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    SubjectRe: [PATCH 14/28] VFS: Introduce Mountpoint file descriptors (resend)
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    Christoph Hellwig wrote:

    > You haven't explained why you actually need it, though.
    >

    Apparently I used the wrong server and a couple patches bounced :\

    I'll try to make the next series more 'forward self-describing' :)
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    This patch introduces the concept of a mountpoint file descriptor (mountfd).
    All interfacing with mountfds are done through the mountfd(2) syscall (which
    currently uses a command op argument to determine what you really are trying
    to do.

    This patch only adds the following abilities:
    - grab a reference to a mountfd given a directory fd (of the root of the
    mountpoint).
    - get the directory fd of the root of the mountpoint given a mountfd.

    NOTE: the entire interface is highly likely to change and
    comments/suggestions are more than welcome as the provided api is mostly a
    prototype at this stage and far from complete.

    Further patches add the following functionalities:
    - attach a mountpoint given a mountfd to a directory (given a dirfd)
    - detach/umount a mountpoint given a mountfd (umount if not busy, forced
    umount, lazy umount)

    NOTE AGAIN: most of the interface is half-baked, and really does require
    input from others. Most of this functionality is not neccesarily needed for
    autofsng, however I thought I'd start the ball rolling for _some_ interface.

    Signed-off-by: Mike Waychison <michael.waychison@sun.com>
    ---

    Documentation/ioctl-number.txt | 1
    arch/i386/kernel/entry.S | 17 ++
    fs/Makefile | 2
    fs/mountfd.c | 256 +++++++++++++++++++++++++++++++++++++++++
    fs/namespace.c | 2
    include/asm-i386/unistd.h | 3
    include/linux/fs.h | 2
    7 files changed, 281 insertions(+), 2 deletions(-)

    Index: linux-2.6.9-quilt/arch/i386/kernel/entry.S
    ===================================================================
    --- linux-2.6.9-quilt.orig/arch/i386/kernel/entry.S 2004-08-14 01:36:32.000000000 -0400
    +++ linux-2.6.9-quilt/arch/i386/kernel/entry.S 2004-10-22 17:17:40.735271440 -0400
    @@ -886,5 +886,22 @@ ENTRY(sys_call_table)
    .long sys_mq_notify
    .long sys_mq_getsetattr
    .long sys_ni_syscall /* reserved for kexec */
    + .long sys_ni_syscall
    + .long sys_ni_syscall /* 285 */
    + .long sys_ni_syscall
    + .long sys_ni_syscall
    + .long sys_ni_syscall
    + .long sys_ni_syscall
    + .long sys_ni_syscall /* 290 */
    + .long sys_ni_syscall
    + .long sys_ni_syscall
    + .long sys_ni_syscall
    + .long sys_ni_syscall
    + .long sys_ni_syscall /* 295 */
    + .long sys_ni_syscall
    + .long sys_ni_syscall
    + .long sys_ni_syscall
    + .long sys_ni_syscall
    + .long sys_mountfd /* 300 */

    syscall_table_size=(.-sys_call_table)
    Index: linux-2.6.9-quilt/include/asm-i386/unistd.h
    ===================================================================
    --- linux-2.6.9-quilt.orig/include/asm-i386/unistd.h 2004-08-14 01:37:25.000000000 -0400
    +++ linux-2.6.9-quilt/include/asm-i386/unistd.h 2004-10-22 17:17:40.735271440 -0400
    @@ -289,8 +289,9 @@
    #define __NR_mq_notify (__NR_mq_open+4)
    #define __NR_mq_getsetattr (__NR_mq_open+5)
    #define __NR_sys_kexec_load 283
    +#define __NR_mountfd 300

    -#define NR_syscalls 284
    +#define NR_syscalls 301

    /* user-visible error numbers are in the range -1 - -124: see <asm-i386/errno.h> */

    Index: linux-2.6.9-quilt/fs/mountfd.c
    ===================================================================
    --- /dev/null 1970-01-01 00:00:00.000000000 +0000
    +++ linux-2.6.9-quilt/fs/mountfd.c 2004-10-22 17:17:40.736271288 -0400
    @@ -0,0 +1,256 @@
    +#include <linux/fs.h>
    +#include <linux/file.h>
    +#include <linux/mount.h>
    +#include <linux/slab.h>
    +#include <linux/stat.h>
    +#include <linux/init.h>
    +#include <linux/security.h>
    +
    +#define MFDFS_MAGIC 0x4A9F2E43
    +#define MFDFS_ROOT_INO 1
    +
    +#define VFSMOUNT(filp) ((struct vfsmount *)((filp)->private_data))
    +
    +static struct vfsmount *mfdfs_mnt;
    +
    +static void mfdfs_read_inode(struct inode *inode);
    +static struct super_operations mfdfs_ops = {
    + .read_inode = mfdfs_read_inode,
    + .statfs = simple_statfs,
    +};
    +
    +static struct super_block *mfdfs_get_sb(struct file_system_type *fs_type,
    + int flags, const char *dev_name, void *data)
    +{
    + return get_sb_pseudo(fs_type, "mountfd:", &mfdfs_ops, MFDFS_MAGIC);
    +}
    +
    +static struct file_system_type mfd_fs_type = {
    + .name = "mfdfs",
    + .get_sb = mfdfs_get_sb,
    + .kill_sb = kill_anon_super,
    +};
    +
    +void __init mfdfs_init(void)
    +{
    + register_filesystem(&mfd_fs_type);
    + mfdfs_mnt = kern_mount(&mfd_fs_type);
    +}
    +
    +/* TODO: change this list into a hashtable */
    +spinlock_t mfd_lock = SPIN_LOCK_UNLOCKED;
    +LIST_HEAD(mfd_list);
    +struct mountfd {
    + struct list_head list;
    + struct vfsmount *mnt;
    + struct dentry *dentry;
    + atomic_t count;
    +};
    +
    +static int mfd_release(struct inode *inode, struct file *file)
    +{
    + struct vfsmount *mnt = VFSMOUNT(file);
    + struct mountfd *mfd, *tofree = NULL;
    +
    + spin_lock(&mfd_lock);
    + list_for_each_entry(mfd, &mfd_list, list) {
    + if (mfd->mnt == mnt) {
    + if (atomic_dec_and_test(&mfd->count)) {
    + tofree = mfd;
    + list_del_init(&tofree->list);
    + }
    + break;
    + }
    + }
    + spin_unlock(&mfd_lock);
    +
    + if (tofree) {
    + d_delete(tofree->dentry);
    + mntsoftput(tofree->mnt);
    + kfree(tofree);
    + }
    + return 0;
    +}
    +
    +static int mfd_ioctl(struct inode *inode, struct file *filp,
    + unsigned int cmd, unsigned long arg);
    +static struct file_operations mfd_file_ops = {
    + .release = mfd_release,
    + .ioctl = mfd_ioctl,
    +};
    +
    +static void mfdfs_read_inode(struct inode *inode)
    +{
    + inode->i_fop = &mfd_file_ops;
    + inode->i_mode = S_IFREG | S_IRUGO | S_IXUGO;
    + inode->i_nlink = 1;
    + inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
    +
    + inode->i_blocks = 0;
    + inode->i_blksize = 1024;
    +}
    +
    +static struct dentry *get_mfd_dentry(struct vfsmount *mnt)
    +{
    + struct mountfd *newmfd, *mfd;
    + struct dentry *dentry;
    + struct qstr qstr;
    + struct inode *inode;
    +
    + /* create an new mfd before we lock */
    + newmfd = kmalloc(sizeof(*newmfd), GFP_KERNEL);
    + if (!newmfd)
    + return ERR_PTR(-ENOMEM);
    + if (!mfdfs_mnt->mnt_root)
    + return ERR_PTR(-ENOMEM);
    + /* TODO: how to make this context dependent in proc/<pid>/fd */
    + qstr.name = "DUMMY";
    + qstr.len = 5;
    + newmfd->dentry = d_alloc(mfdfs_mnt->mnt_root, &qstr);
    + if (IS_ERR(newmfd->dentry)) {
    + struct dentry *err = newmfd->dentry;
    + kfree(newmfd);
    + return err;
    + }
    + inode = iget(mfdfs_mnt->mnt_sb, iunique(mfdfs_mnt->mnt_sb, MFDFS_ROOT_INO));
    + if (IS_ERR(inode)) {
    + int err = PTR_ERR(inode);
    + dput(newmfd->dentry);
    + kfree(newmfd);
    + return ERR_PTR(err);
    + }
    + d_add(newmfd->dentry, inode);
    +
    + spin_lock(&mfd_lock);
    + list_for_each_entry(mfd, &mfd_list, list) {
    + if (mfd->mnt == mnt) {
    + dentry = dget(mfd->dentry);
    + atomic_inc(&mfd->count);
    + spin_unlock(&mfd_lock);
    +
    + d_delete(newmfd->dentry);
    + kfree(newmfd);
    +
    + return dentry;
    + }
    + }
    +
    + newmfd->mnt = mntsoftget(mnt);
    + dentry = newmfd->dentry;
    + INIT_LIST_HEAD(&newmfd->list);
    + list_add(&newmfd->list, &mfd_list);
    + atomic_set(&newmfd->count, 1);
    + spin_unlock(&mfd_lock);
    +
    + return dget(dentry);
    +}
    +
    +static long open_mfd(struct vfsmount *mnt)
    +{
    + struct file *file;
    + int error;
    + int fd;
    +
    + error = -ENOMEM;
    + file = get_empty_filp();
    + if (!file)
    + goto out;
    +
    + error = -ENFILE;
    + fd = get_unused_fd();
    + if (fd < 0)
    + goto out_putfilp;
    +
    + file->private_data = mnt;
    + file->f_dentry = get_mfd_dentry(mnt);
    + if (IS_ERR(file->f_dentry)) {
    + error = PTR_ERR(file->f_dentry);
    + goto out_putfd;
    + }
    + file->f_vfsmnt = mntget(mfdfs_mnt);
    +
    + file->f_op = &mfd_file_ops;
    + file->f_mode = FMODE_READ | FMODE_WRITE;
    + file->f_flags = O_RDWR;
    + file->f_pos = 0;
    +
    + fd_install(fd, file);
    + return fd;
    +
    +out_putfd:
    + put_unused_fd(fd);
    +out_putfilp:
    + put_filp(file);
    +out:
    + return error;
    +}
    +
    +static long mfd_getmfd(int dirfd)
    +{
    + struct file *dir;
    + int error;
    +
    + error = -EBADF;
    + dir = fget(dirfd);
    + if (!dir)
    + goto out;
    +
    + error = -ENOTDIR;
    + if (!S_ISDIR(dir->f_dentry->d_inode->i_mode))
    + goto out_fput;
    +
    + error = -EINVAL;
    + if (dir->f_vfsmnt->mnt_root != dir->f_dentry)
    + goto out_fput;
    +
    + error = open_mfd(dir->f_vfsmnt);
    +
    +out_fput:
    + fput(dir);
    +out:
    + return error;
    +}
    +
    +long mfd_getdirfd(struct file *mountfilp)
    +{
    + long error, dirfd;
    + struct file *filp;
    + struct vfsmount *mnt;
    +
    + mnt = mntget(VFSMOUNT(mountfilp));
    +
    + error = -ENFILE;
    + if ((dirfd = get_unused_fd()) < 0)
    + goto out_filp;
    +
    + filp = dentry_open(dget(mnt->mnt_root), mnt, O_DIRECTORY | O_RDONLY);
    + if (IS_ERR(filp)) {
    + error = PTR_ERR(filp);
    + goto out_filp;
    + }
    +
    + error = dirfd;
    + fd_install(dirfd, filp);
    +
    +out_filp:
    + mntput(mnt);
    + return error;
    +}
    +
    +static int mfd_ioctl(struct inode *inode, struct file *filp,
    + unsigned int cmd, unsigned long arg)
    +{
    + switch (cmd) {
    + case MOUNTFD_IOC_GETDIRFD:
    + return mfd_getdirfd(filp);
    + }
    + return -ENOTTY;
    +}
    +
    +asmlinkage long sys_mountfd(int dirfd)
    +{
    + if (!capable(CAP_SYS_ADMIN))
    + return -EPERM;
    +
    + return mfd_getmfd(dirfd);
    +}
    Index: linux-2.6.9-quilt/fs/Makefile
    ===================================================================
    --- linux-2.6.9-quilt.orig/fs/Makefile 2004-08-14 01:37:14.000000000 -0400
    +++ linux-2.6.9-quilt/fs/Makefile 2004-10-22 17:17:40.736271288 -0400
    @@ -10,7 +10,7 @@ obj-y := open.o read_write.o file_table.
    namei.o fcntl.o ioctl.o readdir.o select.o fifo.o locks.o \
    dcache.o inode.o attr.o bad_inode.o file.o dnotify.o \
    filesystems.o namespace.o seq_file.o xattr.o libfs.o \
    - fs-writeback.o mpage.o direct-io.o aio.o
    + fs-writeback.o mpage.o direct-io.o aio.o mountfd.o

    obj-$(CONFIG_EPOLL) += eventpoll.o
    obj-$(CONFIG_COMPAT) += compat.o
    Index: linux-2.6.9-quilt/fs/namespace.c
    ===================================================================
    --- linux-2.6.9-quilt.orig/fs/namespace.c 2004-10-22 17:17:40.187354736 -0400
    +++ linux-2.6.9-quilt/fs/namespace.c 2004-10-22 17:17:40.738270984 -0400
    @@ -34,6 +34,7 @@ static inline int sysfs_init(void)
    return 0;
    }
    #endif
    +extern void __init mfdfs_init(void);

    /* spinlock for vfsmount related operations, inplace of dcache_lock */
    spinlock_t vfsmount_lock __cacheline_aligned_in_smp = SPIN_LOCK_UNLOCKED;
    @@ -1711,6 +1712,7 @@ void __init mnt_init(unsigned long mempa
    d++;
    i--;
    } while (i);
    + mfdfs_init();
    sysfs_init();
    init_rootfs();
    init_mount_tree();
    Index: linux-2.6.9-quilt/Documentation/ioctl-number.txt
    ===================================================================
    --- linux-2.6.9-quilt.orig/Documentation/ioctl-number.txt 2004-08-14 01:38:04.000000000 -0400
    +++ linux-2.6.9-quilt/Documentation/ioctl-number.txt 2004-10-22 17:17:40.738270984 -0400
    @@ -144,6 +144,7 @@ Code Seq# Include File Comments
    'p' 40-7F linux/nvram.h
    'p' 80-9F user-space parport
    <mailto:tim@cyberelk.net>
    +'p' A0-BF mountpoint file descriptors
    'q' 00-1F linux/videotext.h conflict!
    'q' 80-FF Internet PhoneJACK, Internet LineJACK
    <http://www.quicknet.net>
    Index: linux-2.6.9-quilt/include/linux/fs.h
    ===================================================================
    --- linux-2.6.9-quilt.orig/include/linux/fs.h 2004-10-22 17:17:37.120820920 -0400
    +++ linux-2.6.9-quilt/include/linux/fs.h 2004-10-22 17:17:40.739270832 -0400
    @@ -214,6 +214,8 @@ extern int leases_enable, dir_notify_ena
    #define FIBMAP _IO(0x00,1) /* bmap access */
    #define FIGETBSZ _IO(0x00,2) /* get the block size used for bmap */

    +#define MOUNTFD_IOC_GETDIRFD _IO('p', 0xa0)
    +
    #ifdef __KERNEL__

    #include <linux/list.h>
    \
     
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