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

[Vivek Goyal]

On Tue, Aug 10, 2021 at 05:26:15PM +0200, Hanna Reitz wrote:
> On 10.08.21 17:23, Vivek Goyal wrote:
> > On Tue, Aug 10, 2021 at 10:32:55AM +0200, Hanna Reitz wrote:
> > > On 09.08.21 20:41, Vivek Goyal wrote:
> > > > On Fri, Jul 30, 2021 at 05:01:33PM +0200, Max Reitz wrote:
> > > > > When the inode_file_handles option is set, try to generate a file 
> > > > > handle
> > > > > for new inodes instead of opening an O_PATH FD.
> > > > > 
> > > > > Being able to open these again will require CAP_DAC_READ_SEARCH, so 
> > > > > the
> > > > > description text tells the user they will also need to specify
> > > > > -o modcaps=+dac_read_search.
> > > > > 
> > > > > Generating a file handle returns the mount ID it is valid for.  
> > > > > Opening
> > > > > it will require an FD instead.  We have mount_fds to map an ID to an 
> > > > > FD.
> > > > > get_file_handle() fills the hash map by opening the file we have
> > > > > generated a handle for.  To verify that the resulting FD indeed
> > > > > represents the handle's mount ID, we use statx().  Therefore, using 
> > > > > file
> > > > > handles requires statx() support.
> > > > So opening the file and storing that fd in mount_fds table might be
> > > > a potential problem with inotify work Ioannis is doing.
> > > > 
> > > > So say a file foo.txt was opened O_RDONLY and fd stored in mount_fs. Now
> > > > say user unlinks foo.txt. If notifications are enabled, final 
> > > > notification
> > > > will not be generated till this mount_fds fd is closed.
> > > > 
> > > > Now question is when will this fd be closed? If it closed at some
> > > > later point and then notification is generated, that will break
> > > > notificaitons.
> > > Currently, it is never closed.
> > > 
> > > > In fact even O_PATH fd is delaying notifications due to same reason.
> > > > But its not too bad as we close O_PATH fd pretty quickly after
> > > > unlinking. And we were hoping that file handle support will get rid
> > > > of this problem because we will not keep O_PATH fd open.
> > > > 
> > > > But, IIUC, mount_fds stuff will make it even worse. I did not see
> > > > the code which removes this fd from mount_fds. So I am not sure what's
> > > > the life time of this fd.
> > > The lifetime is forever.  If we wanted to remove it at some point, we’d 
> > > need
> > > to track how many file handles we have open for the given mount fd and 
> > > then
> > > remove it from the table once the count reaches 0, so it would still be
> > > delayed.
> > > 
> > > I think in practice the first thing that is looked up from some mount will
> > > probably be the root directory, which cannot be deleted before everything
> > > else on the mount is gone, so that would work.  We track how many handles
> > > are there, if the whole mount were to be deleted, I hope all lo_inodes are
> > > evicted, the count goes to 0, and we can drop the mount fd.
> > Keeping a reference count on mount_fd object make sense. So we probably
> > maintain this hash table and lookup using mount_id (as you are already
> > doing). All subsequent inodes from same filesystem will use same
> > object. Once all inodes have been flushed out, then mount_fd object
> > should go away as well (allowing for unmount on host).
> > 
> > > I think we can make the assumption that the mount fd is the root directory
> > > certain by, well, looking into mountinfo...  That would result in us 
> > > always
> > > opening the root node of the filesystem, so that first the whole 
> > > filesystem
> > > needs to disappear before it can be deleted (and our mount fd closed) –
> > > which should work, I guess?
> > This seems more reasonable. And I think that's what man page seems to
> > suggest.
> > 
> >         The  mount_id  argument  returns an identifier for the filesystem 
> > mount
> >         that corresponds to pathname.  This corresponds to the first  field 
> >  in
> >         one  of  the  records in /proc/self/mountinfo.  Opening the 
> > pathname in
> >         the fifth field of that record yields a file descriptor for  the  
> > mount
> >         point;  that  file  descriptor  can  be  used  in  a subsequent 
> > call to
> >         open_by_handle_at().
> > 
> > Fifth field seems to be the mount point. man proc says.
> > 
> >                (5)  mount  point:  the  pathname of the mount point 
> > relative to
> >                     the process's root directory.
> > 
> > So opening mount point and saving as mount_fd (if it is not already
> > in hash table) and then take a per inode reference count on mount_fd
> > object looks like will solve the life time issue of mount_fd as
> > well as the issue of temporary failures arising because we can't
> > open a device special file.
> 
> Well, we’ve had this discussion before, and it’s possible that a filesystem
> has a device file as its mount point.

Yes. I think you did modified fuse to do some special trickery. Not sure
where should that be fixed. 

If filesystem is faking, then it can fake a device node as regular
file and fool us into opening it as well?

> 
> 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?

Vivek