Re: [PATCH v3 09/10] virtiofsd: Optionally fill lo_inode.fhandle

[Hanna Reitz <hreitz@redhat.com>]

On 16.08.21 21:44, Vivek Goyal wrote:
> On Wed, Aug 11, 2021 at 08:41:18AM +0200, Hanna Reitz wrote:
>
> [..]
>>>> But given the inotify complications, there’s really a good reason we should
>>>> use mountinfo.
>>>>
>>>>>> It’s a bit tricky because our sandboxing prevents easy access to mountinfo,
>>>>>> but if that’s the only way...
>>>>> yes. We already have lo->proc_self_fd. Maybe we need to keep
>>>>> /proc/self/mountinfo open in lo->proc_self_mountinfo. I am assuming
>>>>> that any mount table changes will still be visible despite the fact
>>>>> I have fd open (and don't have to open new fd to notice new mount/unmount
>>>>> changes).
>>>> Well, yes, that was my idea.  Unfortunately, I wasn’t quite successful yet;
>>>> when I tried keeping the fd open, reading from it would just return 0
>>>> bytes.  Perhaps that’s because we bind-mount /proc/self/fd to /proc so that
>>>> nothing else in /proc is visible. Perhaps we need to bind-mount
>>>> /proc/self/mountinfo into /proc/self/fd before that...
>>> Or perhaps open /proc/self/mountinfo and save fd in lo->proc_mountinfo
>>> before /proc/self/fd is bind mounted on /proc?
>> Yes, I tried that, and then reading would just return 0 bytes.
> Hi Hanna,
>
> I tried this simple patch and I can read /proc/self/mountinfo before
> bind mounting /proc/self/fd and after bind mounting /proc/self/fd. Am
> I missing something.

Yes, but I tried reading it in the main loop (where we’d actually need 
it).  It looks like the umount2(".", MNT_DETACH) in setup_mounts() 
breaks it.

Hanna