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

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Date:   Thu, 04 May 2017 11:22:22 +0100
From:   David Howells <dhowells@...hat.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     dhowells@...hat.com, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, mszeredi@...hat.com
Subject: Re: [PATCH 3/9] VFS: Introduce a mount context

Rasmus Villemoes <linux@...musvillemoes.dk> 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.

David