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    Subject[PATCH 17/27] r-o-bind-mounts-elevate-write-count-opend-files
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    Date

    This is an old patch combined with a couple other ones I've
    been working on. It should the issues that Miklos has
    spotted.

    --

    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.

    We used to do this in may_open(), but Miklos pointed out
    that __dentry_open() is used as well to create filps. This
    will cover even those cases, while a call in may_open()
    would not have.

    There is also an elevated count around the vfs_create() call
    in open_namei(). See the comments for more details, but we
    need this to fix a 'create, remount, fail r/w open()' race.

    Some filesystems forego the use of normal vfs calls to create
    struct files. Make sure that these users elevate the mnt
    writer count because they will get __fput(), and we need
    to make sure they're balanced.

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

    linux-2.6.git-dave/fs/file_table.c | 16 +++++++-
    linux-2.6.git-dave/fs/namei.c | 74 ++++++++++++++++++++++++++++++++-----
    linux-2.6.git-dave/fs/open.c | 36 +++++++++++++++++-
    linux-2.6.git-dave/ipc/mqueue.c | 3 +
    4 files changed, 116 insertions(+), 13 deletions(-)

    diff -puN fs/file_table.c~r-o-bind-mounts-elevate-write-count-opend-files fs/file_table.c
    --- linux-2.6.git/fs/file_table.c~r-o-bind-mounts-elevate-write-count-opend-files 2007-11-01 14:46:16.000000000 -0700
    +++ linux-2.6.git-dave/fs/file_table.c 2007-11-01 14:46:16.000000000 -0700
    @@ -193,6 +193,17 @@ int init_file(struct file *file, struct
    file->f_mapping = dentry->d_inode->i_mapping;
    file->f_mode = mode;
    file->f_op = fop;
    +
    + /*
    + * These mounts don't really matter in practice
    + * for r/o bind mounts. They aren't userspace-
    + * visible. We do this for consistency, and so
    + * that we can do debugging checks at __fput()e
    + */
    + if ((mode & FMODE_WRITE) && !special_file(dentry->d_inode->i_mode)) {
    + error = mnt_want_write(mnt);
    + WARN_ON(error);
    + }
    return error;
    }
    EXPORT_SYMBOL(init_file);
    @@ -230,8 +241,11 @@ void fastcall __fput(struct file *file)
    if (unlikely(S_ISCHR(inode->i_mode) && 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);
    + }
    put_pid(file->f_owner.pid);
    file_kill(file);
    file->f_path.dentry = NULL;
    diff -puN fs/namei.c~r-o-bind-mounts-elevate-write-count-opend-files fs/namei.c
    --- linux-2.6.git/fs/namei.c~r-o-bind-mounts-elevate-write-count-opend-files 2007-11-01 14:46:16.000000000 -0700
    +++ linux-2.6.git-dave/fs/namei.c 2007-11-01 14:46:16.000000000 -0700
    @@ -1620,12 +1620,12 @@ int may_open(struct nameidata *nd, int a
    return -EACCES;

    flag &= ~O_TRUNC;
    - } else if (IS_RDONLY(inode) && (flag & FMODE_WRITE))
    - return -EROFS;
    + }

    error = vfs_permission(nd, acc_mode);
    if (error)
    return error;
    +
    /*
    * An append-only file must be opened in append mode for writing.
    */
    @@ -1721,18 +1721,31 @@ static inline int sys_open_flags_to_name
    return flag;
    }

    +static inline int open_will_write_to_fs(int flag, struct inode *inode)
    +{
    + /*
    + * We'll never write to the fs underlying
    + * a device file.
    + */
    + if (special_file(inode->i_mode))
    + return 0;
    + return (flag & O_TRUNC);
    +}
    +
    /*
    * open_pathname()
    *
    * namei for open - this is in fact almost the whole open-routine.
    *
    - * Note that the low bits of "flag" aren't the same as in the open
    - * system call. See sys_open_flags_to_namei_flags().
    + * Note that the low bits of the passed in "sys_open_flag"
    + * are not the same as in the local variable "flag". See
    + * sys_open_flags_to_namei_flags() for more details.
    * SMP-safe
    */
    struct file *open_pathname(int dfd, const char *pathname,
    int sys_open_flag, int mode)
    {
    + struct file *filp;
    struct nameidata nd;
    int acc_mode, error;
    struct path path;
    @@ -1792,17 +1805,30 @@ 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 = __open_namei_create(&nd, &path, flag, mode);
    + /*
    + * This write is needed to ensure that a
    + * ro->rw transition does not occur between
    + * the time when the file is created and when
    + * a permanent write count is taken through
    + * the 'struct file' in nameidata_to_filp().
    + */
    + error = mnt_want_write(nd.mnt);
    if (error)
    + goto exit_mutex_unlock;
    + error = __open_namei_create(&nd, &path, flag, mode);
    + if (error) {
    + mnt_drop_write(nd.mnt);
    goto exit;
    - return nameidata_to_filp(&nd, sys_open_flag);
    + }
    + filp = nameidata_to_filp(&nd, sys_open_flag);
    + mnt_drop_write(nd.mnt);
    + return filp;
    }

    /*
    @@ -1832,11 +1858,39 @@ do_last:
    if (path.dentry->d_inode && S_ISDIR(path.dentry->d_inode->i_mode))
    goto exit;
    ok:
    + /*
    + * Consider:
    + * 1. may_open() truncates a file
    + * 2. a rw->ro mount transition occurs
    + * 3. nameidata_to_filp() fails due to
    + * the ro mount.
    + * That would be inconsistent, and should
    + * be avoided. Taking this mnt write here
    + * ensures that (2) can not occur.
    + */
    + if (open_will_write_to_fs(flag, nd.dentry->d_inode)) {
    + error = mnt_want_write(nd.mnt);
    + if (error)
    + goto exit;
    + }
    error = may_open(&nd, acc_mode, flag);
    - if (error)
    + if (error) {
    + if (open_will_write_to_fs(flag, nd.dentry->d_inode))
    + mnt_drop_write(nd.mnt);
    goto exit;
    - return nameidata_to_filp(&nd, sys_open_flag);
    + }
    + filp = nameidata_to_filp(&nd, sys_open_flag);
    + /*
    + * It is now safe to drop the mnt write
    + * because the filp has had a write taken
    + * on its behalf.
    + */
    + if (open_will_write_to_fs(flag, nd.dentry->d_inode))
    + mnt_drop_write(nd.mnt);
    + return filp;

    +exit_mutex_unlock:
    + mutex_unlock(&dir->d_inode->i_mutex);
    exit_dput:
    dput_path(&path, &nd);
    exit:
    diff -puN fs/open.c~r-o-bind-mounts-elevate-write-count-opend-files fs/open.c
    --- linux-2.6.git/fs/open.c~r-o-bind-mounts-elevate-write-count-opend-files 2007-11-01 14:46:16.000000000 -0700
    +++ linux-2.6.git-dave/fs/open.c 2007-11-01 14:46:16.000000000 -0700
    @@ -734,6 +734,35 @@ out:
    return error;
    }

    +/*
    + * You have to be very careful that these write
    + * counts get cleaned up in error cases and
    + * upon __fput(). This should probably never
    + * be called outside of __dentry_open().
    + */
    +static inline int __get_file_write_access(struct inode *inode,
    + struct vfsmount *mnt)
    +{
    + int error;
    + error = get_write_access(inode);
    + if (error)
    + return error;
    + /*
    + * Do not take mount writer counts on
    + * special files since no writes to
    + * the mount itself will occur.
    + */
    + if (!special_file(inode->i_mode)) {
    + /*
    + * Balanced in __fput()
    + */
    + error = mnt_want_write(mnt);
    + if (error)
    + put_write_access(inode);
    + }
    + return error;
    +}
    +
    static struct file *__dentry_open(struct dentry *dentry, struct vfsmount *mnt,
    int flags, struct file *f,
    int (*open)(struct inode *, struct file *))
    @@ -746,7 +775,7 @@ static struct file *__dentry_open(struct
    FMODE_PREAD | FMODE_PWRITE;
    inode = dentry->d_inode;
    if (f->f_mode & FMODE_WRITE) {
    - error = get_write_access(inode);
    + error = __get_file_write_access(inode, mnt);
    if (error)
    goto cleanup_file;
    }
    @@ -788,8 +817,11 @@ static struct file *__dentry_open(struct

    cleanup_all:
    fops_put(f->f_op);
    - if (f->f_mode & FMODE_WRITE)
    + if (f->f_mode & FMODE_WRITE) {
    put_write_access(inode);
    + if (!special_file(inode->i_mode))
    + mnt_drop_write(mnt);
    + }
    file_kill(f);
    f->f_path.dentry = NULL;
    f->f_path.mnt = NULL;
    diff -puN ipc/mqueue.c~r-o-bind-mounts-elevate-write-count-opend-files ipc/mqueue.c
    --- linux-2.6.git/ipc/mqueue.c~r-o-bind-mounts-elevate-write-count-opend-files 2007-11-01 14:46:16.000000000 -0700
    +++ linux-2.6.git-dave/ipc/mqueue.c 2007-11-01 14:46:16.000000000 -0700
    @@ -682,6 +682,9 @@ 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);
    }
    _
    -
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