Re: [PATCH 03/14] VFS: Implement a filesystem superblock creation/configuration context [ver #6]

[Miklos Szeredi]

On Fri, Oct 27, 2017 at 6:03 PM, David Howells <dhowells@xxxxxxxxxx> wrote:
> Miklos Szeredi <mszeredi@xxxxxxxxxx> wrote:
>
>> Yes I did mean vfs_parse_sb_flag_option().
>>
>> Yes, I understand its purpose, but it would be cleaner if all the
>> option parsing was done in fc->ops->parse_option().
>>
>> It might be worth introducing the vfs_parse_sb_flag_option(), to be
>> called from ->parse_option().
>
> I was trying to relieve the filesystem of the requirement to have to deal with
> common stuff and also the need to talk directly to the LSM.

No need to talk directly to the LSM:
security_fs_context_parse_option() will do that in VFS code.

How common is common stuff?

dirsync/sync/rw: not handled by all filesystems, those that don't
handle it should reject the option on the new interface

lazytime: handled by generic code, AFAICS, but makes no sense on
read-only fs so those should probably reject it

mand: handled by generic code, but does not make sense for some
filesystems (e.g. those that don't have all the unixy permission
bits).

posixacl: there's no such mount option now.  The options is "acl" and
does not get translated to MS_POSIXACL in mount(8).   Makes zero sense
to add a previously nonexistent option to the new interface.

silent: makes no sense on the new interface, since we should no longer
be putting error messages into the kernel log.

So that leaves async/ro/nolazytime/nomand options to be handled by all
filesystems.

Not sure how to best handle these differences, but the current code
definitely seems lacking, and I cannot imagine a better way than to
pass all options to filesystem's ->parse_option() and add helper(s) to
handle the generic options.

>> > Btw, how would it affect the LSM?
>>
>> LSM would have to reject a "reset" if not enough privileges to
>> *create* a new fs instance, since it essentially requires creating a
>> new config, which is what is done when creating an fs instance.
>
> That's not what I'm asking.  Would the reset change LSM state?  Reset security
> labels and options?

No.   And it wouldn't reset any other option that is immutable (e.g.
server IP address).

Thanks,
Miklos