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From: Denys Vlasenko <vda.linux@googlemail.com>
Date: Mon, 9 Mar 2015 16:15:57 +0100
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Subject: Re: [PATCH 1/2] move offsetofend() from vfio.h to stddef.h
To: Ingo Molnar <mingo@kernel.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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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);
                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.

Nevertheless, I can resend a version where comment survives if you want...