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

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Date:	Mon, 9 Mar 2015 08:30:32 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] move offsetofend() from vfio.h to stddef.h

On Mon, Mar 9, 2015 at 8:28 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Denys Vlasenko <vda.linux@...glemail.com> wrote:
>
>> On Mon, Mar 9, 2015 at 3:58 PM, Ingo Molnar <mingo@...nel.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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