On Mon, Mar 9, 2015 at 8:28 AM, Ingo Molnar <mi...@kernel.org> wrote:
>
> * Denys Vlasenko <vda.li...@googlemail.com> wrote:
>
>> On Mon, Mar 9, 2015 at 3:58 PM, Ingo Molnar <mi...@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?
struct foo {
u64 a;
char b;
};
sizeof(struct foo) will be 16, but offsetofend(struct foo, b) will be
9 on most platforms, right?
--Andy
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