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

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Message-ID: <87le6p6oqe.fsf@suse.de>
Date: Mon, 11 Mar 2024 10:34:49 +0000
From: Luis Henriques <lhenriques@...e.de>
To: Miklos Szeredi <miklos@...redi.hu>
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

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'.

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.

Cheers,
-- 
Luís