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.
>
> 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).
Vivek
