[safe/jmp/linux-2.6] / ipc / util.c

/*
 * linux/ipc/util.c
 * Copyright (C) 1992 Krishna Balasubramanian
 *
 * Sep 1997 - Call suser() last after "normal" permission checks so we
 *            get BSD style process accounting right.
 *            Occurs in several places in the IPC code.
 *            Chris Evans, <chris@ferret.lmh.ox.ac.uk>
 * Nov 1999 - ipc helper functions, unified SMP locking
 *            Manfred Spraul <manfred@colorfullife.com>
 * Oct 2002 - One lock per IPC id. RCU ipc_free for lock-free grow_ary().
 *            Mingming Cao <cmm@us.ibm.com>
 * Mar 2006 - support for audit of ipc object properties
 *            Dustin Kirkland <dustin.kirkland@us.ibm.com>
 * Jun 2006 - namespaces ssupport
 *            OpenVZ, SWsoft Inc.
 *            Pavel Emelianov <xemul@openvz.org>
 */

#include <linux/mm.h>
#include <linux/shm.h>
#include <linux/init.h>
#include <linux/msg.h>
#include <linux/vmalloc.h>
#include <linux/slab.h>
#include <linux/capability.h>
#include <linux/highuid.h>
#include <linux/security.h>
#include <linux/rcupdate.h>
#include <linux/workqueue.h>
#include <linux/seq_file.h>
#include <linux/proc_fs.h>
#include <linux/audit.h>
#include <linux/nsproxy.h>
#include <linux/rwsem.h>
#include <linux/memory.h>
#include <linux/ipc_namespace.h>

#include <asm/unistd.h>

#include "util.h"

struct ipc_proc_iface {
        const char *path;
        const char *header;
        int ids;
        int (*show)(struct seq_file *, void *);
};

struct ipc_namespace init_ipc_ns = {
        .kref = {
                .refcount       = ATOMIC_INIT(2),
        },
};

atomic_t nr_ipc_ns = ATOMIC_INIT(1);


#ifdef CONFIG_MEMORY_HOTPLUG

static int ipc_memory_callback(struct notifier_block *self,
                                unsigned long action, void *arg)
{
        switch (action) {
        case MEM_ONLINE:    /* memory successfully brought online */
        case MEM_OFFLINE:   /* or offline: it's time to recompute msgmni */
                /*
                 * This is done by invoking the ipcns notifier chain with the
                 * IPC_MEMCHANGED event.
                 */
                ipcns_notify(IPCNS_MEMCHANGED);
                break;
        case MEM_GOING_ONLINE:
        case MEM_GOING_OFFLINE:
        case MEM_CANCEL_ONLINE:
        case MEM_CANCEL_OFFLINE:
        default:
                break;
        }

        return NOTIFY_OK;
}

#endif /* CONFIG_MEMORY_HOTPLUG */

/**
 *      ipc_init        -       initialise IPC subsystem
 *
 *      The various system5 IPC resources (semaphores, messages and shared
 *      memory) are initialised
 *      A callback routine is registered into the memory hotplug notifier
 *      chain: since msgmni scales to lowmem this callback routine will be
 *      called upon successful memory add / remove to recompute msmgni.
 */
 
static int __init ipc_init(void)
{
        sem_init();
        msg_init();
        shm_init();
        hotplug_memory_notifier(ipc_memory_callback, IPC_CALLBACK_PRI);
        register_ipcns_notifier(&init_ipc_ns);
        return 0;
}
__initcall(ipc_init);

/**
 *      ipc_init_ids            -       initialise IPC identifiers
 *      @ids: Identifier set
 *
 *      Set up the sequence range to use for the ipc identifier range (limited
 *      below IPCMNI) then initialise the ids idr.
 */
 
void ipc_init_ids(struct ipc_ids *ids)
{
        init_rwsem(&ids->rw_mutex);

        ids->in_use = 0;
        ids->seq = 0;
        {
                int seq_limit = INT_MAX/SEQ_MULTIPLIER;
                if(seq_limit > USHRT_MAX)
                        ids->seq_max = USHRT_MAX;
                 else
                        ids->seq_max = seq_limit;
        }

        idr_init(&ids->ipcs_idr);
}

#ifdef CONFIG_PROC_FS
static const struct file_operations sysvipc_proc_fops;
/**
 *      ipc_init_proc_interface -  Create a proc interface for sysipc types using a seq_file interface.
 *      @path: Path in procfs
 *      @header: Banner to be printed at the beginning of the file.
 *      @ids: ipc id table to iterate.
 *      @show: show routine.
 */
void __init ipc_init_proc_interface(const char *path, const char *header,
                int ids, int (*show)(struct seq_file *, void *))
{
        struct proc_dir_entry *pde;
        struct ipc_proc_iface *iface;

        iface = kmalloc(sizeof(*iface), GFP_KERNEL);
        if (!iface)
                return;
        iface->path     = path;
        iface->header   = header;
        iface->ids      = ids;
        iface->show     = show;

        pde = create_proc_entry(path,
                                S_IRUGO,        /* world readable */
                                NULL            /* parent dir */);
        if (pde) {
                pde->data = iface;
                pde->proc_fops = &sysvipc_proc_fops;
        } else {
                kfree(iface);
        }
}
#endif

/**
 *      ipc_findkey     -       find a key in an ipc identifier set     
 *      @ids: Identifier set
 *      @key: The key to find
 *      
 *      Requires ipc_ids.rw_mutex locked.
 *      Returns the LOCKED pointer to the ipc structure if found or NULL
 *      if not.
 *      If key is found ipc points to the owning ipc structure
 */
 
static struct kern_ipc_perm *ipc_findkey(struct ipc_ids *ids, key_t key)
{
        struct kern_ipc_perm *ipc;
        int next_id;
        int total;

        for (total = 0, next_id = 0; total < ids->in_use; next_id++) {
                ipc = idr_find(&ids->ipcs_idr, next_id);

                if (ipc == NULL)
                        continue;

                if (ipc->key != key) {
                        total++;
                        continue;
                }

                ipc_lock_by_ptr(ipc);
                return ipc;
        }

        return NULL;
}

/**
 *      ipc_get_maxid   -       get the last assigned id
 *      @ids: IPC identifier set
 *
 *      Called with ipc_ids.rw_mutex held.
 */

int ipc_get_maxid(struct ipc_ids *ids)
{
        struct kern_ipc_perm *ipc;
        int max_id = -1;
        int total, id;

        if (ids->in_use == 0)
                return -1;

        if (ids->in_use == IPCMNI)
                return IPCMNI - 1;

        /* Look for the last assigned id */
        total = 0;
        for (id = 0; id < IPCMNI && total < ids->in_use; id++) {
                ipc = idr_find(&ids->ipcs_idr, id);
                if (ipc != NULL) {
                        max_id = id;
                        total++;
                }
        }
        return max_id;
}

/**
 *      ipc_addid       -       add an IPC identifier
 *      @ids: IPC identifier set
 *      @new: new IPC permission set
 *      @size: limit for the number of used ids
 *
 *      Add an entry 'new' to the IPC ids idr. The permissions object is
 *      initialised and the first free entry is set up and the id assigned
 *      is returned. The 'new' entry is returned in a locked state on success.
 *      On failure the entry is not locked and a negative err-code is returned.
 *
 *      Called with ipc_ids.rw_mutex held as a writer.
 */
 
int ipc_addid(struct ipc_ids* ids, struct kern_ipc_perm* new, int size)
{
        int id, err;

        if (size > IPCMNI)
                size = IPCMNI;

        if (ids->in_use >= size)
                return -ENOSPC;

        err = idr_get_new(&ids->ipcs_idr, new, &id);
        if (err)
                return err;

        ids->in_use++;

        new->cuid = new->uid = current->euid;
        new->gid = new->cgid = current->egid;

        new->seq = ids->seq++;
        if(ids->seq > ids->seq_max)
                ids->seq = 0;

        new->id = ipc_buildid(id, new->seq);
        spin_lock_init(&new->lock);
        new->deleted = 0;
        rcu_read_lock();
        spin_lock(&new->lock);
        return id;
}

/**
 *      ipcget_new      -       create a new ipc object
 *      @ns: namespace
 *      @ids: IPC identifer set
 *      @ops: the actual creation routine to call
 *      @params: its parameters
 *
 *      This routine is called by sys_msgget, sys_semget() and sys_shmget()
 *      when the key is IPC_PRIVATE.
 */
static int ipcget_new(struct ipc_namespace *ns, struct ipc_ids *ids,
                struct ipc_ops *ops, struct ipc_params *params)
{
        int err;
retry:
        err = idr_pre_get(&ids->ipcs_idr, GFP_KERNEL);

        if (!err)
                return -ENOMEM;

        down_write(&ids->rw_mutex);
        err = ops->getnew(ns, params);
        up_write(&ids->rw_mutex);

        if (err == -EAGAIN)
                goto retry;

        return err;
}

/**
 *      ipc_check_perms -       check security and permissions for an IPC
 *      @ipcp: ipc permission set
 *      @ops: the actual security routine to call
 *      @params: its parameters
 *
 *      This routine is called by sys_msgget(), sys_semget() and sys_shmget()
 *      when the key is not IPC_PRIVATE and that key already exists in the
 *      ids IDR.
 *
 *      On success, the IPC id is returned.
 *
 *      It is called with ipc_ids.rw_mutex and ipcp->lock held.
 */
static int ipc_check_perms(struct kern_ipc_perm *ipcp, struct ipc_ops *ops,
                        struct ipc_params *params)
{
        int err;

        if (ipcperms(ipcp, params->flg))
                err = -EACCES;
        else {
                err = ops->associate(ipcp, params->flg);
                if (!err)
                        err = ipcp->id;
        }

        return err;
}

/**
 *      ipcget_public   -       get an ipc object or create a new one
 *      @ns: namespace
 *      @ids: IPC identifer set
 *      @ops: the actual creation routine to call
 *      @params: its parameters
 *
 *      This routine is called by sys_msgget, sys_semget() and sys_shmget()
 *      when the key is not IPC_PRIVATE.
 *      It adds a new entry if the key is not found and does some permission
 *      / security checkings if the key is found.
 *
 *      On success, the ipc id is returned.
 */
static int ipcget_public(struct ipc_namespace *ns, struct ipc_ids *ids,
                struct ipc_ops *ops, struct ipc_params *params)
{
        struct kern_ipc_perm *ipcp;
        int flg = params->flg;
        int err;
retry:
        err = idr_pre_get(&ids->ipcs_idr, GFP_KERNEL);

        /*
         * Take the lock as a writer since we are potentially going to add
         * a new entry + read locks are not "upgradable"
         */
        down_write(&ids->rw_mutex);
        ipcp = ipc_findkey(ids, params->key);
        if (ipcp == NULL) {
                /* key not used */
                if (!(flg & IPC_CREAT))
                        err = -ENOENT;
                else if (!err)
                        err = -ENOMEM;
                else
                        err = ops->getnew(ns, params);
        } else {
                /* ipc object has been locked by ipc_findkey() */

                if (flg & IPC_CREAT && flg & IPC_EXCL)
                        err = -EEXIST;
                else {
                        err = 0;
                        if (ops->more_checks)
                                err = ops->more_checks(ipcp, params);
                        if (!err)
                                /*
                                 * ipc_check_perms returns the IPC id on
                                 * success
                                 */
                                err = ipc_check_perms(ipcp, ops, params);
                }
                ipc_unlock(ipcp);
        }
        up_write(&ids->rw_mutex);

        if (err == -EAGAIN)
                goto retry;

        return err;
}


/**
 *      ipc_rmid        -       remove an IPC identifier
 *      @ids: IPC identifier set
 *      @ipcp: ipc perm structure containing the identifier to remove
 *
 *      ipc_ids.rw_mutex (as a writer) and the spinlock for this ID are held
 *      before this function is called, and remain locked on the exit.
 */
 
void ipc_rmid(struct ipc_ids *ids, struct kern_ipc_perm *ipcp)
{
        int lid = ipcid_to_idx(ipcp->id);

        idr_remove(&ids->ipcs_idr, lid);

        ids->in_use--;

        ipcp->deleted = 1;

        return;
}

/**
 *      ipc_alloc       -       allocate ipc space
 *      @size: size desired
 *
 *      Allocate memory from the appropriate pools and return a pointer to it.
 *      NULL is returned if the allocation fails
 */
 
void* ipc_alloc(int size)
{
        void* out;
        if(size > PAGE_SIZE)
                out = vmalloc(size);
        else
                out = kmalloc(size, GFP_KERNEL);
        return out;
}

/**
 *      ipc_free        -       free ipc space
 *      @ptr: pointer returned by ipc_alloc
 *      @size: size of block
 *
 *      Free a block created with ipc_alloc(). The caller must know the size
 *      used in the allocation call.
 */

void ipc_free(void* ptr, int size)
{
        if(size > PAGE_SIZE)
                vfree(ptr);
        else
                kfree(ptr);
}

/*
 * rcu allocations:
 * There are three headers that are prepended to the actual allocation:
 * - during use: ipc_rcu_hdr.
 * - during the rcu grace period: ipc_rcu_grace.
 * - [only if vmalloc]: ipc_rcu_sched.
 * Their lifetime doesn't overlap, thus the headers share the same memory.
 * Unlike a normal union, they are right-aligned, thus some container_of
 * forward/backward casting is necessary:
 */
struct ipc_rcu_hdr
{
        int refcount;
        int is_vmalloc;
        void *data[0];
};


struct ipc_rcu_grace
{
        struct rcu_head rcu;
        /* "void *" makes sure alignment of following data is sane. */
        void *data[0];
};

struct ipc_rcu_sched
{
        struct work_struct work;
        /* "void *" makes sure alignment of following data is sane. */
        void *data[0];
};

#define HDRLEN_KMALLOC          (sizeof(struct ipc_rcu_grace) > sizeof(struct ipc_rcu_hdr) ? \
                                        sizeof(struct ipc_rcu_grace) : sizeof(struct ipc_rcu_hdr))
#define HDRLEN_VMALLOC          (sizeof(struct ipc_rcu_sched) > HDRLEN_KMALLOC ? \
                                        sizeof(struct ipc_rcu_sched) : HDRLEN_KMALLOC)

static inline int rcu_use_vmalloc(int size)
{
        /* Too big for a single page? */
        if (HDRLEN_KMALLOC + size > PAGE_SIZE)
                return 1;
        return 0;
}

/**
 *      ipc_rcu_alloc   -       allocate ipc and rcu space 
 *      @size: size desired
 *
 *      Allocate memory for the rcu header structure +  the object.
 *      Returns the pointer to the object.
 *      NULL is returned if the allocation fails. 
 */
 
void* ipc_rcu_alloc(int size)
{
        void* out;
        /* 
         * We prepend the allocation with the rcu struct, and
         * workqueue if necessary (for vmalloc). 
         */
        if (rcu_use_vmalloc(size)) {
                out = vmalloc(HDRLEN_VMALLOC + size);
                if (out) {
                        out += HDRLEN_VMALLOC;
                        container_of(out, struct ipc_rcu_hdr, data)->is_vmalloc = 1;
                        container_of(out, struct ipc_rcu_hdr, data)->refcount = 1;
                }
        } else {
                out = kmalloc(HDRLEN_KMALLOC + size, GFP_KERNEL);
                if (out) {
                        out += HDRLEN_KMALLOC;
                        container_of(out, struct ipc_rcu_hdr, data)->is_vmalloc = 0;
                        container_of(out, struct ipc_rcu_hdr, data)->refcount = 1;
                }
        }

        return out;
}

void ipc_rcu_getref(void *ptr)
{
        container_of(ptr, struct ipc_rcu_hdr, data)->refcount++;
}

static void ipc_do_vfree(struct work_struct *work)
{
        vfree(container_of(work, struct ipc_rcu_sched, work));
}

/**
 * ipc_schedule_free - free ipc + rcu space
 * @head: RCU callback structure for queued work
 * 
 * Since RCU callback function is called in bh,
 * we need to defer the vfree to schedule_work().
 */
static void ipc_schedule_free(struct rcu_head *head)
{
        struct ipc_rcu_grace *grace;
        struct ipc_rcu_sched *sched;

        grace = container_of(head, struct ipc_rcu_grace, rcu);
        sched = container_of(&(grace->data[0]), struct ipc_rcu_sched,
                                data[0]);

        INIT_WORK(&sched->work, ipc_do_vfree);
        schedule_work(&sched->work);
}

/**
 * ipc_immediate_free - free ipc + rcu space
 * @head: RCU callback structure that contains pointer to be freed
 *
 * Free from the RCU callback context.
 */
static void ipc_immediate_free(struct rcu_head *head)
{
        struct ipc_rcu_grace *free =
                container_of(head, struct ipc_rcu_grace, rcu);
        kfree(free);
}

void ipc_rcu_putref(void *ptr)
{
        if (--container_of(ptr, struct ipc_rcu_hdr, data)->refcount > 0)
                return;

        if (container_of(ptr, struct ipc_rcu_hdr, data)->is_vmalloc) {
                call_rcu(&container_of(ptr, struct ipc_rcu_grace, data)->rcu,
                                ipc_schedule_free);
        } else {
                call_rcu(&container_of(ptr, struct ipc_rcu_grace, data)->rcu,
                                ipc_immediate_free);
        }
}

/**
 *      ipcperms        -       check IPC permissions
 *      @ipcp: IPC permission set
 *      @flag: desired permission set.
 *
 *      Check user, group, other permissions for access
 *      to ipc resources. return 0 if allowed
 */
 
int ipcperms (struct kern_ipc_perm *ipcp, short flag)
{       /* flag will most probably be 0 or S_...UGO from <linux/stat.h> */
        int requested_mode, granted_mode, err;

        if (unlikely((err = audit_ipc_obj(ipcp))))
                return err;
        requested_mode = (flag >> 6) | (flag >> 3) | flag;
        granted_mode = ipcp->mode;
        if (current->euid == ipcp->cuid || current->euid == ipcp->uid)
                granted_mode >>= 6;
        else if (in_group_p(ipcp->cgid) || in_group_p(ipcp->gid))
                granted_mode >>= 3;
        /* is there some bit set in requested_mode but not in granted_mode? */
        if ((requested_mode & ~granted_mode & 0007) && 
            !capable(CAP_IPC_OWNER))
                return -1;

        return security_ipc_permission(ipcp, flag);
}

/*
 * Functions to convert between the kern_ipc_perm structure and the
 * old/new ipc_perm structures
 */

/**
 *      kernel_to_ipc64_perm    -       convert kernel ipc permissions to user
 *      @in: kernel permissions
 *      @out: new style IPC permissions
 *
 *      Turn the kernel object @in into a set of permissions descriptions
 *      for returning to userspace (@out).
 */
 

void kernel_to_ipc64_perm (struct kern_ipc_perm *in, struct ipc64_perm *out)
{
        out->key        = in->key;
        out->uid        = in->uid;
        out->gid        = in->gid;
        out->cuid       = in->cuid;
        out->cgid       = in->cgid;
        out->mode       = in->mode;
        out->seq        = in->seq;
}

/**
 *      ipc64_perm_to_ipc_perm  -       convert new ipc permissions to old
 *      @in: new style IPC permissions
 *      @out: old style IPC permissions
 *
 *      Turn the new style permissions object @in into a compatibility
 *      object and store it into the @out pointer.
 */
 
void ipc64_perm_to_ipc_perm (struct ipc64_perm *in, struct ipc_perm *out)
{
        out->key        = in->key;
        SET_UID(out->uid, in->uid);
        SET_GID(out->gid, in->gid);
        SET_UID(out->cuid, in->cuid);
        SET_GID(out->cgid, in->cgid);
        out->mode       = in->mode;
        out->seq        = in->seq;
}

/**
 * ipc_lock - Lock an ipc structure without rw_mutex held
 * @ids: IPC identifier set
 * @id: ipc id to look for
 *
 * Look for an id in the ipc ids idr and lock the associated ipc object.
 *
 * The ipc object is locked on exit.
 *
 * This is the routine that should be called when the rw_mutex is not already
 * held, i.e. idr tree not protected: it protects the idr tree in read mode
 * during the idr_find().
 */

struct kern_ipc_perm *ipc_lock(struct ipc_ids *ids, int id)
{
        struct kern_ipc_perm *out;
        int lid = ipcid_to_idx(id);

        down_read(&ids->rw_mutex);

        rcu_read_lock();
        out = idr_find(&ids->ipcs_idr, lid);
        if (out == NULL) {
                rcu_read_unlock();
                up_read(&ids->rw_mutex);
                return ERR_PTR(-EINVAL);
        }

        up_read(&ids->rw_mutex);

        spin_lock(&out->lock);
        
        /* ipc_rmid() may have already freed the ID while ipc_lock
         * was spinning: here verify that the structure is still valid
         */
        if (out->deleted) {
                spin_unlock(&out->lock);
                rcu_read_unlock();
                return ERR_PTR(-EINVAL);
        }

        return out;
}

/**
 * ipc_lock_down - Lock an ipc structure with rw_sem held
 * @ids: IPC identifier set
 * @id: ipc id to look for
 *
 * Look for an id in the ipc ids idr and lock the associated ipc object.
 *
 * The ipc object is locked on exit.
 *
 * This is the routine that should be called when the rw_mutex is already
 * held, i.e. idr tree protected.
 */

struct kern_ipc_perm *ipc_lock_down(struct ipc_ids *ids, int id)
{
        struct kern_ipc_perm *out;
        int lid = ipcid_to_idx(id);

        rcu_read_lock();
        out = idr_find(&ids->ipcs_idr, lid);
        if (out == NULL) {
                rcu_read_unlock();
                return ERR_PTR(-EINVAL);
        }

        spin_lock(&out->lock);

        /*
         * No need to verify that the structure is still valid since the
         * rw_mutex is held.
         */
        return out;
}

struct kern_ipc_perm *ipc_lock_check_down(struct ipc_ids *ids, int id)
{
        struct kern_ipc_perm *out;

        out = ipc_lock_down(ids, id);
        if (IS_ERR(out))
                return out;

        if (ipc_checkid(out, id)) {
                ipc_unlock(out);
                return ERR_PTR(-EIDRM);
        }

        return out;
}

struct kern_ipc_perm *ipc_lock_check(struct ipc_ids *ids, int id)
{
        struct kern_ipc_perm *out;

        out = ipc_lock(ids, id);
        if (IS_ERR(out))
                return out;

        if (ipc_checkid(out, id)) {
                ipc_unlock(out);
                return ERR_PTR(-EIDRM);
        }

        return out;
}

/**
 * ipcget - Common sys_*get() code
 * @ns : namsepace
 * @ids : IPC identifier set
 * @ops : operations to be called on ipc object creation, permission checks
 *        and further checks
 * @params : the parameters needed by the previous operations.
 *
 * Common routine called by sys_msgget(), sys_semget() and sys_shmget().
 */
int ipcget(struct ipc_namespace *ns, struct ipc_ids *ids,
                        struct ipc_ops *ops, struct ipc_params *params)
{
        if (params->key == IPC_PRIVATE)
                return ipcget_new(ns, ids, ops, params);
        else
                return ipcget_public(ns, ids, ops, params);
}

#ifdef __ARCH_WANT_IPC_PARSE_VERSION


/**
 *      ipc_parse_version       -       IPC call version
 *      @cmd: pointer to command
 *
 *      Return IPC_64 for new style IPC and IPC_OLD for old style IPC. 
 *      The @cmd value is turned from an encoding command and version into
 *      just the command code.
 */
 
int ipc_parse_version (int *cmd)
{
        if (*cmd & IPC_64) {
                *cmd ^= IPC_64;
                return IPC_64;
        } else {
                return IPC_OLD;
        }
}

#endif /* __ARCH_WANT_IPC_PARSE_VERSION */

#ifdef CONFIG_PROC_FS
struct ipc_proc_iter {
        struct ipc_namespace *ns;
        struct ipc_proc_iface *iface;
};

/*
 * This routine locks the ipc structure found at least at position pos.
 */
static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos,
                                              loff_t *new_pos)
{
        struct kern_ipc_perm *ipc;
        int total, id;

        total = 0;
        for (id = 0; id < pos && total < ids->in_use; id++) {
                ipc = idr_find(&ids->ipcs_idr, id);
                if (ipc != NULL)
                        total++;
        }

        if (total >= ids->in_use)
                return NULL;

        for ( ; pos < IPCMNI; pos++) {
                ipc = idr_find(&ids->ipcs_idr, pos);
                if (ipc != NULL) {
                        *new_pos = pos + 1;
                        ipc_lock_by_ptr(ipc);
                        return ipc;
                }
        }

        /* Out of range - return NULL to terminate iteration */
        return NULL;
}

static void *sysvipc_proc_next(struct seq_file *s, void *it, loff_t *pos)
{
        struct ipc_proc_iter *iter = s->private;
        struct ipc_proc_iface *iface = iter->iface;
        struct kern_ipc_perm *ipc = it;

        /* If we had an ipc id locked before, unlock it */
        if (ipc && ipc != SEQ_START_TOKEN)
                ipc_unlock(ipc);

        return sysvipc_find_ipc(&iter->ns->ids[iface->ids], *pos, pos);
}

/*
 * File positions: pos 0 -> header, pos n -> ipc id = n - 1.
 * SeqFile iterator: iterator value locked ipc pointer or SEQ_TOKEN_START.
 */
static void *sysvipc_proc_start(struct seq_file *s, loff_t *pos)
{
        struct ipc_proc_iter *iter = s->private;
        struct ipc_proc_iface *iface = iter->iface;
        struct ipc_ids *ids;

        ids = &iter->ns->ids[iface->ids];

        /*
         * Take the lock - this will be released by the corresponding
         * call to stop().
         */
        down_read(&ids->rw_mutex);

        /* pos < 0 is invalid */
        if (*pos < 0)
                return NULL;

        /* pos == 0 means header */
        if (*pos == 0)
                return SEQ_START_TOKEN;

        /* Find the (pos-1)th ipc */
        return sysvipc_find_ipc(ids, *pos - 1, pos);
}

static void sysvipc_proc_stop(struct seq_file *s, void *it)
{
        struct kern_ipc_perm *ipc = it;
        struct ipc_proc_iter *iter = s->private;
        struct ipc_proc_iface *iface = iter->iface;
        struct ipc_ids *ids;

        /* If we had a locked structure, release it */
        if (ipc && ipc != SEQ_START_TOKEN)
                ipc_unlock(ipc);

        ids = &iter->ns->ids[iface->ids];
        /* Release the lock we took in start() */
        up_read(&ids->rw_mutex);
}

static int sysvipc_proc_show(struct seq_file *s, void *it)
{
        struct ipc_proc_iter *iter = s->private;
        struct ipc_proc_iface *iface = iter->iface;

        if (it == SEQ_START_TOKEN)
                return seq_puts(s, iface->header);

        return iface->show(s, it);
}

static struct seq_operations sysvipc_proc_seqops = {
        .start = sysvipc_proc_start,
        .stop  = sysvipc_proc_stop,
        .next  = sysvipc_proc_next,
        .show  = sysvipc_proc_show,
};

static int sysvipc_proc_open(struct inode *inode, struct file *file)
{
        int ret;
        struct seq_file *seq;
        struct ipc_proc_iter *iter;

        ret = -ENOMEM;
        iter = kmalloc(sizeof(*iter), GFP_KERNEL);
        if (!iter)
                goto out;

        ret = seq_open(file, &sysvipc_proc_seqops);
        if (ret)
                goto out_kfree;

        seq = file->private_data;
        seq->private = iter;

        iter->iface = PDE(inode)->data;
        iter->ns    = get_ipc_ns(current->nsproxy->ipc_ns);
out:
        return ret;
out_kfree:
        kfree(iter);
        goto out;
}

static int sysvipc_proc_release(struct inode *inode, struct file *file)
{
        struct seq_file *seq = file->private_data;
        struct ipc_proc_iter *iter = seq->private;
        put_ipc_ns(iter->ns);
        return seq_release_private(inode, file);
}

static const struct file_operations sysvipc_proc_fops = {
        .open    = sysvipc_proc_open,
        .read    = seq_read,
        .llseek  = seq_lseek,
        .release = sysvipc_proc_release,
};
#endif /* CONFIG_PROC_FS */