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    Subject[PATCH 4.0 02/58] sysctl: Allow creating permanently empty directories that serve as mountpoints.
    Date
    4.0-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: "Eric W. Biederman" <ebiederm@xmission.com>

    commit f9bd6733d3f11e24f3949becf277507d422ee1eb upstream.

    Add a magic sysctl table sysctl_mount_point that when used to
    create a directory forces that directory to be permanently empty.

    Update the code to use make_empty_dir_inode when accessing permanently
    empty directories.

    Update the code to not allow adding to permanently empty directories.

    Update /proc/sys/fs/binfmt_misc to be a permanently empty directory.

    Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/proc/proc_sysctl.c | 37 +++++++++++++++++++++++++++++++++++++
    include/linux/sysctl.h | 3 +++
    kernel/sysctl.c | 8 +-------
    3 files changed, 41 insertions(+), 7 deletions(-)

    --- a/fs/proc/proc_sysctl.c
    +++ b/fs/proc/proc_sysctl.c
    @@ -19,6 +19,28 @@ static const struct inode_operations pro
    static const struct file_operations proc_sys_dir_file_operations;
    static const struct inode_operations proc_sys_dir_operations;

    +/* Support for permanently empty directories */
    +
    +struct ctl_table sysctl_mount_point[] = {
    + { }
    +};
    +
    +static bool is_empty_dir(struct ctl_table_header *head)
    +{
    + return head->ctl_table[0].child == sysctl_mount_point;
    +}
    +
    +static void set_empty_dir(struct ctl_dir *dir)
    +{
    + dir->header.ctl_table[0].child = sysctl_mount_point;
    +}
    +
    +static void clear_empty_dir(struct ctl_dir *dir)
    +
    +{
    + dir->header.ctl_table[0].child = NULL;
    +}
    +
    void proc_sys_poll_notify(struct ctl_table_poll *poll)
    {
    if (!poll)
    @@ -187,6 +209,17 @@ static int insert_header(struct ctl_dir
    struct ctl_table *entry;
    int err;

    + /* Is this a permanently empty directory? */
    + if (is_empty_dir(&dir->header))
    + return -EROFS;
    +
    + /* Am I creating a permanently empty directory? */
    + if (header->ctl_table == sysctl_mount_point) {
    + if (!RB_EMPTY_ROOT(&dir->root))
    + return -EINVAL;
    + set_empty_dir(dir);
    + }
    +
    dir->header.nreg++;
    header->parent = dir;
    err = insert_links(header);
    @@ -202,6 +235,8 @@ fail:
    erase_header(header);
    put_links(header);
    fail_links:
    + if (header->ctl_table == sysctl_mount_point)
    + clear_empty_dir(dir);
    header->parent = NULL;
    drop_sysctl_table(&dir->header);
    return err;
    @@ -419,6 +454,8 @@ static struct inode *proc_sys_make_inode
    inode->i_mode |= S_IFDIR;
    inode->i_op = &proc_sys_dir_operations;
    inode->i_fop = &proc_sys_dir_file_operations;
    + if (is_empty_dir(head))
    + make_empty_dir_inode(inode);
    }
    out:
    return inode;
    --- a/include/linux/sysctl.h
    +++ b/include/linux/sysctl.h
    @@ -188,6 +188,9 @@ struct ctl_table_header *register_sysctl
    void unregister_sysctl_table(struct ctl_table_header * table);

    extern int sysctl_init(void);
    +
    +extern struct ctl_table sysctl_mount_point[];
    +
    #else /* CONFIG_SYSCTL */
    static inline struct ctl_table_header *register_sysctl_table(struct ctl_table * table)
    {
    --- a/kernel/sysctl.c
    +++ b/kernel/sysctl.c
    @@ -1510,12 +1510,6 @@ static struct ctl_table vm_table[] = {
    { }
    };

    -#if defined(CONFIG_BINFMT_MISC) || defined(CONFIG_BINFMT_MISC_MODULE)
    -static struct ctl_table binfmt_misc_table[] = {
    - { }
    -};
    -#endif
    -
    static struct ctl_table fs_table[] = {
    {
    .procname = "inode-nr",
    @@ -1669,7 +1663,7 @@ static struct ctl_table fs_table[] = {
    {
    .procname = "binfmt_misc",
    .mode = 0555,
    - .child = binfmt_misc_table,
    + .child = sysctl_mount_point,
    },
    #endif
    {



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