linux-ext4 - Re: [PATCH v2 3/3] ovl: fix the parsing of empty string mount parameters

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Date: Tue, 12 Mar 2024 09:50:46 +0100
From: Christian Brauner <brauner@...nel.org>
To: Jan Kara <jack@...e.cz>
Cc: Miklos Szeredi <miklos@...redi.hu>, 
	Luis Henriques <lhenriques@...e.de>, Theodore Ts'o <tytso@....edu>, 
	Andreas Dilger <adilger.kernel@...ger.ca>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Amir Goldstein <amir73il@...il.com>, linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] ovl: fix the parsing of empty string mount
 parameters

On Mon, Mar 11, 2024 at 07:01:27PM +0100, Jan Kara wrote:
> On Mon 11-03-24 15:39:39, Miklos Szeredi wrote:
> > On Mon, 11 Mar 2024 at 14:25, Christian Brauner <brauner@...nel.org> wrote:
> > 
> > > Yeah, so with that I do agree. But have you read my reply to the other
> > > thread? I'd like to hear your thoughs on that. The problem is that
> > > mount(8) currently does:
> > >
> > > fsconfig(3, FSCONFIG_SET_FLAG, "usrjquota", NULL, 0) = -1 EINVAL (Invalid argument)
> > >
> > > for both -o usrjquota and -o usrjquota=
> > 
> > For "-o usrjquota" this seems right.
> > 
> > For "-o usrjquota=" it doesn't.  Flags should never have that "=", so
> > this seems buggy in more than one ways.
> > 
> > > So we need a clear contract with userspace or the in-kernel solution
> > > proposed here. I see the following options:
> > >
> > > (1) Userspace must know that mount options such as "usrjquota" that can
> > >     have no value must be specified as "usrjquota=" when passed to
> > >     mount(8). This in turn means we need to tell Karel to update
> > >     mount(8) to recognize this and infer from "usrjquota=" that it must
> > >     be passed as FSCONFIG_SET_STRING.
> > 
> > Yes, this is what I'm thinking.  Of course this only works if there
> > are no backward compatibility issues, if "-o usrjquota" worked in the
> > past and some systems out there relied on this, then this is not
> > sufficient.
> 
> No, "-o usrjquota" never worked and I'm inclined to keep refusing this
> variant as IMHO it is confusing.

Tbh, I'm not too sure that having empty string options was a good idea
even though it can be useful. I think it would've been better if we had
used a specific phantom value to signify this. But yes, I just filed an
issue on util-linux to get this fixed. I think we should also
util-linux and Karel's up for handling this.

> 
> > > In any case, we need to document what we want:
> > >
> > > https://github.com/brauner/man-pages-md/blob/main/fsconfig.md
> > 
> > What's the plan with these?  It would be good if "man fsconfig" would
> > finally work.
> 
> Yes, merging these into official manpages would be nice.

I'll try to get around to it.