linux-kernel - Re: [RFC PATCH] getvalues(2) prototype

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Date:   Wed, 23 Mar 2022 15:39:36 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Christian Brauner <brauner@...nel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>,
        LSM <linux-security-module@...r.kernel.org>,
        Karel Zak <kzak@...hat.com>, Ian Kent <raven@...maw.net>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        Amir Goldstein <amir73il@...il.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [RFC PATCH] getvalues(2) prototype

On 3/23/2022 7:00 AM, Miklos Szeredi wrote:
> On Wed, 23 Mar 2022 at 14:51, Casey Schaufler <casey@...aufler-ca.com> wrote:
>
>> You also need a way to get a list off what attributes are available
>> and/or a way to get all available attributes. Applications and especially
>> libraries shouldn't have to guess what information is relevant. If the
>> attributes change depending on the filesystem and/or LSM involved, and
>> they do, how can a general purpose library function know what data to
>> ask for?
> Oh, yes.  Even the current prototype does that:
>
> # ~/getvalues / ""
> [] = "mnt" "mntns" "xattr" "data" (len=21)
> # ~/getvalues / "mnt"
> [mnt] = "id" "parentid" "root" "mountpoint" "options" "shared"
> "master" "propagate_from" "unbindable" (len=76)
> # ~/getvalues / "mntns"
> [mntns] = "21" "22" "24" "25" "23" "26" "27" "28" "29" "30" "31" "32" (len=36)
>   ~/getvalues / "mntns:21"
> [mntns:21] = "id" "parentid" "root" "mountpoint" "options" "shared"
> "master" "propagate_from" "unbindable" (len=76)

That requires multiple calls and hierarchy tracking by the caller.
Not to mention that in this case the caller needs to understand
how mount namespaces are being used. I don't see that you've made
anything cleaner. You have discarded the type checking provided
by the "classic" APIs. Elsewhere in this thread the claims of
improved performance have been questioned, but I can't say boo
about that. Is this interface targeted for languages other than C
for which the paradigm might provide (more?) value?

>
> I didn't implement enumeration for "data" and "xattr" but that is
> certainly possible and not even difficult to do.
>
> Thanks,
> Miklos