Re: [PATCH 1/2] move offsetofend() from vfio.h to stddef.h

[Ingo Molnar <mingo@kernel.org>]

* Denys Vlasenko <vda.linux@googlemail.com> wrote:

> On Mon, Mar 9, 2015 at 3:58 PM, Ingo Molnar <mingo@kernel.org> wrote:
> >> -/**
> >> - * offsetofend(TYPE, MEMBER)
> >> - *
> >> - * @TYPE: The type of the structure
> >> - * @MEMBER: The member within the structure to get the end offset of
> >> - *
> >> - * Simple helper macro for dealing with variable sized structures passed
> >> - * from user space.  This allows us to easily determine if the provided
> >> - * structure is sized to include various fields.
> >> - */
> >> -#define offsetofend(TYPE, MEMBER) \
> >> -     (offsetof(TYPE, MEMBER) + sizeof(((TYPE *)0)->MEMBER))
> >
> > So I like it, and because it is not particularly trivial when to use
> > this primitive it was explained nicely in a description in the vfio.h
> > version.
> >
> > But you lost that nice description during the code move!!
> 
> That description was clearly specific to how that macro is used in
> drivers/vfio/*.c, along the lines of
> 
>                 minsz = offsetofend(struct vfio_eeh_pe_op, op);

Hm, but here 'minsz' == sizeof(struct vfio_eeh_pe_op), so the vfio 
usage does not seem to be justified.

>                 if (copy_from_user(&op, (void __user *)arg, minsz))
>                         return -EFAULT;
>                 if (op.argsz < minsz || op.flags)
>                         return -EINVAL;
> 
> But the macro is generic, it has many other uses besides this one.

So I might be missing something, but what generic uses does it have, 
beyond structures that have some rare size related weirdness, such as 
alignment attributes? In 99% of the cases:

   sizeof(struct) == offsetofend(struct, last_member)

right?

> Nevertheless, I can resend a version where comment survives if you 
> want...

So maybe extend it to a description that you think describes its uses 
correctly? People will keep wondering about when to use this.

Thanks,

	Ingo