Re: [PATCH 1/2] move offsetofend() from vfio.h to stddef.h
From: Alex Williamson
Date: Mon Mar 09 2015 - 11:44:58 EST
On Mon, 2015-03-09 at 16:28 +0100, Ingo Molnar wrote:
> * Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx> wrote:
> > On Mon, Mar 9, 2015 at 3:58 PM, Ingo Molnar <mingo@xxxxxxxxxx> 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)
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
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,
> > 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.
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