linux-kernel - Re: [PATCH 11/19] afs: Support fsinfo() [ver #16]

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Date:   Thu, 20 Feb 2020 12:58:52 +0000
From:   David Howells <dhowells@...hat.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
        raven@...maw.net, Miklos Szeredi <mszeredi@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/19] afs: Support fsinfo() [ver #16]

Jann Horn <jannh@...gle.com> wrote:

> Ewww. So basically, having one static set of .fsinfo_attributes is not
> sufficiently flexible for everyone, but instead of allowing the
> filesystem to dynamically provide a list of supported attributes, you
> just duplicate the super_operations? Seems to me like it'd be cleaner
> to add a function pointer to the super_operations that can dynamically
> fill out the supported fsinfo attributes.
>
> It seems to me like the current API is going to be a dead end if you
> ever want to have decent passthrough of these things for e.g. FUSE, or
> overlayfs, or VirtFS?

Ummm...

Would it be sufficient to have a function that returns a list of attributes?
Or does it need to be able to call to vfs_do_fsinfo() if it supports an
attribute?

There are two things I want to be able to do:

 (1) Do the buffer wrangling in the core - which means the core needs to see
     the type of the attribute.  That's fine if, say, afs_fsinfo() can call
     vfs_do_fsinfo() with the definition for any attribute it wants to handle
     and, say, return -ENOPKG otherways so that the core can then fall back to
     its private list.

 (2) Be able to retrieve the list of attributes and/or query an attribute.
     Now, I can probably manage this even through the same interface.  If,
     say, seeing FSINFO_ATTR_FSINFO_ATTRIBUTES causes the handler to simply
     append on the IDs of its own supported attributes (a helper can be
     provided for that).

     If it sees FSINFO_ATR_FSINFO_ATTRIBUTE_INFO, it can just look to see if
     it has the attribute with the ID matching Nth and return that, else
     ENOPKG - again a helper could be provided.

Chaining through overlayfs gets tricky.  You end up with multiple contributory
filesystems with different properties - and any one of those layers could
perhaps be another overlay.  Overlayfs would probably needs to integrate the
info and derive the lowest common set.

David