Re: User-visible context-mount API

[Miklos Szeredi <mszeredi-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Tue, Jan 16, 2018 at 4:40 PM, David Howells <dhowells-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> Miklos Szeredi <mszeredi-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
>
>> >  (6) Adjust a mountpoint's topology flags:
>> >
>> >         mount_set_topology(int dfd, const char *path,
>> >                            unsigned int topology_flags);
>> >
>> >  (7) Reconfigure a mountpoint:
>> >
>> >         mount_reconfigure(int dfd, const char *path,
>> >                           unsigned int mount_flags);
>>
>>
>> What's the fundamental  difference between topology flags and other
>> flags?  Why two syscalls?
>
> Actually, if you look at the do_mount() function, these *are* separate
> things.  If you look at the code:
>
>         if (flags & MS_REMOUNT)
>                 retval = do_remount(&path, flags, sb_flags, mnt_flags,
>                                     data_page);
>
> ^^^ that is mount_reconfigure() and superblock reconfiguration.
>
>         else if (flags & MS_BIND)
>                 retval = do_loopback(&path, dev_name, flags & MS_REC);
>         else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
>                 retval = do_change_type(&path, flags);
>
> ^^^ that is mount_set_topology().
>
>         else if (flags & MS_MOVE)
>                 retval = do_move_mount(&path, dev_name);
>         else
>                 retval = do_new_mount(&path, type_page, sb_flags, mnt_flags,
>                                       dev_name, data_page);
>
> The mount() syscall is actually a function switch with five functions.

Right.

Still, those two (propagation and flags) are properties of the mount.
No fundamental difference in how to handle them, that I see.  Okay, we
have MS_REC handling in the propagation and not in the flags, but
that's something that might make sense for flags as well.

What's more interesting is how MS_PRIVATE + MS_REC semantics are
complete failure in the real world: the logical thing would be to mark
a mount private on the supplied mount AND propagate an umount event to
everywhere else.  But that's not what we do, we completely mess up
propagation that can result in millions of "leaked" mounts.  I've seen
multiple bug reports where this illogical behavior fooled people.  It
can be worked around, of course, but maybe we should fix this on the
new interface and allow a sane private setting.

Thanks,
Miklos


>
> David