative errno.
 */
int __weak set_swbp(struct arch_uprobe *auprobe, struct mm_struct *mm, unsigned long vaddr)
{
        int result;

        result = is_swbp_at_addr(mm, vaddr);
        if (result == 1)
                return -EEXIST;

        if (result)
                return result;

        return write_opcode(auprobe, mm, vaddr, UPROBE_SWBP_INSN);
}

/**
 * set_orig_insn - Restore the original instruction.
 * @mm: the probed process address space.
 * @auprobe: arch specific probepoint information.
 * @vaddr: the virtual address to insert the opcode.
 * @verify: if true, verify existance of breakpoint instruction.
 *
 * For mm @mm, restore the original opcode (opcode) at @vaddr.
 * Return 0 (success) or a negative errno.
 */
int __weak
set_orig_insn(struct arch_uprobe *auprobe, struct mm_struct *mm, unsigned long vaddr, bool verify)
{
        if (verify) {
                int result;

                result = is_swbp_at_addr(mm, vaddr);
                if (!result)
                        return -EINVAL;

                if (result != 1)
                        return result;
        }
        return write_opcode(auprobe, mm, vaddr, *(uprobe_opcode_t *)auprobe->insn);
}

static int match_uprobe(struct uprobe *l, struct uprobe *r)
{
        if (l->inode < r->inode)
                return -1;

        if (l->inode > r->inode)
                return 1;

        if (l->offset < r->offset)
                return -1;

        if (l->offset > r->offset)
                return 1;

        return 0;
}

static struct uprobe *__find_uprobe(struct inode *inode, loff_t offset)
{
        struct uprobe u = { .inode = inode, .offset = offset };
        struct rb_node *n = uprobes_tree.rb_node;
        struct uprobe *uprobe;
        int match;

        while (n) {
                uprobe = rb_entry(n, struct uprobe, rb_node);
                match = match_uprobe(&u, uprobe);
                if (!match) {
                        atomic_inc(&uprobe->ref);
                        return uprobe;
                }

                if (match < 0)
                        n = n->rb_left;
                else
                        n = n->rb_right;
        }
        return NULL;
}

/*
 * Find a uprobe corresponding to a given inode:offset
 * Acquires uprobes_treelock
 */
static struct uprobe *find_uprobe(struct inode *inode, loff_t offset)
{
        struct uprobe *uprobe;
        unsigned long flags;

        spin_lock_irqsave(&uprobes_treelock, flags);
        uprobe = __find_uprobe(inode, offset);
        spin_unlock_irqrestore(&uprobes_treelock, flags);

        return uprobe;
}

static struct uprobe *__insert_uprobe(struct uprobe *uprobe)
{
        struct rb_node **p = &uprobes_tree.rb_node;
        struct rb_node *parent = NULL;
        struct uprobe *u;
        int match;

        while (*p) {
                parent = *p;
                u = rb_entry(parent, struct uprobe, rb_node);
                match = match_uprobe(uprobe, u);
                if (!match) {
                        atomic_inc(&u->ref);
                        return u;
                }

                if (match < 0)
                        p = &parent->rb_left;
                else
                        p = &parent->rb_right;

        }

        u = NULL;
        rb_link_node(&uprobe->rb_node, parent, p);
        rb_insert_color(&uprobe->rb_node, &uprobes_tree);
        /* get access + creation ref */
        atomic_set(&uprobe->ref, 2);

        return u;
}

/*
 * Acquire uprobes_treelock.
 * Matching uprobe already exists in rbtree;
 *      increment (access refcount) and return the matching uprobe.
 *
 * No matching uprobe; insert the uprobe in rb_tree;
 *      get a double refcount (access + creation) and return NULL.
 */
static struct uprobe *insert_uprobe(struct uprobe *uprobe)
{
        unsigned long flags;
        struct uprobe *u;

        spin_lock_irqsave(&uprobes_treelock, flags);
        u = __insert_uprobe(uprobe);
        spin_unlock_irqrestore(&uprobes_treelock, flags);

        /* For now assume that the instruction need not be single-stepped */
        uprobe->flags |= UPROBE_SKIP_SSTEP;

        return u;
}

static void put_uprobe(struct uprobe *uprobe)
{
        if (atomic_dec_and_test(&uprobe->ref))
                kfree(uprobe);
}

static struct uprobe *alloc_uprobe(struct inode *inode, loff_t offset)
{
        struct uprobe *uprobe, *cur_uprobe;

        uprobe = kzalloc(sizeof(struct uprobe), GFP_KERNEL);
        if (!uprobe)
                return NULL;

        uprobe->inode = igrab(inode);
        uprobe->offset = offset;
        init_rwsem(&uprobe->consumer_rwsem);
        INIT_LIST_HEAD(&uprobe->pending_list);

        /* add to uprobes_tree, sorted on inode:offset */
        cur_uprobe = insert_uprobe(uprobe);

        /* a uprobe exists for this inode:offset combination */
        if (cur_uprobe) {
                kfree(uprobe);
                uprobe = cur_uprobe;
                iput(inode);
        } else {
                atomic_inc(&uprobe_events);
        }

        return uprobe;
}

static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
{
        struct uprobe_consumer *uc;

        if (!(uprobe->flags & UPROBE_RUN_HANDLER))
                return;

        down_read(&uprobe->consumer_rwsem);
        for (uc = uprobe->consumers; uc; uc = uc->next) {
                if (!uc->filter || uc->filter(uc, current))
                        uc->handler(uc, regs);
        }
        up_read(&uprobe->consumer_rwsem);
}

/* Returns the previous consumer */
static struct uprobe_consumer *
consumer_add(struct uprobe *uprobe, struct uprobe_consumer *uc)
{
        down_write(&uprobe->consumer_rwsem);
        uc->next = uprobe->consumers;
        uprobe->consumers = uc;
        up_write(&uprobe->consumer_rwsem);

        return uc->next;
}

/*
 * For uprobe @uprobe, delete the consumer @uc.
 * Return true if the @uc is deleted successfully
 * or return false.
 */
static bool consumer_del(struct uprobe *uprobe, struct uprobe_consumer *uc)
{
        struct uprobe_consumer **con;
        bool ret = false;

        down_write(&uprobe->consumer_rwsem);
        for (con = &uprobe->consumers; *con; con = &(*con)->next) {
                if (*con == uc) {
                        *con = uc->next;
                        ret = true;
                        break;
                }
        }
        up_write(&uprobe->consumer_rwsem);

        return ret;
}

static int
__copy_insn(struct address_space *mapping, struct vm_area_struct *vma, char *insn,
                        unsigned long nbytes, unsigned long offset)
{
        struct file *filp = vma->vm_file;
        struct page *page;
        void *vaddr;
        unsigned long off1;
        unsigned long idx;

        if (!filp)
                return -EINVAL;

        idx = (unsigned long)(offset >> PAGE_CACHE_SHIFT);
        off1 = offset &= ~PAGE_MASK;

        /*
         * Ensure that the page that has the original instruction is
         * populated and in page-cache.
         */
        page = read_mapping_page(mapping, idx, filp);
        if (IS_ERR(page))
                return PTR_ERR(page);

        vaddr = kmap_atomic(page);
        memcpy(insn, vaddr + off1, nbytes);
        kunmap_atomic(vaddr);
        page_cache_release(page);

        return 0;
}

static int
copy_insn(struct uprobe *uprobe, struct vm_area_struct *vma, unsigned long addr)
{
        struct address_space *mapping;
        unsigned long nbytes;
        int bytes;

        addr &= ~PAGE_MASK;
        nbytes = PAGE_SIZE - addr;
        mapping = uprobe->inode->i_mapping;

        /* Instruction at end of binary; copy only available bytes */
        if (uprobe->offset + MAX_UINSN_BYTES > uprobe->inode->i_size)
                bytes = uprobe->inode->i_size - uprobe->offset;
        else
                bytes = MAX_UINSN_BYTES;

        /* Instruction at the page-boundary; copy bytes in second page */
        if (nbytes < bytes) {
                if (__copy_insn(mapping, vma, uprobe->arch.insn + nbytes,
                                bytes - nbytes, uprobe->offset + nbytes))
                        return -ENOMEM;

                bytes = nbytes;
        }
        return __copy_insn(mapping, vma, uprobe->arch.insn, bytes, uprobe->offset);
}

/*
 * How mm->uprobes_state.count gets updated
 * uprobe_mmap() increments the count if
 *      - it successfully adds a breakpoint.
 *      - it cannot add a breakpoint, but sees that there is a underlying
 *        breakpoint (via a is_swbp_at_addr()).
 *
 * uprobe_munmap() decrements the count if
 *      - it sees a underlying breakpoint, (via is_swbp_at_addr)
 *        (Subsequent uprobe_unregister wouldnt find the breakpoint
 *        unless a uprobe_mmap kicks in, since the old vma would be
 *        dropped just after uprobe_munmap.)
 *
 * uprobe_register increments the count if:
 *      - it successfully adds a breakpoint.
 *
 * uprobe_unregister decrements the count if:
 *      - it sees a underlying breakpoint and removes successfully.
 *        (via is_swbp_at_addr)
 *        (Subsequent uprobe_munmap wouldnt find the breakpoint
 *        since there is no underlying breakpoint after the
 *        breakpoint removal.)
 */
static int
install_breakpoint(struct uprobe *uprobe, struct mm_struct *mm,
                        struct vm_area_struct *vma, loff_t vaddr)
{
        unsigned long addr;
        int ret;

        /*
         * If probe is being deleted, unregister thread could be done with
         * the vma-rmap-walk through. Adding a probe now can be fatal since
         * nobody will be able to cleanup. Also we could be from fork or
         * mremap path, where the probe might have already been inserted.
         * Hence behave as if probe already existed.
         */
        if (!uprobe->consumers)
                return -EEXIST;

        addr = (unsigned long)vaddr;

        if (!(uprobe->flags & UPROBE_COPY_INSN)) {
                ret = copy_insn(uprobe, vma, addr);
                if (ret)
                        return ret;

                if (is_swbp_insn((uprobe_opcode_t *)uprobe->arch.insn))
                        return -EEXIST;

                ret = arch_uprobe_analyze_insn(&uprobe->arch, mm);
                if (ret)
                        return ret;

                uprobe->flags |= UPROBE_COPY_INSN;
        }

        /*
         * Ideally, should be updating the probe count after the breakpoint
         * has been successfully inserted. However a thread could hit the
         * breakpoint we just inserted even before the probe count is
         * incremented. If this is the first breakpoint placed, breakpoint
         * notifier might ignore uprobes and pass the trap to the thread.
         * Hence increment before and decrement on failure.
         */
        atomic_inc(&mm->uprobes_state.count);
        ret = set_swbp(&uprobe->arch, mm, addr);
        if (ret)
                atomic_dec(&mm->uprobes_state.count);

        return ret;
}

static void
remove_breakpoint(struct uprobe *uprobe, struct mm_struct *mm, loff_t vaddr)
{
        if (!set_orig_insn(&uprobe->arch, mm, (unsigned long)vaddr, true))
                atomic_dec(&mm->uprobes_state.count);
}

/*
 * There could be threads that have hit the breakpoint and are entering the
 * notifier code and trying to acquire the uprobes_treelock. The thread
 * calling delete_uprobe() that is removing the uprobe from the rb_tree can
 * race with these threads and might acquire the uprobes_treelock compared
 * to some of the breakpoint hit threads. In such a case, the breakpoint
 * hit threads will not find the uprobe. The current unregistering thread
 * waits till all other threads have hit a breakpoint, to acquire the
 * uprobes_treelock before the uprobe is removed from the rbtree.
 */
static void delete_uprobe(struct uprobe *uprobe)
{
        unsigned long flags;

        synchronize_srcu(&uprobes_srcu);
        spin_lock_irqsave(&uprobes_treelock, flags);
        rb_erase(&uprobe->rb_node, &uprobes_tree);
        spin_unlock_irqrestore(&uprobes_treelock, flags);
        iput(uprobe->inode);
        put_uprobe(uprobe);
        atomic_dec(&uprobe_events);
}

static struct vma_info *
__find_next_vma_info(struct address_space *mapping, struct list_head *head,
                        struct vma_info *vi, loff_t offset, bool is_register)
{
        struct prio_tree_iter iter;
        struct vm_area_struct *vma;
        struct vma_info *tmpvi;
        unsigned long pgoff;
        int existing_vma;
        loff_t vaddr;

        pgoff = offset >> PAGE_SHIFT;

        vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
                if (!valid_vma(vma, is_register))
                        continue;

                existing_vma = 0;
                vaddr = vma_address(vma, offset);

                list_for_each_entry(tmpvi, head, probe_list) {
                        if (tmpvi->mm == vma->vm_mm && tmpvi->vaddr == vaddr) {
                                existing_vma = 1;
                                break;
                        }
                }

                /*
                 * Another vma needs a probe to be installed. However skip
                 * installing the probe if the vma is about to be unlinked.
                 */
                if (!existing_vma && atomic_inc_not_zero(&vma->vm_mm->mm_users)) {
                        vi->mm = vma->vm_mm;
                        vi->vaddr = vaddr;
                        list_add(&vi->probe_list, head);

                        return vi;
                }
        }

        return NULL;
}

/*
 * Iterate in the rmap prio tree  and find a vma where a probe has not
 * yet been inserted.
 */
static struct vma_info *
find_next_vma_info(struct address_space *mapping, struct list_head *head,
                loff_t offset, bool is_register)
{
        struct vma_info *vi, *retvi;

        vi = kzalloc(sizeof(struct vma_info), GFP_KERNEL);
        if (!vi)
                return ERR_PTR(-ENOMEM);

        mutex_lock(&mapping->i_mmap_mutex);
        retvi = __find_next_vma_info(mapping, head, vi, offset, is_register);
        mutex_unlock(&mapping->i_mmap_mutex);

        if (!retvi)
                kfree(vi);

        return retvi;
}

static int register_for_each_vma(struct uprobe *uprobe, bool is_register)
{
        struct list_head try_list;
        struct vm_area_struct *vma;
        struct address_space *mapping;
        struct vma_info *vi, *tmpvi;
        struct mm_struct *mm;
        loff_t vaddr;
        int ret;

        mapping = uprobe->inode->i_mapping;
        INIT_LIST_HEAD(&try_list);

        ret = 0;

        for (;;) {
                vi = find_next_vma_info(mapping, &try_list, uprobe->offset, is_register);
                if (!vi)
                        break;

                if (IS_ERR(vi)) {
                        ret = PTR_ERR(vi);
                        break;
                }

                mm = vi->mm;
                down_read(&mm->mmap_sem);
                vma = find_vma(mm, (unsigned long)vi->vaddr);
                if (!vma || !valid_vma(vma, is_register)) {
                        list_del(&vi->probe_list);
                        kfree(vi);
                        up_read(&mm->mmap_sem);
                        mmput(mm);
                        continue;
                }
                vaddr = vma_address(vma, uprobe->offset);
                if (vma->vm_file->f_mapping->host != uprobe->inode ||
                                                vaddr != vi->vaddr) {
                        list_del(&vi->probe_list);
                        kfree(vi);
                        up_read(&mm->mmap_sem);
                        mmput(mm);
                        continue;
                }

                if (is_register)
                        ret = install_breakpoint(uprobe, mm, vma, vi->vaddr);
                else
                        remove_breakpoint(uprobe, mm, vi->vaddr);

                up_read(&mm->mmap_sem);
                mmput(mm);
                if (is_register) {
                        if (ret && ret == -EEXIST)
                                ret = 0;
                        if (ret)
                                break;
                }
        }

        list_for_each_entry_safe(vi, tmpvi, &try_list, probe_list) {
                list_del(&vi->probe_list);
                kfree(vi);
        }

        return ret;
}

static int __uprobe_register(struct uprobe *uprobe)
{
        return register_for_each_vma(uprobe, true);
}

static void __uprobe_unregister(struct uprobe *uprobe)
{
        if (!register_for_each_vma(uprobe, false))
                delete_uprobe(uprobe);

        /* TODO : cant unregister? schedule a worker thread */
}

/*
 * uprobe_register - register a probe
 * @inode: the file in which the probe has to be placed.
 * @offset: offset from the start of the file.
 * @uc: information on howto handle the probe..
 *
 * Apart from the access refcount, uprobe_register() takes a creation
 * refcount (thro alloc_uprobe) if and only if this @uprobe is getting
 * inserted into the rbtree (i.e first consumer for a @inode:@offset
 * tuple).  Creation refcount stops uprobe_unregister from freeing the
 * @uprobe even before the register operation is complete. Creation
 * refcount is released when the last @uc for the @uprobe
 * unregisters.
 *
 * Return errno if it cannot successully install probes
 * else return 0 (success)
 */
int uprobe_register(struct inode *inode, loff_t offset, struct uprobe_consumer *uc)
{
        struct uprobe *uprobe;
        int ret;

        if (!inode || !uc || uc->next)
                return -EINVAL;

        if (offset > i_size_read(inode))
                return -EINVAL;

        ret = 0;
        mutex_lock(uprobes_hash(inode));
        uprobe = alloc_uprobe(inode, offset);

        if (uprobe && !consumer_add(uprobe, uc)) {
                ret = __uprobe_register(uprobe);
                if (ret) {
                        uprobe->consumers = NULL;
                        __uprobe_unregister(uprobe);
                } else {
                        uprobe->flags |= UPROBE_RUN_HANDLER;
                }
        }

        mutex_unlock(uprobes_hash(inode));
        put_uprobe(uprobe);

        return ret;
}

/*
 * uprobe_unregister - unregister a already registered probe.
 * @inode: the file in which the probe has to be removed.
 * @offset: offset from the start of the file.
 * @uc: identify which probe if multiple probes are colocated.
 */
void uprobe_unregister(struct inode *inode, loff_t offset, struct uprobe_consumer *uc)
{
        struct uprobe *uprobe;

        if (!inode || !uc)
                return;

        uprobe = find_uprobe(inode, offset);
        if (!uprobe)
                return;

        mutex_lock(uprobes_hash(inode));

        if (consumer_del(uprobe, uc)) {
                if (!uprobe->consumers) {
                        __uprobe_unregister(uprobe);
                        uprobe->flags &= ~UPROBE_RUN_HANDLER;
                }
        }

        mutex_unlock(uprobes_hash(inode));
        if (uprobe)
                put_uprobe(uprobe);
}

/*
 * Of all the nodes that correspond to the given inode, return the node
 * with the least offset.
 */
static struct rb_node *find_least_offset_node(struct inode *inode)
{
        struct uprobe u = { .inode = inode, .offset = 0};
        struct rb_node *n = uprobes_tree.rb_node;
        struct rb_node *close_node = NULL;
        struct uprobe *uprobe;
        int match;

        while (n) {
                uprobe = rb_entry(n, struct uprobe, rb_node);
                match = match_uprobe(&u, uprobe);

                if (uprobe->inode == inode)
                        close_node = n;

                if (!match)
                        return close_node;

                if (match < 0)
                        n = n->rb_left;
                else
                        n = n->rb_right;
        }

        return close_node;
}

/*
 * For a given inode, build a list of probes that need to be inserted.
 */
static void build_probe_list(struct inode *inode, struct list_head *head)
{
        struct uprobe *uprobe;
        unsigned long flags;
        struct rb_node *n;

        spin_lock_irqsave(&uprobes_treelock, flags);

        n = find_least_offset_node(inode);

        for (; n; n = rb_next(n)) {
                uprobe = rb_entry(n, struct uprobe, rb_node);
                if (uprobe->inode != inode)
                        break;

                list_add(&uprobe->pending_list, head);
                atomic_inc(&uprobe->ref);
        }

        spin_unlock_irqrestore(&uprobes_treelock, flags);
}

/*
 * Called from mmap_region.
 * called with mm->mmap_sem acquired.
 *
 * Return -ve no if we fail to insert probes and we cannot
 * bail-out.
 * Return 0 otherwise. i.e:
 *
 *      - successful insertion of probes
 *      - (or) no possible probes to be inserted.
 *      - (or) insertion of probes failed but we can bail-out.
 */
int uprobe_mmap(struct vm_area_struct *vma)
{
        struct list_head tmp_list;
        struct uprobe *uprobe, *u;
        struct inode *inode;
        int ret, count;

        if (!atomic_read(&uprobe_events) || !valid_vma(vma, true))
                return 0;

        inode = vma->vm_file->f_mapping->host;
        if (!inode)
                return 0;

        INIT_LIST_HEAD(&tmp_list);
        mutex_lock(uprobes_mmap_hash(inode));
        build_probe_list(inode, &tmp_list);

        ret = 0;
        count = 0;

        list_for_each_entry_safe(uprobe, u, &tmp_list, pending_list) {
                loff_t vaddr;

                list_del(&uprobe->pending_list);
                if (!ret) {
                        vaddr = vma_address(vma, uprobe->offset);

                        if (vaddr < vma->vm_start || vaddr >= vma->vm_end) {
                                put_uprobe(uprobe);
                                continue;
                        }

                        ret = install_breakpoint(uprobe, vma->vm_mm, vma, vaddr);

                        /* Ignore double add: */
                        if (ret == -EEXIST) {
                                ret = 0;

                                if (!is_swbp_at_addr(vma->vm_mm, vaddr))
                                        continue;

                                /*
                                 * Unable to insert a breakpoint, but
                                 * breakpoint lies underneath. Increment the
                                 * probe count.
                                 */
                                atomic_inc(&vma->vm_mm->uprobes_state.count);
                        }

                        if (!ret)
                                count++;
                }
                put_uprobe(uprobe);
        }

        mutex_unlock(uprobes_mmap_hash(inode));

        if (ret)
                atomic_sub(count, &vma->vm_mm->uprobes_state.count);

        return ret;
}

/*
 * Called in context of a munmap of a vma.
 */
void uprobe_munmap(struct vm_area_struct *vma)
{
        struct list_head tmp_list;
        struct uprobe *uprobe, *u;
        struct inode *inode;

        if (!atomic_read(&uprobe_events) || !valid_vma(vma, false))
                return;

        if (!atomic_read(&vma->vm_mm->uprobes_state.count))
                return;

        inode = vma->vm_file->f_mapping->host;
        if (!inode)
                return;

        INIT_LIST_HEAD(&tmp_list);
        mutex_lock(uprobes_mmap_hash(inode));
        build_probe_list(inode, &tmp_list);

        list_for_each_entry_safe(uprobe, u, &tmp_list, pending_list) {
                loff_t vaddr;

                list_del(&uprobe->pending_list);
                vaddr = vma_address(vma, uprobe->offset);

                if (vaddr >= vma->vm_start && vaddr < vma->vm_end) {
                        /*
                         * An unregister could have removed the probe before
                         * unmap. So check before we decrement the count.
                         */
                        if (is_swbp_at_addr(vma->vm_mm, vaddr) == 1)
                                atomic_dec(&vma->vm_mm->uprobes_state.count);
                }
                put_uprobe(uprobe);
        }
        mutex_unlock(uprobes_mmap_hash(inode));
}

/* Slot allocation for XOL */
static int xol_add_vma(struct xol_area *area)
{
        struct mm_struct *mm;
        int ret;

        area->page = alloc_page(GFP_HIGHUSER);
        if (!area->page)
                return -ENOMEM;

        ret = -EALREADY;
        mm = current->mm;

        down_write(&mm->mmap_sem);
        if (mm->uprobes_state.xol_area)
                goto fail;

        ret = -ENOMEM;

        /* Try to map as high as possible, this is only a hint. */
        area->vaddr = get_unmapped_area(NULL, TASK_SIZE - PAGE_SIZE, PAGE_SIZE, 0, 0);
        if (area->vaddr & ~PAGE_MASK) {
                ret = area->vaddr;
                goto fail;
        }

        ret = install_special_mapping(mm, area->vaddr, PAGE_SIZE,
                                VM_EXEC|VM_MAYEXEC|VM_DONTCOPY|VM_IO, &area->page);
        if (ret)
                goto fail;

        smp_wmb();      /* pairs with get_xol_area() */
        mm->uprobes_state.xol_area = area;
        ret = 0;

fail:
        up_write(&mm->mmap_sem);
        if (ret)
                __free_page(area->page);

        return ret;
}

static struct xol_area *get_xol_area(struct mm_struct *mm)
{
        struct xol_area *area;

        area = mm->uprobes_state.xol_area;
        smp_read_barrier_depends();     /* pairs with wmb in xol_add_vma() */

        return area;
}

/*
 * xol_alloc_area - Allocate process's xol_area.
 * This area will be used for storing instructions for execution out of
 * line.
 *
 * Returns the allocated area or NULL.
 */
static struct xol_area *xol_alloc_area(void)
{
        struct xol_area *area;

        area = kzalloc(sizeof(*area), GFP_KERNEL);
        if (unlikely(!area))
                return NULL;

        area->bitmap = kzalloc(BITS_TO_LONGS(UINSNS_PER_PAGE) * sizeof(long), GFP_KERNEL);

        if (!area->bitmap)
                goto fail;

        init_waitqueue_head(&area->wq);
        if (!xol_add_vma(area))
                return area;

fail:
        kfree(area->bitmap);
        kfree(area);

        return get_xol_area(current->mm);
}

/*
 * uprobe_clear_state - Free the area allocated for slots.
 */
void uprobe_clear_state(struct mm_struct *mm)
{
        struct xol_area *area = mm->uprobes_state.xol_area;

        if (!area)
                return;

        put_page(area->page);
        kfree(area->bitmap);
        kfree(area);
}

/*
 * uprobe_reset_state - Free the area allocated for slots.
 */
void uprobe_reset_state(struct mm_struct *mm)
{
        mm->uprobes_state.xol_area = NULL;
        atomic_set(&mm->uprobes_state.count, 0);
}

/*
 *  - search for a free slot.
 */
static unsigned long xol_take_insn_slot(struct xol_area *area)
{
        unsigned long slot_addr;
        int slot_nr;

        do {
                slot_nr = find_first_zero_bit(area->bitmap, UINSNS_PER_PAGE);
                if (slot_nr < UINSNS_PER_PAGE) {
                        if (!test_and_set_bit(slot_nr, area->bitmap))
                                break;

                        slot_nr = UINSNS_PER_PAGE;
                        continue;
                }
                wait_event(area->wq, (atomic_read(&area->slot_count) < UINSNS_PER_PAGE));
        } while (slot_nr >= UINSNS_PER_PAGE);

        slot_addr = area->vaddr + (slot_nr * UPROBE_XOL_SLOT_BYTES);
        atomic_inc(&area->slot_count);

        return slot_addr;
}

/*
 * xol_get_insn_slot - If was not allocated a slot, then
 * allocate a slot.
 * Returns the allocated slot address or 0.
 */
static unsigned long xol_get_insn_slot(struct uprobe *uprobe, unsigned long slot_addr)
{
        struct xol_area *area;
        unsigned long offset;
        void *vaddr;

        area = get_xol_area(current->mm);
        if (!area) {
                area = xol_alloc_area();
                if (!area)
                        return 0;
        }
        current->utask->xol_vaddr = xol_take_insn_slot(area);

        /*
         * Initialize the slot if xol_vaddr points to valid
         * instruction slot.
         */
        if (unlikely(!current->utask->xol_vaddr))
                return 0;

        current->utask->vaddr = slot_addr;
        offset = current->utask->xol_vaddr & ~PAGE_MASK;
        vaddr = kmap_atomic(area->page);
        memcpy(vaddr + offset, uprobe->arch.insn, MAX_UINSN_BYTES);
        kunmap_atomic(vaddr);

        return current->utask->xol_vaddr;
}

/*
 * xol_free_insn_slot - If slot was earlier allocated by
 * @xol_get_insn_slot(), make the slot available for
 * subsequent requests.
 */
static void xol_free_insn_slot(struct task_struct *tsk)
{
        struct xol_area *area;
        unsigned long vma_end;
        unsigned long slot_addr;

        if (!tsk->mm || !tsk->mm->uprobes_state.xol_area || !tsk->utask)
                return;

        slot_addr = tsk->utask->xol_vaddr;

        if (unlikely(!slot_addr || IS_ERR_VALUE(slot_addr)))
                return;

        area = tsk->mm->uprobes_state.xol_area;
        vma_end = area->vaddr + PAGE_SIZE;
        if (area->vaddr <= slot_addr && slot_addr < vma_end) {
                unsigned long offset;
                int slot_nr;

                offset = slot_addr - area->vaddr;
                slot_nr = offset / UPROBE_XOL_SLOT_BYTES;
                if (slot_nr >= UINSNS_PER_PAGE)
                        return;

                clear_bit(slot_nr, area->bitmap);
                atomic_dec(&area->slot_count);
                if (waitqueue_active(&area->wq))
                        wake_up(&area->wq);

                tsk->utask->xol_vaddr = 0;
        }
}

/**
 * uprobe_get_swbp_addr - compute address of swbp given post-swbp regs
 * @regs: Reflects the saved state of the task after it has hit a breakpoint
 * instruction.
 * Return the address of the breakpoint instruction.
 */
unsigned long __weak uprobe_get_swbp_addr(struct pt_regs *regs)
{
        return instruction_pointer(regs) - UPROBE_SWBP_INSN_SIZE;
}

/*
 * Called with no locks held.
 * Called in context of a exiting or a exec-ing thread.
 */
void uprobe_free_utask(struct task_struct *t)
{
        struct uprobe_task *utask = t->utask;

        if (t->uprobe_srcu_id != -1)
                srcu_read_unlock_raw(&uprobes_srcu, t->uprobe_srcu_id);

        if (!utask)
                return;

        if (utask->active_uprobe)
                put_uprobe(utask->active_uprobe);

        xol_free_insn_slot(t);
        kfree(utask);
        t->utask = NULL;
}

/*
 * Called in context of a new clone/fork from copy_process.
 */
void uprobe_copy_process(struct task_struct *t)
{
        t->utask = NULL;
        t->uprobe_srcu_id = -1;
}

/*
 * Allocate a uprobe_task object for the task.
 * Called when the thread hits a breakpoint for the first time.
 *
 * Returns:
 * - pointer to new uprobe_task on success
 * - NULL otherwise
 */
static struct uprobe_task *add_utask(void)
{
        struct uprobe_task *utask;

        utask = kzalloc(sizeof *utask, GFP_KERNEL);
        if (unlikely(!utask))
                return NULL;

        utask->active_uprobe = NULL;
        current->utask = utask;
        return utask;
}

/* Prepare to single-step probed instruction out of line. */
static int
pre_ssout(struct uprobe *uprobe, struct pt_regs *regs, unsigned long vaddr)
{
        if (xol_get_insn_slot(uprobe, vaddr) && !arch_uprobe_pre_xol(&uprobe->arch, regs))
                return 0;

        return -EFAULT;
}

/*
 * If we are singlestepping, then ensure this thread is not connected to
 * non-fatal signals until completion of singlestep.  When xol insn itself
 * triggers the signal,  restart the original insn even if the task is
 * already SIGKILL'ed (since coredump should report the correct ip).  This
 * is even more important if the task has a handler for SIGSEGV/etc, The
 * _same_ instruction should be repeated again after return from the signal
 * handler, and SSTEP can never finish in this case.
 */
bool uprobe_deny_signal(void)
{
        struct task_struct *t = current;
        struct uprobe_task *utask = t->utask;

        if (likely(!utask || !utask->active_uprobe))
                return false;

        WARN_ON_ONCE(utask->state != UTASK_SSTEP);

        if (signal_pending(t)) {
                spin_lock_irq(&t->sighand->siglock);
                clear_tsk_thread_flag(t, TIF_SIGPENDING);
                spin_unlock_irq(&t->sighand->siglock);

                if (__fatal_signal_pending(t) || arch_uprobe_xol_was_trapped(t)) {
                        utask->state = UTASK_SSTEP_TRAPPED;
                        set_tsk_thread_flag(t, TIF_UPROBE);
                        set_tsk_thread_flag(t, TIF_NOTIFY_RESUME);
                }
        }

        return true;
}

/*
 * Avoid singlestepping the original instruction if the original instruction
 * is a NOP or can be emulated.
 */
static bool can_skip_sstep(struct uprobe *uprobe, struct pt_regs *regs)
{
        if (arch_uprobe_skip_sstep(&uprobe->arch, regs))
                return true;

        uprobe->flags &= ~UPROBE_SKIP_SSTEP;
        return false;
}

/*
 * Run handler and ask thread to singlestep.
 * Ensure all non-fatal signals cannot interrupt thread while it singlesteps.
 */
static void handle_swbp(struct pt_regs *regs)
{
        struct vm_area_struct *vma;
        struct uprobe_task *utask;
        struct uprobe *uprobe;
        struct mm_struct *mm;
        unsigned long bp_vaddr;

        uprobe = NULL;
        bp_vaddr = uprobe_get_swbp_addr(regs);
        mm = current->mm;
        down_read(&mm->mmap_sem);
        vma = find_vma(mm, bp_vaddr);

        if (vma && vma->vm_start <= bp_vaddr && valid_vma(vma, false)) {
                struct inode *inode;
                loff_t offset;

                inode = vma->vm_file->f_mapping->host;
                offset = bp_vaddr - vma->vm_start;
                offset += (vma->vm_pgoff << PAGE_SHIFT);
                uprobe = find_uprobe(inode, offset);
        }

        srcu_read_unlock_raw(&uprobes_srcu, current->uprobe_srcu_id);
        current->uprobe_srcu_id = -1;
        up_read(&mm->mmap_sem);

        if (!uprobe) {
                /* No matching uprobe; signal SIGTRAP. */
                send_sig(SIGTRAP, current, 0);
                return;
        }

        utask = current->utask;
        if (!utask) {
                utask = add_utask();
                /* Cannot allocate; re-execute the instruction. */
                if (!utask)
                        goto cleanup_ret;
        }
        utask->active_uprobe = uprobe;
        handler_chain(uprobe, regs);
        if (uprobe->flags & UPROBE_SKIP_SSTEP && can_skip_sstep(uprobe, regs))
                goto cleanup_ret;

        utask->state = UTASK_SSTEP;
        if (!pre_ssout(uprobe, regs, bp_vaddr)) {
                user_enable_single_step(current);
                return;
        }

cleanup_ret:
        if (utask) {
                utask->active_uprobe = NULL;
                utask->state = UTASK_RUNNING;
        }
        if (uprobe) {
                if (!(uprobe->flags & UPROBE_SKIP_SSTEP))

                        /*
                         * cannot singlestep; cannot skip instruction;
                         * re-execute the instruction.
                         */
                        instruction_pointer_set(regs, bp_vaddr);

                put_uprobe(uprobe);
        }
}

/*
 * Perform required fix-ups and disable singlestep.
 * Allow pending signals to take effect.
 */
static void handle_singlestep(struct uprobe_task *utask, struct pt_regs *regs)
{
        struct uprobe *uprobe;

        uprobe = utask->active_uprobe;
        if (utask->state == UTASK_SSTEP_ACK)
                arch_uprobe_post_xol(&uprobe->arch, regs);
        else if (utask->state == UTASK_SSTEP_TRAPPED)
                arch_uprobe_abort_xol(&uprobe->arch, regs);
        else
                WARN_ON_ONCE(1);

        put_uprobe(uprobe);
        utask->active_uprobe = NULL;
        utask->state = UTASK_RUNNING;
        user_disable_single_step(current);
        xol_free_insn_slot(current);

        spin_lock_irq(&current->sighand->siglock);
        recalc_sigpending(); /* see uprobe_deny_signal() */
        spin_unlock_irq(&current->sighand->siglock);
}

/*
 * On breakpoint hit, breakpoint notifier sets the TIF_UPROBE flag.  (and on
 * subsequent probe hits on the thread sets the state to UTASK_BP_HIT) and
 * allows the thread to return from interrupt.
 *
 * On singlestep exception, singlestep notifier sets the TIF_UPROBE flag and
 * also sets the state to UTASK_SSTEP_ACK and allows the thread to return from
 * interrupt.
 *
 * While returning to userspace, thread notices the TIF_UPROBE flag and calls
 * uprobe_notify_resume().
 */
void uprobe_notify_resume(struct pt_regs *regs)
{
        struct uprobe_task *utask;

        utask = current->utask;
        if (!utask || utask->state == UTASK_BP_HIT)
                handle_swbp(regs);
        else
                handle_singlestep(utask, regs);
}

/*
 * uprobe_pre_sstep_notifier gets called from interrupt context as part of
 * notifier mechanism. Set TIF_UPROBE flag and indicate breakpoint hit.
 */
int uprobe_pre_sstep_notifier(struct pt_regs *regs)
{
        struct uprobe_task *utask;

        if (!current->mm || !atomic_read(&current->mm->uprobes_state.count))
                /* task is currently not uprobed */
                return 0;

        utask = current->utask;
        if (utask)
                utask->state = UTASK_BP_HIT;

        set_thread_flag(TIF_UPROBE);
        current->uprobe_srcu_id = srcu_read_lock_raw(&uprobes_srcu);

        return 1;
}

/*
 * uprobe_post_sstep_notifier gets called in interrupt context as part of notifier
 * mechanism. Set TIF_UPROBE flag and indicate completion of singlestep.
 */
int uprobe_post_sstep_notifier(struct pt_regs *regs)
{
        struct uprobe_task *utask = current->utask;

        if (!current->mm || !utask || !utask->active_uprobe)
                /* task is currently not uprobed */
                return 0;

        utask->state = UTASK_SSTEP_ACK;
        set_thread_flag(TIF_UPROBE);
        return 1;
}

static struct notifier_block uprobe_exception_nb = {
        .notifier_call          = arch_uprobe_exception_notify,
        .priority               = INT_MAX-1,    /* notified after kprobes, kgdb */
};

static int __init init_uprobes(void)
{
        int i;

        for (i = 0; i < UPROBES_HASH_SZ; i++) {
                mutex_init(&uprobes_mutex[i]);
                mutex_init(&uprobes_mmap_mutex[i]);
        }
        init_srcu_struct(&uprobes_srcu);

        return register_die_notifier(&uprobe_exception_nb);
}
module_init(init_uprobes);

static void __exit exit_uprobes(void)
{
}
module_exit(exit_uprobes);