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

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Date:   Fri, 21 Feb 2020 13:26:51 +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:

> Hm - I guess just returning a list of attributes ought to be fine?
> Then AFS can just return one of its two statically-allocated attribute
> lists there, and a filesystem with more complicated circumstances
> (like FUSE or overlayfs or whatever) can compute a heap-allocated list
> on mount that is freed when the superblock goes away, or something
> like that?

I've changed it so that the core calls into the filesystem with no buffer
allocated first.  If the fs finds an appropriate attribute, it calls a helper
to handle it.  As there's no buffer, this will just return the size.

If the fs doesn't have a handler, it returns -EOPNOTSUPP and the core looks
for a common attribute instead and calls the helper on that if found.

At this point, if a valid length was returned and if userspace didn't specify
a buffer, we just return the proposed size to userspace.

If userspace did specify a buffer, then core will allocate a buffer of the
requested size and call into the filesystem again.  The helper will call the
->get() function to retrieve the value.  The ->get() function returns the
size.

If the returned size exceeds the buffer size, a bigger buffer will be
allocated and it will repeat the last step.

A simple example looks like:

	int ext4_fsinfo(struct path *path, struct fsinfo_context *ctx)
	{
		return fsinfo_get_attribute(path, ctx, ext4_fsinfo_attributes);
	}

where the ext4_fsinfo_attributes is an array of attribute defs.  The helper,
fsinfo_get_attribute() scans the list.  The helper can be called multiple
times if there's more than one list to process.  The caller should stop if one
doesn't return -EOPNOTSUPP.

When the attribute IDs are being listed, the helper will detect that and just
add all the IDs to the list, returning -EOPNOTSUPP when it's done so that all
the attributes get listed.

When the metadata for an attribute is being retrieved, the helper detects that
and searches the given table for that attribute.  If it finds it, it will
return information about that attribute rather than calling the attribute
helper.

David