Re: [PATCH 07/14] fsinfo: Allow mount information to be queried [ver #18]

[Miklos Szeredi]

On Mon, Mar 9, 2020 at 3:02 PM David Howells <dhowells@xxxxxxxxxx> wrote:
>
> Allow mount information, including information about the topology tree to
> be queried with the fsinfo() system call.  Setting AT_FSINFO_QUERY_MOUNT
> allows overlapping mounts to be queried by indicating that the syscall
> should interpet the pathname as a number indicating the mount ID.
>
> To this end, a number of fsinfo() attributes are provided:
>
>  (1) FSINFO_ATTR_MOUNT_INFO.
>
>      This is a structure providing information about a mount, including:
>
>         - Mounted superblock ID (mount ID uniquifier).
>         - Mount ID (can be used with AT_FSINFO_QUERY_MOUNT).
>         - Parent mount ID.
>         - Mount attributes (eg. R/O, NOEXEC).
>         - Mount change/notification counter.
>
>      Note that the parent mount ID is overridden to the ID of the queried
>      mount if the parent lies outside of the chroot or dfd tree.
>
>  (2) FSINFO_ATTR_MOUNT_PATH.
>
>      This a string providing information about a bind mount relative the
>      the root that was bound off, though it may get overridden by the
>      filesystem (NFS unconditionally sets it to "/", for example).
>
>  (3) FSINFO_ATTR_MOUNT_POINT.
>
>      This is a string indicating the name of the mountpoint within the
>      parent mount, limited to the parent's mounted root and the chroot.
>
>  (4) FSINFO_ATTR_MOUNT_POINT_FULL.
>
>      This is a string indicating the full path of the mountpoint, limited to
>      the chroot.
>
>  (5) FSINFO_ATTR_MOUNT_CHILDREN.
>
>      This produces an array of structures, one for each child and capped
>      with one for the argument mount (checked after listing all the
>      children).  Each element contains the mount ID and the change counter
>      of the respective mount object.
>
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> ---
>
>  fs/d_path.c                 |    2
>  fs/fsinfo.c                 |   14 +++
>  fs/internal.h               |   10 ++
>  fs/namespace.c              |  177 +++++++++++++++++++++++++++++++++++++++++++
>  include/uapi/linux/fsinfo.h |   36 +++++++++
>  samples/vfs/test-fsinfo.c   |   43 ++++++++++
>  6 files changed, 281 insertions(+), 1 deletion(-)
>
> diff --git a/fs/d_path.c b/fs/d_path.c
> index 0f1fc1743302..4c203f64e45e 100644
> --- a/fs/d_path.c
> +++ b/fs/d_path.c
> @@ -229,7 +229,7 @@ static int prepend_unreachable(char **buffer, int *buflen)
>         return prepend(buffer, buflen, "(unreachable)", 13);
>  }
>
> -static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
> +void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
>  {
>         unsigned seq;
>
> diff --git a/fs/fsinfo.c b/fs/fsinfo.c
> index bafeb73feaf4..6d2bc03998e4 100644
> --- a/fs/fsinfo.c
> +++ b/fs/fsinfo.c
> @@ -236,6 +236,14 @@ static int fsinfo_generic_seq_read(struct path *path, struct fsinfo_context *ctx
>                         ret = sb->s_op->show_options(&m, path->mnt->mnt_root);
>                 break;
>
> +       case FSINFO_ATTR_MOUNT_PATH:
> +               if (sb->s_op->show_path) {
> +                       ret = sb->s_op->show_path(&m, path->mnt->mnt_root);
> +               } else {
> +                       seq_dentry(&m, path->mnt->mnt_root, " \t\n\\");
> +               }
> +               break;
> +
>         case FSINFO_ATTR_FS_STATISTICS:
>                 if (sb->s_op->show_stats)
>                         ret = sb->s_op->show_stats(&m, path->mnt->mnt_root);
> @@ -270,6 +278,12 @@ static const struct fsinfo_attribute fsinfo_common_attributes[] = {
>
>         FSINFO_LIST     (FSINFO_ATTR_FSINFO_ATTRIBUTES, (void *)123UL),
>         FSINFO_VSTRUCT_N(FSINFO_ATTR_FSINFO_ATTRIBUTE_INFO, (void *)123UL),
> +
> +       FSINFO_VSTRUCT  (FSINFO_ATTR_MOUNT_INFO,        fsinfo_generic_mount_info),
> +       FSINFO_STRING   (FSINFO_ATTR_MOUNT_PATH,        fsinfo_generic_seq_read),
> +       FSINFO_STRING   (FSINFO_ATTR_MOUNT_POINT,       fsinfo_generic_mount_point),
> +       FSINFO_STRING   (FSINFO_ATTR_MOUNT_POINT_FULL,  fsinfo_generic_mount_point_full),
> +       FSINFO_LIST     (FSINFO_ATTR_MOUNT_CHILDREN,    fsinfo_generic_mount_children),
>         {}
>  };
>
> diff --git a/fs/internal.h b/fs/internal.h
> index abbd5299e7dc..1a318dc85f2f 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -15,6 +15,7 @@ struct mount;
>  struct shrink_control;
>  struct fs_context;
>  struct user_namespace;
> +struct fsinfo_context;
>
>  /*
>   * block_dev.c
> @@ -47,6 +48,11 @@ extern int __block_write_begin_int(struct page *page, loff_t pos, unsigned len,
>   */
>  extern void __init chrdev_init(void);
>
> +/*
> + * d_path.c
> + */
> +extern void get_fs_root_rcu(struct fs_struct *fs, struct path *root);
> +
>  /*
>   * fs_context.c
>   */
> @@ -93,6 +99,10 @@ extern void __mnt_drop_write_file(struct file *);
>  extern void dissolve_on_fput(struct vfsmount *);
>  extern int lookup_mount_object(struct path *, int, struct path *);
>  extern int fsinfo_generic_mount_source(struct path *, struct fsinfo_context *);
> +extern int fsinfo_generic_mount_info(struct path *, struct fsinfo_context *);
> +extern int fsinfo_generic_mount_point(struct path *, struct fsinfo_context *);
> +extern int fsinfo_generic_mount_point_full(struct path *, struct fsinfo_context *);
> +extern int fsinfo_generic_mount_children(struct path *, struct fsinfo_context *);
>
>  /*
>   * fs_struct.c
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 54e8eb93fdd6..a6cb8c6b004f 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -4149,4 +4149,181 @@ int lookup_mount_object(struct path *root, int mnt_id, struct path *_mntpt)
>         goto out_unlock;
>  }
>
> +/*
> + * Retrieve information about the nominated mount.
> + */
> +int fsinfo_generic_mount_info(struct path *path, struct fsinfo_context *ctx)
> +{
> +       struct fsinfo_mount_info *p = ctx->buffer;
> +       struct super_block *sb;
> +       struct mount *m;
> +       struct path root;
> +       unsigned int flags;
> +
> +       m = real_mount(path->mnt);
> +       sb = m->mnt.mnt_sb;
> +
> +       p->sb_unique_id         = sb->s_unique_id;
> +       p->mnt_unique_id        = m->mnt_unique_id;
> +       p->mnt_id               = m->mnt_id;
> +       p->parent_id            = m->mnt_parent->mnt_id;
> +
> +       get_fs_root(current->fs, &root);
> +       if (path->mnt == root.mnt) {
> +               p->parent_id = p->mnt_id;
> +       } else {
> +               rcu_read_lock();
> +               if (!are_paths_connected(&root, path))
> +                       p->parent_id = p->mnt_id;
> +               rcu_read_unlock();
> +       }
> +       if (IS_MNT_SHARED(m))
> +               p->group_id = m->mnt_group_id;
> +       if (IS_MNT_SLAVE(m)) {
> +               int master = m->mnt_master->mnt_group_id;
> +               int dom = get_dominating_id(m, &root);

This isn't safe without namespace_sem or mount_lock.

Thanks,
Miklos