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

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 2015-03-09 at 16:28 +0100, Ingo Molnar wrote:
> * 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?

The idea in the vfio code is to allow the structure to evolve over time
while maintaining compatibility.  We effectively create a header in the
structure with the base functionality and flags and structure size can
tell us what optional fields are present.  A better examples is
drivers/vfio/pci/vfio_pci.c:vfio_pci_ioctl(), particularly
VFIO_DEVICE_SET_IRQS where we read what's being provided in the "header"
and then read beyond the minimum structure size if directed.  Thanks,

Alex

> > 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
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