Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <d0894565-9783-b398-0faf-60bfb96837ce@schaufler-ca.com>
Date:   Wed, 23 Mar 2022 15:39:36 -0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101
 Thunderbird/91.7.0
Subject: Re: [RFC PATCH] getvalues(2) prototype
Content-Language: en-US
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     Christian Brauner <brauner@kernel.org>,
        Miklos Szeredi <mszeredi@redhat.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Linux API <linux-api@vger.kernel.org>,
        linux-man <linux-man@vger.kernel.org>,
        LSM <linux-security-module@vger.kernel.org>,
        Karel Zak <kzak@redhat.com>, Ian Kent <raven@themaw.net>,
        David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <christian@brauner.io>,
        Amir Goldstein <amir73il@gmail.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        Casey Schaufler <casey@schaufler-ca.com>
References: <20220322192712.709170-1-mszeredi@redhat.com>
 <20220323114215.pfrxy2b6vsvqig6a@wittgenstein>
 <CAJfpegsCKEx41KA1S2QJ9gX9BEBG4_d8igA0DT66GFH2ZanspA@mail.gmail.com>
 <d3333dbe-b4b7-8eb9-4a50-8526d95b5394@schaufler-ca.com>
 <CAJfpegvwTmaw0bp70-nYQAvs8T=wYyxnDEoA=rOvX8HDZnxCTg@mail.gmail.com>
From:   Casey Schaufler <casey@schaufler-ca.com>
In-Reply-To: <CAJfpegvwTmaw0bp70-nYQAvs8T=wYyxnDEoA=rOvX8HDZnxCTg@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 3/23/2022 7:00 AM, Miklos Szeredi wrote:
> On Wed, 23 Mar 2022 at 14:51, Casey Schaufler <casey@schaufler-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