Linux 4.14.124

[linux/fpc-iii.git] / ipc / util.c

// SPDX-License-Identifier: GPL-2.0
/*
 * 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>
 *
 * General sysv ipc locking scheme:
 *      rcu_read_lock()
 *          obtain the ipc object (kern_ipc_perm) by looking up the id in an idr
 *          tree.
 *          - perform initial checks (capabilities, auditing and permission,
 *            etc).
 *          - perform read-only operations, such as STAT, INFO commands.
 *            acquire the ipc lock (kern_ipc_perm.lock) through
 *            ipc_lock_object()
 *              - perform data updates, such as SET, RMID commands and
 *                mechanism-specific operations (semop/semtimedop,
 *                msgsnd/msgrcv, shmat/shmdt).
 *          drop the ipc lock, through ipc_unlock_object().
 *      rcu_read_unlock()
 *
 *  The ids->rwsem must be taken when:
 *      - creating, removing and iterating the existing entries in ipc
 *        identifier sets.
 *      - iterating through files under /proc/sysvipc/
 *
 *  Note that sems have a special fast path that avoids kern_ipc_perm.lock -
 *  see sem_lock().
 */

#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/notifier.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 *);
};

/**
 * ipc_init - initialise ipc subsystem
 *
 * The various sysv 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)
{
        int err_sem, err_msg;

        err_sem = sem_init();
        WARN(err_sem, "ipc: sysv sem_init failed: %d\n", err_sem);
        err_msg = msg_init();
        WARN(err_msg, "ipc: sysv msg_init failed: %d\n", err_msg);
        shm_init();

        return err_msg ? err_msg : err_sem;
}
device_initcall(ipc_init);

static const struct rhashtable_params ipc_kht_params = {
        .head_offset            = offsetof(struct kern_ipc_perm, khtnode),
        .key_offset             = offsetof(struct kern_ipc_perm, key),
        .key_len                = FIELD_SIZEOF(struct kern_ipc_perm, key),
        .locks_mul              = 1,
        .automatic_shrinking    = true,
};

/**
 * ipc_init_ids - initialise ipc identifiers
 * @ids: ipc identifier set
 *
 * Set up the sequence range to use for the ipc identifier range (limited
 * below IPCMNI) then initialise the keys hashtable and ids idr.
 */
int ipc_init_ids(struct ipc_ids *ids)
{
        int err;
        ids->in_use = 0;
        ids->seq = 0;
        ids->next_id = -1;
        init_rwsem(&ids->rwsem);
        err = rhashtable_init(&ids->key_ht, &ipc_kht_params);
        if (err)
                return err;
        idr_init(&ids->ipcs_idr);
        ids->tables_initialized = true;
        return 0;
}

#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 = proc_create_data(path,
                               S_IRUGO,        /* world readable */
                               NULL,           /* parent dir */
                               &sysvipc_proc_fops,
                               iface);
        if (!pde)
                kfree(iface);
}
#endif

/**
 * ipc_findkey  - find a key in an ipc identifier set
 * @ids: ipc identifier set
 * @key: key to find
 *
 * Returns the locked pointer to the ipc structure if found or NULL
 * otherwise. If key is found ipc points to the owning ipc structure
 *
 * Called with writer ipc_ids.rwsem held.
 */
static struct kern_ipc_perm *ipc_findkey(struct ipc_ids *ids, key_t key)
{
        struct kern_ipc_perm *ipcp = NULL;

        if (likely(ids->tables_initialized))
                ipcp = rhashtable_lookup_fast(&ids->key_ht, &key,
                                              ipc_kht_params);

        if (ipcp) {
                rcu_read_lock();
                ipc_lock_object(ipcp);
                return ipcp;
        }

        return NULL;
}

/**
 * ipc_get_maxid - get the last assigned id
 * @ids: ipc identifier set
 *
 * Called with ipc_ids.rwsem 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 writer ipc_ids.rwsem held.
 */
int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int size)
{
        kuid_t euid;
        kgid_t egid;
        int id, err;
        int next_id = ids->next_id;

        if (size > IPCMNI)
                size = IPCMNI;

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

        idr_preload(GFP_KERNEL);

        refcount_set(&new->refcount, 1);
        spin_lock_init(&new->lock);
        new->deleted = false;
        rcu_read_lock();
        spin_lock(&new->lock);

        current_euid_egid(&euid, &egid);
        new->cuid = new->uid = euid;
        new->gid = new->cgid = egid;

        id = idr_alloc(&ids->ipcs_idr, new,
                       (next_id < 0) ? 0 : ipcid_to_idx(next_id), 0,
                       GFP_NOWAIT);
        idr_preload_end();

        if (id >= 0 && new->key != IPC_PRIVATE) {
                err = rhashtable_insert_fast(&ids->key_ht, &new->khtnode,
                                             ipc_kht_params);
                if (err < 0) {
                        idr_remove(&ids->ipcs_idr, id);
                        id = err;
                }
        }
        if (id < 0) {
                spin_unlock(&new->lock);
                rcu_read_unlock();
                return id;
        }

        ids->in_use++;

        if (next_id < 0) {
                new->seq = ids->seq++;
                if (ids->seq > IPCID_SEQ_MAX)
                        ids->seq = 0;
        } else {
                new->seq = ipcid_to_seqx(next_id);
                ids->next_id = -1;
        }

        new->id = ipc_buildid(id, new->seq);
        return id;
}

/**
 * ipcget_new - create a new ipc object
 * @ns: ipc namespace
 * @ids: ipc identifier 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,
                const struct ipc_ops *ops, struct ipc_params *params)
{
        int err;

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

/**
 * ipc_check_perms - check security and permissions for an ipc object
 * @ns: ipc namespace
 * @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
 * ds IDR.
 *
 * On success, the ipc id is returned.
 *
 * It is called with ipc_ids.rwsem and ipcp->lock held.
 */
static int ipc_check_perms(struct ipc_namespace *ns,
                           struct kern_ipc_perm *ipcp,
                           const struct ipc_ops *ops,
                           struct ipc_params *params)
{
        int err;

        if (ipcperms(ns, 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: ipc namespace
 * @ids: ipc identifier 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,
                const struct ipc_ops *ops, struct ipc_params *params)
{
        struct kern_ipc_perm *ipcp;
        int flg = params->flg;
        int err;

        /*
         * Take the lock as a writer since we are potentially going to add
         * a new entry + read locks are not "upgradable"
         */
        down_write(&ids->rwsem);
        ipcp = ipc_findkey(ids, params->key);
        if (ipcp == NULL) {
                /* key not used */
                if (!(flg & IPC_CREAT))
                        err = -ENOENT;
                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(ns, ipcp, ops, params);
                }
                ipc_unlock(ipcp);
        }
        up_write(&ids->rwsem);

        return err;
}

/**
 * ipc_kht_remove - remove an ipc from the key hashtable
 * @ids: ipc identifier set
 * @ipcp: ipc perm structure containing the key to remove
 *
 * ipc_ids.rwsem (as a writer) and the spinlock for this ID are held
 * before this function is called, and remain locked on the exit.
 */
static void ipc_kht_remove(struct ipc_ids *ids, struct kern_ipc_perm *ipcp)
{
        if (ipcp->key != IPC_PRIVATE)
                rhashtable_remove_fast(&ids->key_ht, &ipcp->khtnode,
                                       ipc_kht_params);
}

/**
 * ipc_rmid - remove an ipc identifier
 * @ids: ipc identifier set
 * @ipcp: ipc perm structure containing the identifier to remove
 *
 * ipc_ids.rwsem (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);
        ipc_kht_remove(ids, ipcp);
        ids->in_use--;
        ipcp->deleted = true;
}

/**
 * ipc_set_key_private - switch the key of an existing ipc to IPC_PRIVATE
 * @ids: ipc identifier set
 * @ipcp: ipc perm structure containing the key to modify
 *
 * ipc_ids.rwsem (as a writer) and the spinlock for this ID are held
 * before this function is called, and remain locked on the exit.
 */
void ipc_set_key_private(struct ipc_ids *ids, struct kern_ipc_perm *ipcp)
{
        ipc_kht_remove(ids, ipcp);
        ipcp->key = IPC_PRIVATE;
}

int ipc_rcu_getref(struct kern_ipc_perm *ptr)
{
        return refcount_inc_not_zero(&ptr->refcount);
}

void ipc_rcu_putref(struct kern_ipc_perm *ptr,
                        void (*func)(struct rcu_head *head))
{
        if (!refcount_dec_and_test(&ptr->refcount))
                return;

        call_rcu(&ptr->rcu, func);
}

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

        audit_ipc_obj(ipcp);
        requested_mode = (flag >> 6) | (flag >> 3) | flag;
        granted_mode = ipcp->mode;
        if (uid_eq(euid, ipcp->cuid) ||
            uid_eq(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) &&
            !ns_capable(ns->user_ns, 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        = from_kuid_munged(current_user_ns(), in->uid);
        out->gid        = from_kgid_munged(current_user_ns(), in->gid);
        out->cuid       = from_kuid_munged(current_user_ns(), in->cuid);
        out->cgid       = from_kgid_munged(current_user_ns(), 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_obtain_object_idr
 * @ids: ipc identifier set
 * @id: ipc id to look for
 *
 * Look for an id in the ipc ids idr and return associated ipc object.
 *
 * Call inside the RCU critical section.
 * The ipc object is *not* locked on exit.
 */
struct kern_ipc_perm *ipc_obtain_object_idr(struct ipc_ids *ids, int id)
{
        struct kern_ipc_perm *out;
        int lid = ipcid_to_idx(id);

        if (unlikely(!ids->tables_initialized))
                return ERR_PTR(-EINVAL);

        out = idr_find(&ids->ipcs_idr, lid);
        if (!out)
                return ERR_PTR(-EINVAL);

        return out;
}

/**
 * ipc_lock - lock an ipc structure without rwsem 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 successful exit.
 */
struct kern_ipc_perm *ipc_lock(struct ipc_ids *ids, int id)
{
        struct kern_ipc_perm *out;

        rcu_read_lock();
        out = ipc_obtain_object_idr(ids, id);
        if (IS_ERR(out))
                goto err;

        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.
         * Upon races with RMID, return -EIDRM, thus indicating that
         * the ID points to a removed identifier.
         */
        if (ipc_valid_object(out))
                return out;

        spin_unlock(&out->lock);
        out = ERR_PTR(-EIDRM);
err:
        rcu_read_unlock();
        return out;
}

/**
 * ipc_obtain_object_check
 * @ids: ipc identifier set
 * @id: ipc id to look for
 *
 * Similar to ipc_obtain_object_idr() but also checks
 * the ipc object reference counter.
 *
 * Call inside the RCU critical section.
 * The ipc object is *not* locked on exit.
 */
struct kern_ipc_perm *ipc_obtain_object_check(struct ipc_ids *ids, int id)
{
        struct kern_ipc_perm *out = ipc_obtain_object_idr(ids, id);

        if (IS_ERR(out))
                goto out;

        if (ipc_checkid(out, id))
                return ERR_PTR(-EINVAL);
out:
        return out;
}

/**
 * ipcget - Common sys_*get() code
 * @ns: namespace
 * @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,
                        const 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);
}

/**
 * ipc_update_perm - update the permissions of an ipc object
 * @in:  the permission given as input.
 * @out: the permission of the ipc to set.
 */
int ipc_update_perm(struct ipc64_perm *in, struct kern_ipc_perm *out)
{
        kuid_t uid = make_kuid(current_user_ns(), in->uid);
        kgid_t gid = make_kgid(current_user_ns(), in->gid);
        if (!uid_valid(uid) || !gid_valid(gid))
                return -EINVAL;

        out->uid = uid;
        out->gid = gid;
        out->mode = (out->mode & ~S_IRWXUGO)
                | (in->mode & S_IRWXUGO);

        return 0;
}

/**
 * ipcctl_pre_down_nolock - retrieve an ipc and check permissions for some IPC_XXX cmd
 * @ns:  ipc namespace
 * @ids:  the table of ids where to look for the ipc
 * @id:   the id of the ipc to retrieve
 * @cmd:  the cmd to check
 * @perm: the permission to set
 * @extra_perm: one extra permission parameter used by msq
 *
 * This function does some common audit and permissions check for some IPC_XXX
 * cmd and is called from semctl_down, shmctl_down and msgctl_down.
 * It must be called without any lock held and:
 *
 *   - retrieves the ipc with the given id in the given table.
 *   - performs some audit and permission check, depending on the given cmd
 *   - returns a pointer to the ipc object or otherwise, the corresponding
 *     error.
 *
 * Call holding the both the rwsem and the rcu read lock.
 */
struct kern_ipc_perm *ipcctl_pre_down_nolock(struct ipc_namespace *ns,
                                        struct ipc_ids *ids, int id, int cmd,
                                        struct ipc64_perm *perm, int extra_perm)
{
        kuid_t euid;
        int err = -EPERM;
        struct kern_ipc_perm *ipcp;

        ipcp = ipc_obtain_object_check(ids, id);
        if (IS_ERR(ipcp)) {
                err = PTR_ERR(ipcp);
                goto err;
        }

        audit_ipc_obj(ipcp);
        if (cmd == IPC_SET)
                audit_ipc_set_perm(extra_perm, perm->uid,
                                   perm->gid, perm->mode);

        euid = current_euid();
        if (uid_eq(euid, ipcp->cuid) || uid_eq(euid, ipcp->uid)  ||
            ns_capable(ns->user_ns, CAP_SYS_ADMIN))
                return ipcp; /* successful lookup */
err:
        return ERR_PTR(err);
}

#ifdef CONFIG_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 /* CONFIG_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;
                        rcu_read_lock();
                        ipc_lock_object(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->rwsem);

        /* 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->rwsem);
}

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) {
                seq_puts(s, iface->header);
                return 0;
        }

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

static const 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)
{
        struct ipc_proc_iter *iter;

        iter = __seq_open_private(file, &sysvipc_proc_seqops, sizeof(*iter));
        if (!iter)
                return -ENOMEM;

        iter->iface = PDE_DATA(inode);
        iter->ns    = get_ipc_ns(current->nsproxy->ipc_ns);

        return 0;
}

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 */