lkml.org 
[lkml]   [2006]   [Jun]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Subject[RFC][PATCH 12/20] tricky: elevate write count files are open()ed
    From
    Date

    This is the first really tricky patch in the series. It
    elevates the writer count on a mount each time a
    non-special file is opened for write.

    This is not completely apparent in the patch because the
    two if() conditions in may_open() above the
    mnt_want_write() call are, combined, equivalent to
    special_file().

    There is also an elevated count around the vfs_create()
    call in open_namei(). The count needs to be kept elevated
    all the way into the may_open() call. Otherwise, when the
    write is dropped, a ro->rw transisition could occur. This
    would lead to having rw access on the newly created file,
    while the vfsmount is ro. That is bad.

    Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
    ---

    lxc-dave/fs/file_table.c | 5 ++++-
    lxc-dave/fs/namei.c | 22 ++++++++++++++++++----
    lxc-dave/ipc/mqueue.c | 3 +++
    3 files changed, 25 insertions(+), 5 deletions(-)

    diff -puN fs/file_table.c~C-elevate-writers-opens-part1 fs/file_table.c
    --- lxc/fs/file_table.c~C-elevate-writers-opens-part1 2006-06-16 15:58:06.000000000 -0700
    +++ lxc-dave/fs/file_table.c 2006-06-16 15:58:06.000000000 -0700
    @@ -180,8 +180,11 @@ void fastcall __fput(struct file *file)
    if (unlikely(inode->i_cdev != NULL))
    cdev_put(inode->i_cdev);
    fops_put(file->f_op);
    - if (file->f_mode & FMODE_WRITE)
    + if (file->f_mode & FMODE_WRITE) {
    put_write_access(inode);
    + if(!special_file(inode->i_mode))
    + mnt_drop_write(mnt);
    + }
    file_kill(file);
    file->f_dentry = NULL;
    file->f_vfsmnt = NULL;
    diff -puN fs/namei.c~C-elevate-writers-opens-part1 fs/namei.c
    --- lxc/fs/namei.c~C-elevate-writers-opens-part1 2006-06-16 15:58:06.000000000 -0700
    +++ lxc-dave/fs/namei.c 2006-06-16 15:58:06.000000000 -0700
    @@ -1512,8 +1512,17 @@ int may_open(struct nameidata *nd, int a
    return -EACCES;

    flag &= ~O_TRUNC;
    - } else if (IS_RDONLY(inode) && (flag & FMODE_WRITE))
    - return -EROFS;
    + } else if (flag & FMODE_WRITE) {
    + /*
    + * effectively: !special_file()
    + * balanced by __fput()
    + */
    + error = mnt_want_write(nd->mnt);
    + if (error)
    + return error;
    + if (IS_RDONLY(inode))
    + return -EROFS;
    + }
    /*
    * An append-only file must be opened in append mode for writing.
    */
    @@ -1652,14 +1661,17 @@ do_last:
    }

    if (IS_ERR(nd->intent.open.file)) {
    - mutex_unlock(&dir->d_inode->i_mutex);
    error = PTR_ERR(nd->intent.open.file);
    - goto exit_dput;
    + goto exit_mutex_unlock;
    }

    /* Negative dentry, just create the file */
    if (!path.dentry->d_inode) {
    + error = mnt_want_write(nd->mnt);
    + if (error)
    + goto exit_mutex_unlock;
    error = open_namei_create(nd, &path, flag, mode);
    + mnt_drop_write(nd->mnt);
    if (error)
    goto exit;
    return 0;
    @@ -1695,6 +1707,8 @@ ok:
    goto exit;
    return 0;

    +exit_mutex_unlock:
    + mutex_unlock(&dir->d_inode->i_mutex);
    exit_dput:
    dput_path(&path, nd);
    exit:
    diff -puN ipc/mqueue.c~C-elevate-writers-opens-part1 ipc/mqueue.c
    --- lxc/ipc/mqueue.c~C-elevate-writers-opens-part1 2006-06-16 15:58:06.000000000 -0700
    +++ lxc-dave/ipc/mqueue.c 2006-06-16 15:58:06.000000000 -0700
    @@ -679,6 +679,9 @@ asmlinkage long sys_mq_open(const char _
    goto out;
    filp = do_open(dentry, oflag);
    } else {
    + error = mnt_want_write(mqueue_mnt);
    + if (error)
    + goto out;
    filp = do_create(mqueue_mnt->mnt_root, dentry,
    oflag, mode, u_attr);
    }
    diff -L fs/namei.c. -puN /dev/null /dev/null
    _
    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/
    \
     
     \ /
      Last update: 2006-06-17 01:17    [W:2.448 / U:0.032 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site