[Virtio-fs] [RFC 2/2] virtiofsd: Set st_rdev for sub-mount points

[Max Reitz]

On 07.05.20 16:29, Miklos Szeredi wrote:
> On Thu, May 7, 2020 at 2:18 PM Max Reitz <mreitz at redhat.com> wrote:
>>
>> On 07.05.20 13:43, Miklos Szeredi wrote:
>>> On Thu, May 7, 2020 at 12:56 PM Max Reitz <mreitz at redhat.com> wrote:
>>>>
>>>> On 06.05.20 18:04, Dr. David Alan Gilbert wrote:
>>>>> * Max Reitz (mreitz at redhat.com) wrote:
>>>>>> Whenever we encounter a directory with an st_dev that differs from that
>>>>>> of its parent, we set st_rdev accordingly so the guest can create a
>>>>>> submount for it.
>>>>>>
>>>>>> Make this behavior optional, so submounts are only announced to the
>>>>>> guest with the announce_submounts option.  Some users may prefer the
>>>>>> current behavior, so that the guest learns nothing about the host mount
>>>>>> structure.
>>>>>>
>>>>>> Signed-off-by: Max Reitz <mreitz at redhat.com>
>>>>>
>>>>> Does this need to be wired to a flag in the INIT message (like say
>>>>> FUSE_ASYNC_READ) to indicate that the kernel/daemon supports this, or is
>>>>> it really safe just to start sending the changed rdev?
>>>>
>>>> I supposed it to be safe, given that rdev is never used for anything but
>>>> device files, so basically it was just a reserved field for directories.
>>>
>>> You assume only directories can be mounted, but that's not so.  A
>>> device can also be the source or destination of a bind mount.
>>
>> Well, I mostly thought this case wouldn’t be too important.  But I
>> suppose it doesn’t make sense to decide now that it’s never going to be
>> important.
>>
>>> We need to extend fuse_entry_out with a flags field.  We could
>>> possibly reuse the upper bits of entry_valid (i.e. timeouts above say
>>> INT_MAX seconds could be taken as  "infinity").  Unfortunately that's
>>> not as easy as splitting it into two fields due to endianness :(
>>
>> For the time being, a single bit would be sufficient to signify whether
>> there should be a submount at all.  We could add a field with a submount
>> ID (i.e., st_dev from the host) in the future when we decide we
>> want/need to ensure that two inodes with the same st_dev on the host
>> also have the same st_dev in the guest.
> 
> Hmm, do we need a separate bit, then?  Couldn't the kernel decide
> based on st_dev whether a new submount is needed?  I.e. return root
> st_dev in INIT reply, store it in root fuse_mount and lookups compare
> attr.dev to fm->st_dev.

That was the idea, but there is no fuse_attr.dev, which is why I
(ab)used fuse_attr.rdev instead.

Max

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