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

[Christian Brauner]

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")

and misses a:

Cc: stable@xxxxxxxxxxxxxxx

I'll fix this up for you though.