linux-kernel - Re: [PATCH] fs: avoid empty option when generating legacy mount string

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Message-ID: <d5377e0b-b02e-40dd-945b-368a3b8c05ce@t-8ch.de>
Date:   Wed, 7 Jun 2023 21:44:35 +0200
From:   Thomas Weißschuh <thomas@...ch.de>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        David Howells <dhowells@...hat.com>,
        Karel Zak <kzag@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH] fs: avoid empty option when generating legacy mount
 string

On 2023-06-07 21:39:01+0200, Christian Brauner wrote:
> On Wed, Jun 07, 2023 at 07:28:48PM +0200, Thomas Weißschuh wrote:
> > As each option string fragment is always prepended with a comma it would
> > happen that the whole string always starts with a comma.
> > This could be interpreted by filesystem drivers as an empty option and
> > may produce errors.
> > 
> > For example the NTFS driver from ntfs.ko behaves like this and fails when
> > mounted via the new API.
> > 
> > Link: https://github.com/util-linux/util-linux/issues/2298
> 
> Yeah, the old ntfs driver implements its own option parser. It
> overwrites/splits at ',' returning '\0' and then trips over this.
> 
> Contrast with e.g., ovl_next_op() which does the same thing but skips
> over '\0' in ovl_parse_opt().
> 
> So arguably also a bug in ntfs parsing. But there's no reason we should
> prepend ',' for legacy mount option strings.
> 
> And yeah, I can easily verify this...
> 
> Using my custom move-mount tool I originally wrote for another patchset
> but which is handy to pass mount options via the new mount api _system_
> calls and not via mount():
> https://github.com/brauner/move-mount-beneath
> 
> I can do:
> 
>         sudo ./move-mount -f overlay -olowerdir=/mnt/a:/mnt/b,upperdir=/mnt/upper,workdir=/mnt/work /mnt/merged
> 
> and clearly see:
> 
>         > sudo bpftrace -e 'kfunc:legacy_get_tree { @m = args->fc; printf("%s\n", str(((struct legacy_fs_context *)@m->fs_private)->legacy_data)); }'
>         Attaching 1 probe...
>         ,lowerdir=/mnt/a:/mnt/b,upperdir=/mnt/upper,workdir=/mnt/work
> 
> > Fixes: 3e1aeb00e6d1 ("vfs: Implement a filesystem superblock creation/configuration context")
> 
> Should be:
> 
> Fixes: commit 3e1aeb00e6d1 ("vfs: Implement a filesystem superblock creation/configuration context")

AFAIK the Fixes: tag does not use the "commit" keyword. Only inline
commit references.

This is how it's currently documented in
Documentation/process/submitting-patches.rst.

> and misses a:
> 
> Cc: stable@...r.kernel.org

This was fixed in v2.

> I'll fix this up for you though.

Thanks!


Thomas