Re: [PATCH 3/9] VFS: Introduce a mount context

From: David Howells
Date: Thu May 04 2017 - 06:22:36 EST

Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> wrote:

> > + if (fs_type->fsopen && fs_type->mc_size < sizeof(*mc))
> > + BUG();
> So ->mc_size can be 0 (i.e. not explicitly initialized) if fs_type does
> not have ->fsopen. OK.

I need to be able to handle filesystems that don't support this yet. Once all
filesystems support this, I would be able to take away the max_t() thing.

> > + if (!src->ops->dup)
> > + return ERR_PTR(-ENOTSUPP);
> > +
> > + mc = kmemdup(src, src->fs_type->mc_size, GFP_KERNEL);
> So this assumes that vfs_dup_mount_context is only used if ->mc_size is
> explicitly initialized. A max_t here as well probably wouldn't hurt.

If you don't provide an ->fsopen() op, you can't set src->ops, you don't see a
mount context and you can't call this function. If you did supply an
->fsopen() op, the BUG() would've got you if you didn't set ->mc_size.

> > + unsigned short mc_size; /* Size of mount context to allocate */
> Any particular reason to use a short? The struct doesn't pack any better.

But it leaves a hole someone else can use. I try not to use fields larger
than I need to.