sdk_linux/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 INFO command, that
 *          do not demand atomicity
 *          acquire the ipc lock (kern_ipc_perm.lock) through
 *          ipc_lock_object()
 *        - perform read-only operations that demand atomicity,
 *          such as STAT command.
 *        - 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 <linux/rhashtable.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)
{
    proc_mkdir("sysvipc", NULL);
    sem_init();
    msg_init();
    shm_init();

    return 0;
}
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 = sizeof_field(struct kern_ipc_perm, key),
    .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 ipc_mni) then initialise the keys hashtable and ids idr.
 */
void ipc_init_ids(struct ipc_ids *ids)
{
    ids->in_use = 0;
    ids->seq = 0;
    init_rwsem(&ids->rwsem);
    rhashtable_init(&ids->key_ht, &ipc_kht_params);
    idr_init(&ids->ipcs_idr);
    ids->max_idx = -1;
    ids->last_idx = -1;
#ifdef CONFIG_CHECKPOINT_RESTORE
    ids->next_id = -1;
#endif
}

#ifdef CONFIG_PROC_FS
static const struct proc_ops sysvipc_proc_ops;
/**
 * 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_ops, 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;

    ipcp = rhashtable_lookup_fast(&ids->key_ht, &key, ipc_kht_params);
    if (!ipcp) {
        return NULL;
    }

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

/*
 * Insert new IPC object into idr tree, and set sequence number and id
 * in the correct order.
 * Especially:
 * - the sequence number must be set before inserting the object into the idr,
 *   because the sequence number is accessed without a lock.
 * - the id can/must be set after inserting the object into the idr.
 *   All accesses must be done after getting kern_ipc_perm.lock.
 *
 * The caller must own kern_ipc_perm.lock.of the new object.
 * On error, the function returns a (negative) error code.
 *
 * To conserve sequence number space, especially with extended ipc_mni,
 * the sequence number is incremented only when the returned ID is less than
 * the last one.
 */
static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
{
    int idx, next_id = -1;

#ifdef CONFIG_CHECKPOINT_RESTORE
    next_id = ids->next_id;
    ids->next_id = -1;
#endif

    /*
     * As soon as a new object is inserted into the idr,
     * ipc_obtain_object_idr() or ipc_obtain_object_check() can find it,
     * and the lockless preparations for ipc operations can start.
     * This means especially: permission checks, audit calls, allocation
     * of undo structures, ...
     *
     * Thus the object must be fully initialized, and if something fails,
     * then the full tear-down sequence must be followed.
     * (i.e.: set new->deleted, reduce refcount, call_rcu())
     */

    if (next_id < 0) { /* !CHECKPOINT_RESTORE or next_id is unset */
        int max_idx;

        max_idx = max(ids->in_use * 0x3 / 0x2, ipc_min_cycle);
        max_idx = min(max_idx, ipc_mni);

        /* allocate the idx, with a NULL struct kern_ipc_perm */
        idx = idr_alloc_cyclic(&ids->ipcs_idr, NULL, 0, max_idx, GFP_NOWAIT);
        if (idx >= 0) {
            /*
             * idx got allocated successfully.
             * Now calculate the sequence number and set the
             * pointer for real.
             */
            if (idx <= ids->last_idx) {
                ids->seq++;
                if (ids->seq >= ipcid_seq_max()) {
                    ids->seq = 0;
                }
            }
            ids->last_idx = idx;

            new->seq = ids->seq;
            /* no need for smp_wmb(), this is done
             * inside idr_replace, as part of
             * rcu_assign_pointer
             */
            idr_replace(&ids->ipcs_idr, new, idx);
        }
    } else {
        new->seq = ipcid_to_seqx(next_id);
        idx = idr_alloc(&ids->ipcs_idr, new, ipcid_to_idx(next_id), 0, GFP_NOWAIT);
    }
    if (idx >= 0) {
        new->id = (new->seq << ipcmni_seq_shift()) + idx;
    }
    return idx;
}

/**
 * ipc_addid - add an ipc identifier
 * @ids: ipc identifier set
 * @new: new ipc permission set
 * @limit: 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 index 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.
 * The caller must use ipc_rcu_putref() to free the identifier.
 *
 * Called with writer ipc_ids.rwsem held.
 */
int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
{
    kuid_t euid;
    kgid_t egid;
    int idx, err;

    /* 1) Initialize the refcount so that ipc_rcu_putref works */
    refcount_set(&new->refcount, 1);

    if (limit > ipc_mni) {
        limit = ipc_mni;
    }

    if (ids->in_use >= limit) {
        return -ENOSPC;
    }

    idr_preload(GFP_KERNEL);

    spin_lock_init(&new->lock);
    rcu_read_lock();
    spin_lock(&new->lock);

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

    new->deleted = false;

    idx = ipc_idr_alloc(ids, new);
    idr_preload_end();

    if (idx >= 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, idx);
            idx = err;
        }
    }
    if (idx < 0) {
        new->deleted = true;
        spin_unlock(&new->lock);
        rcu_read_unlock();
        return idx;
    }

    ids->in_use++;
    if (idx > ids->max_idx) {
        ids->max_idx = idx;
    }
    return idx;
}

/**
 * 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) {
        WARN_ON_ONCE(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 idx = ipcid_to_idx(ipcp->id);

    WARN_ON_ONCE(idr_remove(&ids->ipcs_idr, idx) != ipcp);
    ipc_kht_remove(ids, ipcp);
    ids->in_use--;
    ipcp->deleted = true;

    if (unlikely(idx == ids->max_idx)) {
        do {
            idx--;
            if (idx == -1) {
                break;
            }
        } while (!idr_find(&ids->ipcs_idr, idx));
        ids->max_idx = idx;
    }
}

/**
 * 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;
}

bool 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 >> 0x6) | (flag >> 0x3) | flag;
    granted_mode = ipcp->mode;
    if (uid_eq(euid, ipcp->cuid) || uid_eq(euid, ipcp->uid)) {
        granted_mode >>= 0x6;
    } else if (in_group_p(ipcp->cgid) || in_group_p(ipcp->gid)) {
        granted_mode >>= 0x3;
    }
    /* is there some bit set in requested_mode but not in granted_mode? */
    if ((requested_mode & ~granted_mode & 0x7) && !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 idx = ipcid_to_idx(id);

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

    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
 * sequence number.
 *
 * 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_obtain_check - retrieve an ipc object and check permissions
 * @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:
 *   - retrieves the ipc object 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_obtain_check(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 pid_namespace *pid_ns;
    struct ipc_proc_iface *iface;
};

struct pid_namespace *ipc_seq_pid_ns(struct seq_file *s)
{
    struct ipc_proc_iter *iter = s->private;
    return iter->pid_ns;
}

/*
 * 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++;
        }
    }

    ipc = NULL;
    if (total >= ids->in_use) {
        goto out;
    }

    for (; pos < ipc_mni; pos++) {
        ipc = idr_find(&ids->ipcs_idr, pos);
        if (ipc != NULL) {
            rcu_read_lock();
            ipc_lock_object(ipc);
            break;
        }
    }
out:
    *new_pos = pos + 1;
    return ipc;
}

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);
    iter->pid_ns = get_pid_ns(task_active_pid_ns(current));

    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);
    put_pid_ns(iter->pid_ns);
    return seq_release_private(inode, file);
}

static const struct proc_ops sysvipc_proc_ops = {
    .proc_flags = PROC_ENTRY_PERMANENT,
    .proc_open = sysvipc_proc_open,
    .proc_read = seq_read,
    .proc_lseek = seq_lseek,
    .proc_release = sysvipc_proc_release,
};
#endif /* CONFIG_PROC_FS */