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

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Date: Mon, 11 Mar 2024 19:01:27 +0100
From: Jan Kara <jack@...e.cz>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: Christian Brauner <brauner@...nel.org>,
	Luis Henriques <lhenriques@...e.de>, Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
	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 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.

> > 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.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR