Re: Feature bug with the new mount API: no way of doing read only bind mounts

[Christian Brauner <christian.brauner@ubuntu.com>]

On Thu, Nov 21, 2019 at 08:10:39AM +0000, David Howells wrote:
> James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
> 
> > I was looking to use the read only bind mount as a template for
> > reimplementing shiftfs when I discovered that you can't actually create a
> > read only bind mount with the new API.  The problem is that fspick() will
> > only reconfigure the underlying superblock, which you don't want because you
> > only want the bound subtree to become read only and open_tree()/move_mount()
> > doesn't give you any facility to add or change options on the bind.
> 
> You'd use open_tree() with OPEN_TREE_CLONE and possibly AT_RECURSIVE rather
> than fspick().  fspick() is, as you observed, more for reconfiguring the
> superblock.
> 
> What is missing is a mount_setattr() syscall - something like:
> 
> 	mount_setattr(int dfd, const char *path, unsigned int at_flags,
> 		      unsigned int attr_change_mask, unsigned int attrs);
> 
> which would allow what you want to be done like:
> 
> 	fd = open_tree(AT_FDCWD, "/my/source/", OPEN_TREE_CLONE);
> 	mount_setattr(fd, "", AT_EMPTY_PATH | AT_RECURSIVE,
> 		      MOUNT_ATTR_RDONLY, MOUNT_ATTR_RDONLY);
> 	move_mount(fd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
> 
> Christian: you said you wanted to have a look at doing this - is that still
> your intention?

Yes, it is. I can't put an exact time-frame on this rn.
Also, I thought we've agreed a while back that the flags would move into
a struct since mount is gaining flags quickly too. :)

Christian