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
