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

[Andy Lutomirski]

On Mon, Mar 9, 2015 at 8:28 AM, Ingo Molnar <mingo@xxxxxxxxxx> 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)
>
> 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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