linux-kernel - Re: file metadata via fs API

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Date:   Tue, 18 Aug 2020 11:51:25 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Steven Whitehouse <swhiteho@...hat.com>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>, Karel Zak <kzak@...hat.com>,
        Jeff Layton <jlayton@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Christian Brauner <christian@...uner.io>,
        Lennart Poettering <lennart@...ttering.net>,
        Linux API <linux-api@...r.kernel.org>,
        Ian Kent <raven@...maw.net>,
        LSM <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: file metadata via fs API

On Tue, Aug 18, 2020 at 5:50 AM Miklos Szeredi <miklos@...redi.hu> wrote:
>
> How do you propose handling variable size attributes, like the list of
> fs options?

I really REALLY think those things should just be ASCII data.

I think marshalling binary data is actively evil and wrong. It's great
for well-specified wire protocols. It's great for internal
communication in user space. It's *NOT* great for a kernel system call
interface.

One single simple binary structure? Sure. That's how system calls
work. I'm not claiming that things like "stat()" are wrong because
they take binary data.

But marshalling random binary structures into some buffer? Let's avoid
that. Particularly for these kinds of fairly free-form things like fs
options.

Those things *are* strings, most of them. Exactly because it needs a
level of flexibility that binary data just doesn't have.

So I'd suggest something that is very much like "statfsat()", which
gets a buffer and a length, and returns an extended "struct statfs"
*AND* just a string description at the end.

And if you don't pass a sufficiently big buffer, it will not do some
random "continuations". No state between system calls. It gets
truncated, and you need to pass a bigger buffer, kind of like
"snprintf()".

I think people who have problems parsing plain ASCII text are just
wrong. It's not that expensive. The thing that makes /proc/mounts
expensive is not the individual lines - it's that there are a lot of
them.

                     Linus