linux-kernel - Re: [PATCH 1/2] include: linux: Regularise the use of FIELD

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Date:   Mon, 15 Apr 2019 04:12:16 -0600
From:   William Kucharski <william.kucharski@...cle.com>
To:     Shyam Saini <shyam.saini@...rulasolutions.com>
Cc:     Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        keescook@...omium.org, linux-arm-kernel@...ts.infradead.org,
        linux-mips@...r.kernel.org, intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        netdev@...r.kernel.org, linux-ext4@...r.kernel.org,
        devel@...ts.orangefs.org, linux-mm@...ck.org,
        linux-sctp@...r.kernel.org, bpf@...r.kernel.org,
        kvm@...r.kernel.org, mayhs11saini@...il.com
Subject: Re: [PATCH 1/2] include: linux: Regularise the use of FIELD_SIZEOF
 macro



> On Apr 14, 2019, at 3:14 AM, Shyam Saini <shyam.saini@...rulasolutions.com> wrote:
> 
> Currently, there are 3 different macros, namely sizeof_field, SIZEOF_FIELD
> and FIELD_SIZEOF which are used to calculate the size of a member of
> structure, so to bring uniformity in entire kernel source tree lets use
> FIELD_SIZEOF and replace all occurrences of other two macros with this.
> 
> For this purpose, redefine FIELD_SIZEOF in include/linux/stddef.h and
> tools/testing/selftests/bpf/bpf_util.h and remove its defination from
> include/linux/kernel.h


> --- a/include/linux/stddef.h
> +++ b/include/linux/stddef.h
> @@ -20,6 +20,15 @@ enum {
> #endif
> 
> /**
> + * FIELD_SIZEOF - get the size of a struct's field
> + * @t: the target struct
> + * @f: the target struct's field
> + * Return: the size of @f in the struct definition without having a
> + * declared instance of @t.
> + */
> +#define FIELD_SIZEOF(t, f) (sizeof(((t *)0)->f))
> +
> +/**
>  * sizeof_field(TYPE, MEMBER)
>  *
>  * @TYPE: The structure containing the field of interest
> @@ -34,6 +43,6 @@ enum {
>  * @MEMBER: The member within the structure to get the end offset of
>  */
> #define offsetofend(TYPE, MEMBER) \
> -	(offsetof(TYPE, MEMBER)	+ sizeof_field(TYPE, MEMBER))
> +	(offsetof(TYPE, MEMBER)	+ FIELD_SIZEOF(TYPE, MEMBER))

If you're doing this, why are you leaving the definition of sizeof_field() in
stddef.h untouched?

Given the way this has worked historically, if you are leaving it in place for
source compatibility reasons, shouldn't it be redefined in terms of
FIELD_SIZEOF(), e.g.:

#define sizeof_field(TYPE, MEMBER) FIELD_SIZEOF(TYPE, MEMBER)