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

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Message-ID: <87cys0x5pu.fsf@suse.de>
Date: Mon, 11 Mar 2024 13:23:25 +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 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.

No, not really.  The same thing happens without the '=':

mount -t ext4 -o usrjquota /dev/loop0p1 /mnt/ 
mount: /mnt: wrong fs type, bad option, bad superblock on /dev/loop0p1, missing codepage or helper program, or other error.
       dmesg(1) may have more information after failed mount system call.

The parsing code gets a FSCONFIG_SET_FLAG instead of FSCONFIG_SET_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.
>
> 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.

Fine with me.  But it'd be nice to double-check (by testing) that when
overlayfs gets a 'lowerdir' without a value it really is doing what you'd
expect it to do.

Cheers,
-- 
Luís