linux-kernel - Re: [PATCH 06/18] fsinfo: Add a uniquifier ID to struct mount [ver #21]

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Date:   Tue, 4 Aug 2020 12:41:08 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     David Howells <dhowells@...hat.com>
Cc:     viro@...iv.linux.org.uk, torvalds@...ux-foundation.org,
        raven@...maw.net, mszeredi@...hat.com, christian@...uner.io,
        jannh@...gle.com, darrick.wong@...cle.com, kzak@...hat.com,
        jlayton@...hat.com, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/18] fsinfo: Add a uniquifier ID to struct mount [ver
 #21]

On Mon, Aug 03, 2020 at 02:37:16PM +0100, David Howells wrote:
> Add a uniquifier ID to struct mount that is effectively unique over the
> kernel lifetime to deal around mnt_id values being reused.  This can then
> be exported through fsinfo() to allow detection of replacement mounts that
> happen to end up with the same mount ID.
> 
> The normal mount handle is still used for referring to a particular mount.
> 
> The mount notification is then changed to convey these unique mount IDs
> rather than the mount handle.
> 
> Signed-off-by: David Howells <dhowells@...hat.com>
> ---
> 
>  fs/mount.h        |    3 +++
>  fs/mount_notify.c |    4 ++--
>  fs/namespace.c    |    3 +++
>  3 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/mount.h b/fs/mount.h
> index 85456a5f5a3a..1037781be055 100644
> --- a/fs/mount.h
> +++ b/fs/mount.h
> @@ -79,6 +79,9 @@ struct mount {
>  	int mnt_expiry_mark;		/* true if marked for expiry */
>  	struct hlist_head mnt_pins;
>  	struct hlist_head mnt_stuck_children;
> +#ifdef CONFIG_FSINFO
> +	u64	mnt_unique_id;		/* ID unique over lifetime of kernel */
> +#endif

Not sure if it's worth making conditional.

>  #ifdef CONFIG_MOUNT_NOTIFICATIONS
>  	struct watch_list *mnt_watchers; /* Watches on dentries within this mount */
>  #endif
> diff --git a/fs/mount_notify.c b/fs/mount_notify.c
> index 44f570e4cebe..d8ba66ed5f77 100644
> --- a/fs/mount_notify.c
> +++ b/fs/mount_notify.c
> @@ -90,7 +90,7 @@ void notify_mount(struct mount *trigger,
>  	n.watch.type	= WATCH_TYPE_MOUNT_NOTIFY;
>  	n.watch.subtype	= subtype;
>  	n.watch.info	= info_flags | watch_sizeof(n);
> -	n.triggered_on	= trigger->mnt_id;
> +	n.triggered_on	= trigger->mnt_unique_id;
>  
>  	switch (subtype) {
>  	case NOTIFY_MOUNT_EXPIRY:
> @@ -102,7 +102,7 @@ void notify_mount(struct mount *trigger,
>  	case NOTIFY_MOUNT_UNMOUNT:
>  	case NOTIFY_MOUNT_MOVE_FROM:
>  	case NOTIFY_MOUNT_MOVE_TO:
> -		n.auxiliary_mount	= aux->mnt_id;
> +		n.auxiliary_mount = aux->mnt_unique_id;

Hmm, so we now have two ID's:

 - one can be used to look up the mount
 - one is guaranteed to be unique

With this change the mount cannot be looked up with FSINFO_FLAGS_QUERY_MOUNT,
right?

Should we be merging the two ID's into a single one which has both properties?

>  		break;
>  
>  	default:
> diff --git a/fs/namespace.c b/fs/namespace.c
> index b2b9920ffd3c..1db8a64cd76f 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -115,6 +115,9 @@ static int mnt_alloc_id(struct mount *mnt)
>  	if (res < 0)
>  		return res;
>  	mnt->mnt_id = res;
> +#ifdef CONFIG_FSINFO
> +	mnt->mnt_unique_id = atomic64_inc_return(&vfs_unique_counter);
> +#endif
>  	return 0;
>  }
>  
> 
>