uipc_shm.c

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/*-
 * Copyright (c) 2006, 2011 Robert N. M. Watson
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * Support for shared swap-backed anonymous memory objects via
 * shm_open(2) and shm_unlink(2).  While most of the implementation is
 * here, vm_mmap.c contains mapping logic changes.
 *
 * TODO:
 *
 * (1) Need to export data to a userland tool via a sysctl.  Should ipcs(1)
 *     and ipcrm(1) be expanded or should new tools to manage both POSIX
 *     kernel semaphores and POSIX shared memory be written?
 *
 * (2) Add support for this file type to fstat(1).
 *
 * (3) Resource limits?  Does this need its own resource limits or are the
 *     existing limits in mmap(2) sufficient?
 */

#include <sys/cdefs.h>
__FBSDID("$BSDSUniX$");

#include "opt_capsicum.h"

#include <sys/param.h>
#include <sys/capability.h>
#include <sys/conf.h>
#include <sys/fcntl.h>
#include <sys/file.h>
#include <sys/filedesc.h>
#include <sys/fnv_hash.h>
#include <sys/kernel.h>
#include <sys/lock.h>
#include <sys/malloc.h>
#include <sys/mman.h>
#include <sys/mutex.h>
#include <sys/priv.h>
#include <sys/proc.h>
#include <sys/refcount.h>
#include <sys/resourcevar.h>
#include <sys/stat.h>
#include <sys/sysctl.h>
#include <sys/sysproto.h>
#include <sys/systm.h>
#include <sys/sx.h>
#include <sys/time.h>
#include <sys/vnode.h>

#include <security/mac/mac_framework.h>

#include <vm/vm.h>
#include <vm/vm_param.h>
#include <vm/pmap.h>
#include <vm/vm_extern.h>
#include <vm/vm_map.h>
#include <vm/vm_kern.h>
#include <vm/vm_object.h>
#include <vm/vm_page.h>
#include <vm/vm_pageout.h>
#include <vm/vm_pager.h>
#include <vm/swap_pager.h>

struct shm_mapping {
        char            *sm_path;
        Fnv32_t         sm_fnv;
        struct shmfd    *sm_shmfd;
        LIST_ENTRY(shm_mapping) sm_link;
};

static MALLOC_DEFINE(M_SHMFD, "shmfd", "shared memory file descriptor");
static LIST_HEAD(, shm_mapping) *shm_dictionary;
static struct sx shm_dict_lock;
static struct mtx shm_timestamp_lock;
static u_long shm_hash;
static struct unrhdr *shm_ino_unr;
static dev_t shm_dev_ino;

#define SHM_HASH(fnv)   (&shm_dictionary[(fnv) & shm_hash])

static int      shm_access(struct shmfd *shmfd, struct ucred *ucred, int flags);
static struct shmfd *shm_alloc(struct ucred *ucred, mode_t mode);
static void     shm_init(void *arg);
static void     shm_drop(struct shmfd *shmfd);
static struct shmfd *shm_hold(struct shmfd *shmfd);
static void     shm_insert(char *path, Fnv32_t fnv, struct shmfd *shmfd);
static struct shmfd *shm_lookup(char *path, Fnv32_t fnv);
static int      shm_remove(char *path, Fnv32_t fnv, struct ucred *ucred);
static int      shm_dotruncate(struct shmfd *shmfd, off_t length);

static fo_rdwr_t        shm_read;
static fo_rdwr_t        shm_write;
static fo_truncate_t    shm_truncate;
static fo_ioctl_t       shm_ioctl;
static fo_poll_t        shm_poll;
static fo_kqfilter_t    shm_kqfilter;
static fo_stat_t        shm_stat;
static fo_close_t       shm_close;
static fo_chmod_t       shm_chmod;
static fo_chown_t       shm_chown;

/* File descriptor operations. */
static struct fileops shm_ops = {
        .fo_read = shm_read,
        .fo_write = shm_write,
        .fo_truncate = shm_truncate,
        .fo_ioctl = shm_ioctl,
        .fo_poll = shm_poll,
        .fo_kqfilter = shm_kqfilter,
        .fo_stat = shm_stat,
        .fo_close = shm_close,
        .fo_chmod = shm_chmod,
        .fo_chown = shm_chown,
        .fo_flags = DFLAG_PASSABLE
};

FEATURE(posix_shm, "POSIX shared memory");

static int
shm_read(struct file *fp, struct uio *uio, struct ucred *active_cred,
    int flags, struct thread *td)
{

        return (EOPNOTSUPP);
}

static int
shm_write(struct file *fp, struct uio *uio, struct ucred *active_cred,
    int flags, struct thread *td)
{

        return (EOPNOTSUPP);
}

static int
shm_truncate(struct file *fp, off_t length, struct ucred *active_cred,
    struct thread *td)
{
        struct shmfd *shmfd;
#ifdef MAC
        int error;
#endif

        shmfd = fp->f_data;
#ifdef MAC
        error = mac_posixshm_check_truncate(active_cred, fp->f_cred, shmfd);
        if (error)
                return (error);
#endif
        return (shm_dotruncate(shmfd, length));
}

static int
shm_ioctl(struct file *fp, u_long com, void *data,
    struct ucred *active_cred, struct thread *td)
{

        return (EOPNOTSUPP);
}

static int
shm_poll(struct file *fp, int events, struct ucred *active_cred,
    struct thread *td)
{

        return (EOPNOTSUPP);
}

static int
shm_kqfilter(struct file *fp, struct knote *kn)
{

        return (EOPNOTSUPP);
}

static int
shm_stat(struct file *fp, struct stat *sb, struct ucred *active_cred,
    struct thread *td)
{
        struct shmfd *shmfd;
#ifdef MAC
        int error;
#endif

        shmfd = fp->f_data;

#ifdef MAC
        error = mac_posixshm_check_stat(active_cred, fp->f_cred, shmfd);
        if (error)
                return (error);
#endif
        
        /*
         * Attempt to return sanish values for fstat() on a memory file
         * descriptor.
         */
        bzero(sb, sizeof(*sb));
        sb->st_blksize = PAGE_SIZE;
        sb->st_size = shmfd->shm_size;
        sb->st_blocks = (sb->st_size + sb->st_blksize - 1) / sb->st_blksize;
        mtx_lock(&shm_timestamp_lock);
        sb->st_atim = shmfd->shm_atime;
        sb->st_ctim = shmfd->shm_ctime;
        sb->st_mtim = shmfd->shm_mtime;
        sb->st_birthtim = shmfd->shm_birthtime;
        sb->st_mode = S_IFREG | shmfd->shm_mode;                /* XXX */
        sb->st_uid = shmfd->shm_uid;
        sb->st_gid = shmfd->shm_gid;
        mtx_unlock(&shm_timestamp_lock);
        sb->st_dev = shm_dev_ino;
        sb->st_ino = shmfd->shm_ino;

        return (0);
}

static int
shm_close(struct file *fp, struct thread *td)
{
        struct shmfd *shmfd;

        shmfd = fp->f_data;
        fp->f_data = NULL;
        shm_drop(shmfd);

        return (0);
}

static int
shm_dotruncate(struct shmfd *shmfd, off_t length)
{
        vm_object_t object;
        vm_page_t m, ma[1];
        vm_pindex_t idx, nobjsize;
        vm_ooffset_t delta;
        int base, rv;

        object = shmfd->shm_object;
        VM_OBJECT_LOCK(object);
        if (length == shmfd->shm_size) {
                VM_OBJECT_UNLOCK(object);
                return (0);
        }
        nobjsize = OFF_TO_IDX(length + PAGE_MASK);

        /* Are we shrinking?  If so, trim the end. */
        if (length < shmfd->shm_size) {
                /*
                 * Disallow any requests to shrink the size if this
                 * object is mapped into the kernel.
                 */
                if (shmfd->shm_kmappings > 0) {
                        VM_OBJECT_UNLOCK(object);
                        return (EBUSY);
                }

                /*
                 * Zero the truncated part of the last page.
                 */
                base = length & PAGE_MASK;
                if (base != 0) {
                        idx = OFF_TO_IDX(length);
retry:
                        m = vm_page_lookup(object, idx);
                        if (m != NULL) {
                                if ((m->oflags & VPO_BUSY) != 0 ||
                                    m->busy != 0) {
                                        vm_page_sleep(m, "shmtrc");
                                        goto retry;
                                }
                        } else if (vm_pager_has_page(object, idx, NULL, NULL)) {
                                m = vm_page_alloc(object, idx, VM_ALLOC_NORMAL);
                                if (m == NULL) {
                                        VM_OBJECT_UNLOCK(object);
                                        VM_WAIT;
                                        VM_OBJECT_LOCK(object);
                                        goto retry;
                                } else if (m->valid != VM_PAGE_BITS_ALL) {
                                        ma[0] = m;
                                        rv = vm_pager_get_pages(object, ma, 1,
                                            0);
                                        m = vm_page_lookup(object, idx);
                                } else
                                        /* A cached page was reactivated. */
                                        rv = VM_PAGER_OK;
                                vm_page_lock(m);
                                if (rv == VM_PAGER_OK) {
                                        vm_page_deactivate(m);
                                        vm_page_unlock(m);
                                        vm_page_wakeup(m);
                                } else {
                                        vm_page_free(m);
                                        vm_page_unlock(m);
                                        VM_OBJECT_UNLOCK(object);
                                        return (EIO);
                                }
                        }
                        if (m != NULL) {
                                pmap_zero_page_area(m, base, PAGE_SIZE - base);
                                KASSERT(m->valid == VM_PAGE_BITS_ALL,
                                    ("shm_dotruncate: page %p is invalid", m));
                                vm_page_dirty(m);
                                vm_pager_page_unswapped(m);
                        }
                }
                delta = ptoa(object->size - nobjsize);

                /* Toss in memory pages. */
                if (nobjsize < object->size)
                        vm_object_page_remove(object, nobjsize, object->size,
                            0);

                /* Toss pages from swap. */
                if (object->type == OBJT_SWAP)
                        swap_pager_freespace(object, nobjsize, delta);

                /* Free the swap accounted for shm */
                swap_release_by_cred(delta, object->cred);
                object->charge -= delta;
        } else {
                /* Attempt to reserve the swap */
                delta = ptoa(nobjsize - object->size);
                if (!swap_reserve_by_cred(delta, object->cred)) {
                        VM_OBJECT_UNLOCK(object);
                        return (ENOMEM);
                }
                object->charge += delta;
        }
        shmfd->shm_size = length;
        mtx_lock(&shm_timestamp_lock);
        vfs_timestamp(&shmfd->shm_ctime);
        shmfd->shm_mtime = shmfd->shm_ctime;
        mtx_unlock(&shm_timestamp_lock);
        object->size = nobjsize;
        VM_OBJECT_UNLOCK(object);
        return (0);
}

/*
 * shmfd object management including creation and reference counting
 * routines.
 */
static struct shmfd *
shm_alloc(struct ucred *ucred, mode_t mode)
{
        struct shmfd *shmfd;
        int ino;

        shmfd = malloc(sizeof(*shmfd), M_SHMFD, M_WAITOK | M_ZERO);
        shmfd->shm_size = 0;
        shmfd->shm_uid = ucred->cr_uid;
        shmfd->shm_gid = ucred->cr_gid;
        shmfd->shm_mode = mode;
        shmfd->shm_object = vm_pager_allocate(OBJT_DEFAULT, NULL,
            shmfd->shm_size, VM_PROT_DEFAULT, 0, ucred);
        KASSERT(shmfd->shm_object != NULL, ("shm_create: vm_pager_allocate"));
        VM_OBJECT_LOCK(shmfd->shm_object);
        vm_object_clear_flag(shmfd->shm_object, OBJ_ONEMAPPING);
        vm_object_set_flag(shmfd->shm_object, OBJ_NOSPLIT);
        VM_OBJECT_UNLOCK(shmfd->shm_object);
        vfs_timestamp(&shmfd->shm_birthtime);
        shmfd->shm_atime = shmfd->shm_mtime = shmfd->shm_ctime =
            shmfd->shm_birthtime;
        ino = alloc_unr(shm_ino_unr);
        if (ino == -1)
                shmfd->shm_ino = 0;
        else
                shmfd->shm_ino = ino;
        refcount_init(&shmfd->shm_refs, 1);
#ifdef MAC
        mac_posixshm_init(shmfd);
        mac_posixshm_create(ucred, shmfd);
#endif

        return (shmfd);
}

static struct shmfd *
shm_hold(struct shmfd *shmfd)
{

        refcount_acquire(&shmfd->shm_refs);
        return (shmfd);
}

static void
shm_drop(struct shmfd *shmfd)
{

        if (refcount_release(&shmfd->shm_refs)) {
#ifdef MAC
                mac_posixshm_destroy(shmfd);
#endif
                vm_object_deallocate(shmfd->shm_object);
                if (shmfd->shm_ino != 0)
                        free_unr(shm_ino_unr, shmfd->shm_ino);
                free(shmfd, M_SHMFD);
        }
}

/*
 * Determine if the credentials have sufficient permissions for a
 * specified combination of FREAD and FWRITE.
 */
static int
shm_access(struct shmfd *shmfd, struct ucred *ucred, int flags)
{
        accmode_t accmode;
        int error;

        accmode = 0;
        if (flags & FREAD)
                accmode |= VREAD;
        if (flags & FWRITE)
                accmode |= VWRITE;
        mtx_lock(&shm_timestamp_lock);
        error = vaccess(VREG, shmfd->shm_mode, shmfd->shm_uid, shmfd->shm_gid,
            accmode, ucred, NULL);
        mtx_unlock(&shm_timestamp_lock);
        return (error);
}

/*
 * Dictionary management.  We maintain an in-kernel dictionary to map
 * paths to shmfd objects.  We use the FNV hash on the path to store
 * the mappings in a hash table.
 */
static void
shm_init(void *arg)
{

        mtx_init(&shm_timestamp_lock, "shm timestamps", NULL, MTX_DEF);
        sx_init(&shm_dict_lock, "shm dictionary");
        shm_dictionary = hashinit(1024, M_SHMFD, &shm_hash);
        shm_ino_unr = new_unrhdr(1, INT32_MAX, NULL);
        KASSERT(shm_ino_unr != NULL, ("shm fake inodes not initialized"));
        shm_dev_ino = devfs_alloc_cdp_inode();
        KASSERT(shm_dev_ino > 0, ("shm dev inode not initialized"));
}
SYSINIT(shm_init, SI_SUB_SYSV_SHM, SI_ORDER_ANY, shm_init, NULL);

static struct shmfd *
shm_lookup(char *path, Fnv32_t fnv)
{
        struct shm_mapping *map;

        LIST_FOREACH(map, SHM_HASH(fnv), sm_link) {
                if (map->sm_fnv != fnv)
                        continue;
                if (strcmp(map->sm_path, path) == 0)
                        return (map->sm_shmfd);
        }

        return (NULL);
}

static void
shm_insert(char *path, Fnv32_t fnv, struct shmfd *shmfd)
{
        struct shm_mapping *map;

        map = malloc(sizeof(struct shm_mapping), M_SHMFD, M_WAITOK);
        map->sm_path = path;
        map->sm_fnv = fnv;
        map->sm_shmfd = shm_hold(shmfd);
        shmfd->shm_path = path;
        LIST_INSERT_HEAD(SHM_HASH(fnv), map, sm_link);
}

static int
shm_remove(char *path, Fnv32_t fnv, struct ucred *ucred)
{
        struct shm_mapping *map;
        int error;

        LIST_FOREACH(map, SHM_HASH(fnv), sm_link) {
                if (map->sm_fnv != fnv)
                        continue;
                if (strcmp(map->sm_path, path) == 0) {
#ifdef MAC
                        error = mac_posixshm_check_unlink(ucred, map->sm_shmfd);
                        if (error)
                                return (error);
#endif
                        error = shm_access(map->sm_shmfd, ucred,
                            FREAD | FWRITE);
                        if (error)
                                return (error);
                        map->sm_shmfd->shm_path = NULL;
                        LIST_REMOVE(map, sm_link);
                        shm_drop(map->sm_shmfd);
                        free(map->sm_path, M_SHMFD);
                        free(map, M_SHMFD);
                        return (0);
                }
        }

        return (ENOENT);
}

/* System calls. */
int
sys_shm_open(struct thread *td, struct shm_open_args *uap)
{
        struct filedesc *fdp;
        struct shmfd *shmfd;
        struct file *fp;
        char *path;
        Fnv32_t fnv;
        mode_t cmode;
        int fd, error;

#ifdef CAPABILITY_MODE
        /*
         * shm_open(2) is only allowed for anonymous objects.
         */
        if (IN_CAPABILITY_MODE(td) && (uap->path != SHM_ANON))
                return (ECAPMODE);
#endif

        if ((uap->flags & O_ACCMODE) != O_RDONLY &&
            (uap->flags & O_ACCMODE) != O_RDWR)
                return (EINVAL);

        if ((uap->flags & ~(O_ACCMODE | O_CREAT | O_EXCL | O_TRUNC)) != 0)
                return (EINVAL);

        fdp = td->td_proc->p_fd;
        cmode = (uap->mode & ~fdp->fd_cmask) & ACCESSPERMS;

        error = falloc(td, &fp, &fd, O_CLOEXEC);
        if (error)
                return (error);

        /* A SHM_ANON path pointer creates an anonymous object. */
        if (uap->path == SHM_ANON) {
                /* A read-only anonymous object is pointless. */
                if ((uap->flags & O_ACCMODE) == O_RDONLY) {
                        fdclose(fdp, fp, fd, td);
                        fdrop(fp, td);
                        return (EINVAL);
                }
                shmfd = shm_alloc(td->td_ucred, cmode);
        } else {
                path = malloc(MAXPATHLEN, M_SHMFD, M_WAITOK);
                error = copyinstr(uap->path, path, MAXPATHLEN, NULL);

                /* Require paths to start with a '/' character. */
                if (error == 0 && path[0] != '/')
                        error = EINVAL;
                if (error) {
                        fdclose(fdp, fp, fd, td);
                        fdrop(fp, td);
                        free(path, M_SHMFD);
                        return (error);
                }

                fnv = fnv_32_str(path, FNV1_32_INIT);
                sx_xlock(&shm_dict_lock);
                shmfd = shm_lookup(path, fnv);
                if (shmfd == NULL) {
                        /* Object does not yet exist, create it if requested. */
                        if (uap->flags & O_CREAT) {
#ifdef MAC
                                error = mac_posixshm_check_create(td->td_ucred,
                                    path);
                                if (error == 0) {
#endif
                                        shmfd = shm_alloc(td->td_ucred, cmode);
                                        shm_insert(path, fnv, shmfd);
#ifdef MAC
                                }
#endif
                        } else {
                                free(path, M_SHMFD);
                                error = ENOENT;
                        }
                } else {
                        /*
                         * Object already exists, obtain a new
                         * reference if requested and permitted.
                         */
                        free(path, M_SHMFD);
                        if ((uap->flags & (O_CREAT | O_EXCL)) ==
                            (O_CREAT | O_EXCL))
                                error = EEXIST;
                        else {
#ifdef MAC
                                error = mac_posixshm_check_open(td->td_ucred,
                                    shmfd, FFLAGS(uap->flags & O_ACCMODE));
                                if (error == 0)
#endif
                                error = shm_access(shmfd, td->td_ucred,
                                    FFLAGS(uap->flags & O_ACCMODE));
                        }

                        /*
                         * Truncate the file back to zero length if
                         * O_TRUNC was specified and the object was
                         * opened with read/write.
                         */
                        if (error == 0 &&
                            (uap->flags & (O_ACCMODE | O_TRUNC)) ==
                            (O_RDWR | O_TRUNC)) {
#ifdef MAC
                                error = mac_posixshm_check_truncate(
                                        td->td_ucred, fp->f_cred, shmfd);
                                if (error == 0)
#endif
                                        shm_dotruncate(shmfd, 0);
                        }
                        if (error == 0)
                                shm_hold(shmfd);
                }
                sx_xunlock(&shm_dict_lock);

                if (error) {
                        fdclose(fdp, fp, fd, td);
                        fdrop(fp, td);
                        return (error);
                }
        }

        finit(fp, FFLAGS(uap->flags & O_ACCMODE), DTYPE_SHM, shmfd, &shm_ops);

        td->td_retval[0] = fd;
        fdrop(fp, td);

        return (0);
}

int
sys_shm_unlink(struct thread *td, struct shm_unlink_args *uap)
{
        char *path;
        Fnv32_t fnv;
        int error;

        path = malloc(MAXPATHLEN, M_TEMP, M_WAITOK);
        error = copyinstr(uap->path, path, MAXPATHLEN, NULL);
        if (error) {
                free(path, M_TEMP);
                return (error);
        }

        fnv = fnv_32_str(path, FNV1_32_INIT);
        sx_xlock(&shm_dict_lock);
        error = shm_remove(path, fnv, td->td_ucred);
        sx_xunlock(&shm_dict_lock);
        free(path, M_TEMP);

        return (error);
}

/*
 * mmap() helper to validate mmap() requests against shm object state
 * and give mmap() the vm_object to use for the mapping.
 */
int
shm_mmap(struct shmfd *shmfd, vm_size_t objsize, vm_ooffset_t foff,
    vm_object_t *obj)
{

        /*
         * XXXRW: This validation is probably insufficient, and subject to
         * sign errors.  It should be fixed.
         */
        if (foff >= shmfd->shm_size ||
            foff + objsize > round_page(shmfd->shm_size))
                return (EINVAL);

        mtx_lock(&shm_timestamp_lock);
        vfs_timestamp(&shmfd->shm_atime);
        mtx_unlock(&shm_timestamp_lock);
        vm_object_reference(shmfd->shm_object);
        *obj = shmfd->shm_object;
        return (0);
}

static int
shm_chmod(struct file *fp, mode_t mode, struct ucred *active_cred,
    struct thread *td)
{
        struct shmfd *shmfd;
        int error;

        error = 0;
        shmfd = fp->f_data;
        mtx_lock(&shm_timestamp_lock);
        /*
         * SUSv4 says that x bits of permission need not be affected.
         * Be consistent with our shm_open there.
         */
#ifdef MAC
        error = mac_posixshm_check_setmode(active_cred, shmfd, mode);
        if (error != 0)
                goto out;
#endif
        error = vaccess(VREG, shmfd->shm_mode, shmfd->shm_uid,
            shmfd->shm_gid, VADMIN, active_cred, NULL);
        if (error != 0)
                goto out;
        shmfd->shm_mode = mode & ACCESSPERMS;
out:
        mtx_unlock(&shm_timestamp_lock);
        return (error);
}

static int
shm_chown(struct file *fp, uid_t uid, gid_t gid, struct ucred *active_cred,
    struct thread *td)
{
        struct shmfd *shmfd;
        int error;

        error = 0;
        shmfd = fp->f_data;
        mtx_lock(&shm_timestamp_lock);
#ifdef MAC
        error = mac_posixshm_check_setowner(active_cred, shmfd, uid, gid);
        if (error != 0)
                goto out;
#endif
        if (uid == (uid_t)-1)
                uid = shmfd->shm_uid;
        if (gid == (gid_t)-1)
                 gid = shmfd->shm_gid;
        if (((uid != shmfd->shm_uid && uid != active_cred->cr_uid) ||
            (gid != shmfd->shm_gid && !groupmember(gid, active_cred))) &&
            (error = priv_check_cred(active_cred, PRIV_VFS_CHOWN, 0)))
                goto out;
        shmfd->shm_uid = uid;
        shmfd->shm_gid = gid;
out:
        mtx_unlock(&shm_timestamp_lock);
        return (error);
}

/*
 * Helper routines to allow the backing object of a shared memory file
 * descriptor to be mapped in the kernel.
 */
int
shm_map(struct file *fp, size_t size, off_t offset, void **memp)
{
        struct shmfd *shmfd;
        vm_offset_t kva, ofs;
        vm_object_t obj;
        int rv;

        if (fp->f_type != DTYPE_SHM)
                return (EINVAL);
        shmfd = fp->f_data;
        obj = shmfd->shm_object;
        VM_OBJECT_LOCK(obj);
        /*
         * XXXRW: This validation is probably insufficient, and subject to
         * sign errors.  It should be fixed.
         */
        if (offset >= shmfd->shm_size ||
            offset + size > round_page(shmfd->shm_size)) {
                VM_OBJECT_UNLOCK(obj);
                return (EINVAL);
        }

        shmfd->shm_kmappings++;
        vm_object_reference_locked(obj);
        VM_OBJECT_UNLOCK(obj);

        /* Map the object into the kernel_map and wire it. */
        kva = vm_map_min(kernel_map);
        ofs = offset & PAGE_MASK;
        offset = trunc_page(offset);
        size = round_page(size + ofs);
        rv = vm_map_find(kernel_map, obj, offset, &kva, size,
            VMFS_OPTIMAL_SPACE, VM_PROT_READ | VM_PROT_WRITE,
            VM_PROT_READ | VM_PROT_WRITE, 0);
        if (rv == KERN_SUCCESS) {
                rv = vm_map_wire(kernel_map, kva, kva + size,
                    VM_MAP_WIRE_SYSTEM | VM_MAP_WIRE_NOHOLES);
                if (rv == KERN_SUCCESS) {
                        *memp = (void *)(kva + ofs);
                        return (0);
                }
                vm_map_remove(kernel_map, kva, kva + size);
        } else
                vm_object_deallocate(obj);

        /* On failure, drop our mapping reference. */
        VM_OBJECT_LOCK(obj);
        shmfd->shm_kmappings--;
        VM_OBJECT_UNLOCK(obj);

        return (vm_mmap_to_errno(rv));
}

/*
 * We require the caller to unmap the entire entry.  This allows us to
 * safely decrement shm_kmappings when a mapping is removed.
 */
int
shm_unmap(struct file *fp, void *mem, size_t size)
{
        struct shmfd *shmfd;
        vm_map_entry_t entry;
        vm_offset_t kva, ofs;
        vm_object_t obj;
        vm_pindex_t pindex;
        vm_prot_t prot;
        boolean_t wired;
        vm_map_t map;
        int rv;

        if (fp->f_type != DTYPE_SHM)
                return (EINVAL);
        shmfd = fp->f_data;
        kva = (vm_offset_t)mem;
        ofs = kva & PAGE_MASK;
        kva = trunc_page(kva);
        size = round_page(size + ofs);
        map = kernel_map;
        rv = vm_map_lookup(&map, kva, VM_PROT_READ | VM_PROT_WRITE, &entry,
            &obj, &pindex, &prot, &wired);
        if (rv != KERN_SUCCESS)
                return (EINVAL);
        if (entry->start != kva || entry->end != kva + size) {
                vm_map_lookup_done(map, entry);
                return (EINVAL);
        }
        vm_map_lookup_done(map, entry);
        if (obj != shmfd->shm_object)
                return (EINVAL);
        vm_map_remove(map, kva, kva + size);
        VM_OBJECT_LOCK(obj);
        KASSERT(shmfd->shm_kmappings > 0, ("shm_unmap: object not mapped"));
        shmfd->shm_kmappings--;
        VM_OBJECT_UNLOCK(obj);
        return (0);
}

void
shm_path(struct shmfd *shmfd, char *path, size_t size)
{

        if (shmfd->shm_path == NULL)
                return;
        sx_slock(&shm_dict_lock);
        if (shmfd->shm_path != NULL)
                strlcpy(path, shmfd->shm_path, size);
        sx_sunlock(&shm_dict_lock);
}