Re: read only bind mount ignores ready only

[Miklos Szeredi <miklos@szeredi.hu>]

On Thu, Dec 12, 2013 at 1:05 PM, Karel Zak <kzak@redhat.com> wrote:
>
>  [CC: kernel guys]
>
> On Wed, Dec 11, 2013 at 09:37:57AM -0500, Phillip Susi wrote:
>> It seems that the kernel has a bug where it silently ignores the
>> MS_RDONLY flag when creating a bind mount.  mount issues a warning
>> that the mount point appears to be read-write even though you
>> requested read only.  The reporter suggests a patch to automatically
>> attempt to remount with MS_RDONLY before issuing this warning to work
>> around the kernel bug.  What do you think?
>
> I have it implemented, so
>
>  mount --bind --read-only /mnt /mnt
>
> is interpreted as two requests (two mount(2) calls)
>
>  mount --bind /mnt /mnt
>  mount -o remount,bind,ro /tmp
>
> it works as expected, but it does not work with MS_REC (recursive)
> because kernel currently does not support
>
>   MS_REMOUNT|MS_BIND|MS_REC|...
>
> it means that
>
>   mount --rbind --read-only /mnt /mnt
>
> creates only top-level read-only mountpoint, the rest is unchanged.
>
>
> Miklos would be possible to fix kernel to accept MS_REC for
> MS_REMOUNT|MS_BIND|MS_RDONLY operation? Please.

I really hate the current mount(2) API. It's a gigantic hack, and it's
nearing the end of its life anyway due to flags running out.

So instead of adding more hacks, I think it would be better to think
about adding a couple of syscalls that have clearly defined semantics.

Thanks,
Miklos