linux-kernel - Re: [PATCH] vfs: ensure mount source is set to "none" if empty string specified

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Message-ID: <3A273752-B947-4272-BA3D-6DA33253A4C4@oracle.com>
Date: Tue, 16 Jul 2024 23:12:38 +0000
From: Prakash Sangappa <prakash.sangappa@...cle.com>
To: Christian Brauner <brauner@...nel.org>
CC: "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "viro@...iv.linux.org.uk"
	<viro@...iv.linux.org.uk>
Subject: Re: [PATCH] vfs: ensure mount source is set to "none" if empty string
 specified

> On Jul 15, 2024, at 5:35 AM, Christian Brauner <brauner@...nel.org> wrote:
> 
>> The issue can be easily reproduced.
>> #mount -t tmpfs "" /tmp/tdir
>> #grep "/tmp/tdir" /proc/$$/mountinfo
> 
> The kernel has accepted "" before the new mount api was introduced. So
> the regression was showing "none" when userspace requested "" which got
> fixed. The patch proposed right here would reintroduce the regression:

The commit messages do not indicate that ’none’ caused a regression and was changed back to being empty string again, unless I missed it.

I was under the impression ’none’ was added to address issues with parsing mount entries in /proc/<pid>mountinfo. 
Below was a  patch proposal describing the parsing issues with libmount and other software because the mount source is empty.  Don’t think this patch was merged.

https://patchwork.kernel.org/project/linux-fsdevel/patch/1524109641-45617-1-git-send-email-cofyc.jackson@gmail.com/

Due to changes in v5.4 timeframe the mount entry started showing ’none’ for source name.

Our Database application ran into an issue with libmount  when moving from running on  v5.4* kernel to v5.15* kernel version. 

> 
> (1) 4.15
>    root@b1:~# cat /proc/self/mountinfo | grep mnt
>    386 28 0:52 / /mnt rw,relatime shared:223 - tmpfs  rw
> 
> (2) 5.4
>    root@f1:~# cat /proc/self/mountinfo | grep mnt
>    584 31 0:55 / /mnt rw,relatime shared:336 - tmpfs none rw
> 
> (3) 6.10-rc6
>    root@...alhost:~# cat /proc/self/mountinfo | grep mnt
>    62 130 0:60 / /mnt rw,relatime shared:135 - tmpfs  rw,inode64