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 11:53:03 +0100
From: Miklos Szeredi <miklos@...redi.hu>
To: Luis Henriques <lhenriques@...e.de>
Cc: "Theodore Ts'o" <tytso@....edu>, Andreas Dilger <adilger.kernel@...ger.ca>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, 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 Mar 2024 at 11:34, Luis Henriques <lhenriques@...e.de> wrote:
>
> Miklos Szeredi <miklos@...redi.hu> writes:
>
> > On Thu, 7 Mar 2024 at 19:17, Luis Henriques <lhenriques@...e.de> wrote:
> >>
> >> This patch fixes the usage of mount parameters that are defined as strings
> >> but which can be empty.  Currently, only 'lowerdir' parameter is in this
> >> situation for overlayfs.  But since userspace can pass it in as 'flag'
> >> type (when it doesn't have a value), the parsing will fail because a
> >> 'string' type is assumed.
> >
> > I don't really get why allowing a flag value instead of an empty
> > string value is fixing anything.
> >
> > It just makes the API more liberal, but for what gain?
>
> The point is that userspace may be passing this parameter as a flag and
> not as a string.  I came across this issue with ext4, by doing something
> as simple as:
>
>     mount -t ext4 -o usrjquota= /dev/sda1 /mnt/
>
> (actually, the trigger was fstest ext4/053)
>
> The above mount should succeed.  But it fails because 'usrjquota' is set
> to a 'flag' type, not 'string'.

The above looks like a misparsing, since the equals sign clearly
indicates that this is not a flag.

> Note that I couldn't find a way to reproduce the same issue in overlayfs
> with this 'lowerdir' parameter.  But looking at the code the issue is
> similar.

In overlayfs the empty lowerdir parameter has a special meaning when
lowerdirs are appended instead of parsed in one go.   As such it won't
be used from /etc/fstab for example, as that would just result in a
failed mount.

I don't see a reason to allow it as a flag for overlayfs, since that
just add ambiguity to the API.

Thanks,
Miklos