Re: [PATCH 15/18] fsinfo: Add an attribute that lists all the visible mounts in a namespace [ver #21]

[Miklos Szeredi <miklos@szeredi.hu>]

On Mon, Aug 03, 2020 at 02:38:34PM +0100, David Howells wrote:
> Add a filesystem attribute that exports a list of all the visible mounts in
> a namespace, given the caller's chroot setting.  The returned list is an
> array of:
> 
> 	struct fsinfo_mount_child {
> 		__u64	mnt_unique_id;
> 		__u32	mnt_id;
> 		__u32	parent_id;
> 		__u32	mnt_notify_sum;
> 		__u32	sb_notify_sum;
> 	};
> 
> where each element contains a once-in-a-system-lifetime unique ID, the
> mount ID (which may get reused), the parent mount ID and sums of the
> notification/change counters for the mount and its superblock.

The change counters are currently conditional on CONFIG_MOUNT_NOTIFICATIONS.
Is this is intentional?

> 
> This works with a read lock on the namespace_sem, but ideally would do it
> under the RCU read lock only.
> 
> Signed-off-by: David Howells <dhowells@redhat.com>
> ---
> 
>  fs/fsinfo.c                 |    1 +
>  fs/internal.h               |    1 +
>  fs/namespace.c              |   37 +++++++++++++++++++++++++++++++++++++
>  include/uapi/linux/fsinfo.h |    4 ++++
>  samples/vfs/test-fsinfo.c   |   22 ++++++++++++++++++++++
>  5 files changed, 65 insertions(+)
> 
> diff --git a/fs/fsinfo.c b/fs/fsinfo.c
> index 0540cce89555..f230124ffdf5 100644
> --- a/fs/fsinfo.c
> +++ b/fs/fsinfo.c
> @@ -296,6 +296,7 @@ static const struct fsinfo_attribute fsinfo_common_attributes[] = {
>  	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),
> +	FSINFO_LIST	(FSINFO_ATTR_MOUNT_ALL,		fsinfo_generic_mount_all),
>  	{}
>  };
>  
> diff --git a/fs/internal.h b/fs/internal.h
> index cb5edcc7125a..267b4aaf0271 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -102,6 +102,7 @@ extern int fsinfo_generic_mount_topology(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 *);
> +extern int fsinfo_generic_mount_all(struct path *, struct fsinfo_context *);
>  
>  /*
>   * fs_struct.c
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 122c12f9512b..1f2e06507244 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -4494,4 +4494,41 @@ int fsinfo_generic_mount_children(struct path *path, struct fsinfo_context *ctx)
>  	return ctx->usage;
>  }
>  
> +/*
> + * Return information about all the mounts in the namespace referenced by the
> + * path.
> + */
> +int fsinfo_generic_mount_all(struct path *path, struct fsinfo_context *ctx)
> +{
> +	struct mnt_namespace *ns;
> +	struct mount *m, *p;
> +	struct path chroot;
> +	bool allow;
> +
> +	m = real_mount(path->mnt);
> +	ns = m->mnt_ns;
> +
> +	get_fs_root(current->fs, &chroot);
> +	rcu_read_lock();
> +	allow = are_paths_connected(&chroot, path) || capable(CAP_SYS_ADMIN);
> +	rcu_read_unlock();
> +	path_put(&chroot);
> +	if (!allow)
> +		return -EPERM;
> +
> +	down_read(&namespace_sem);
> +
> +	list_for_each_entry(p, &ns->list, mnt_list) {

This is missing locking and check added by commit 9f6c61f96f2d ("proc/mounts:
add cursor").

> +		struct path mnt_root;
> +
> +		mnt_root.mnt	= &p->mnt;
> +		mnt_root.dentry	= p->mnt.mnt_root;
> +		if (are_paths_connected(path, &mnt_root))
> +			fsinfo_store_mount(ctx, p, p == m);
> +	}
> +
> +	up_read(&namespace_sem);
> +	return ctx->usage;
> +}
> +
>  #endif /* CONFIG_FSINFO */
> diff --git a/include/uapi/linux/fsinfo.h b/include/uapi/linux/fsinfo.h
> index 81329de6905e..e40192d98648 100644
> --- a/include/uapi/linux/fsinfo.h
> +++ b/include/uapi/linux/fsinfo.h
> @@ -37,6 +37,7 @@
>  #define FSINFO_ATTR_MOUNT_POINT_FULL	0x203	/* Absolute path of mount (string) */
>  #define FSINFO_ATTR_MOUNT_TOPOLOGY	0x204	/* Mount object topology */
>  #define FSINFO_ATTR_MOUNT_CHILDREN	0x205	/* Children of this mount (list) */
> +#define FSINFO_ATTR_MOUNT_ALL		0x206	/* List all mounts in a namespace (list) */
>  
>  #define FSINFO_ATTR_AFS_CELL_NAME	0x300	/* AFS cell name (string) */
>  #define FSINFO_ATTR_AFS_SERVER_NAME	0x301	/* Name of the Nth server (string) */
> @@ -128,6 +129,8 @@ struct fsinfo_mount_topology {
>  /*
>   * Information struct element for fsinfo(FSINFO_ATTR_MOUNT_CHILDREN).
>   * - An extra element is placed on the end representing the parent mount.
> + *
> + * Information struct element for fsinfo(FSINFO_ATTR_MOUNT_ALL).
>   */
>  struct fsinfo_mount_child {
>  	__u64	mnt_unique_id;		/* Kernel-lifetime unique mount ID */
> @@ -139,6 +142,7 @@ struct fsinfo_mount_child {
>  };
>  
>  #define FSINFO_ATTR_MOUNT_CHILDREN__STRUCT struct fsinfo_mount_child
> +#define FSINFO_ATTR_MOUNT_ALL__STRUCT struct fsinfo_mount_child
>  
>  /*
>   * Information struct for fsinfo(FSINFO_ATTR_STATFS).
> diff --git a/samples/vfs/test-fsinfo.c b/samples/vfs/test-fsinfo.c
> index 374825ab85b0..596fa5e71762 100644
> --- a/samples/vfs/test-fsinfo.c
> +++ b/samples/vfs/test-fsinfo.c
> @@ -365,6 +365,27 @@ static void dump_fsinfo_generic_mount_children(void *reply, unsigned int size)
>  	       (unsigned long long)r->mnt_notify_sum, mp);
>  }
>  
> +static void dump_fsinfo_generic_mount_all(void *reply, unsigned int size)
> +{
> +	struct fsinfo_mount_child *r = reply;
> +	ssize_t mplen;
> +	char path[32], *mp;
> +
> +	struct fsinfo_params params = {
> +		.flags		= FSINFO_FLAGS_QUERY_MOUNT,
> +		.request	= FSINFO_ATTR_MOUNT_POINT_FULL,
> +	};
> +
> +	sprintf(path, "%u", r->mnt_id);
> +	mplen = get_fsinfo(path, "FSINFO_ATTR_MOUNT_POINT_FULL", &params, (void **)&mp);
> +	if (mplen < 0)
> +		mp = "-";
> +
> +	printf("%5x %5x %12llx %10llu %s\n",
> +	       r->mnt_id, r->parent_id, (unsigned long long)r->mnt_unique_id,
> +	       r->mnt_notify_sum, mp);
> +}
> +
>  static void dump_afs_fsinfo_server_address(void *reply, unsigned int size)
>  {
>  	struct fsinfo_afs_server_address *f = reply;
> @@ -492,6 +513,7 @@ static const struct fsinfo_attribute fsinfo_attributes[] = {
>  	FSINFO_STRING_N	(FSINFO_ATTR_MOUNT_POINT,	string),
>  	FSINFO_STRING_N	(FSINFO_ATTR_MOUNT_POINT_FULL,	string),
>  	FSINFO_LIST	(FSINFO_ATTR_MOUNT_CHILDREN,	fsinfo_generic_mount_children),
> +	FSINFO_LIST	(FSINFO_ATTR_MOUNT_ALL,		fsinfo_generic_mount_all),
>  
>  	FSINFO_STRING	(FSINFO_ATTR_AFS_CELL_NAME,	string),
>  	FSINFO_STRING	(FSINFO_ATTR_AFS_SERVER_NAME,	string),
> 
>