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

[Max Reitz]

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.

Max

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