netdev - Re: [PATCH -next 1/2] string.h: Introduce memset

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Date:   Tue, 7 Dec 2021 20:28:29 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Xiu Jianfeng <xiujianfeng@...wei.com>
Cc:     <keescook@...omium.org>, <laniel_francis@...vacyrequired.com>,
        <andriy.shevchenko@...ux.intel.com>, <adobriyan@...il.com>,
        <linux@...ck-us.net>, <andreyknvl@...il.com>, <dja@...ens.net>,
        <ast@...nel.org>, <daniel@...earbox.net>, <andrii@...nel.org>,
        <kafai@...com>, <songliubraving@...com>, <yhs@...com>,
        <john.fastabend@...il.com>, <kpsingh@...nel.org>,
        <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
        <bpf@...r.kernel.org>
Subject: Re: [PATCH -next 1/2] string.h: Introduce memset_range() for wiping
 members

On Wed, 8 Dec 2021 11:04:50 +0800 Xiu Jianfeng <xiujianfeng@...wei.com> wrote:

> Motivated by memset_after() and memset_startat(), introduce a new helper,
> memset_range() that takes the target struct instance, the byte to write,
> and two member names where zeroing should start and end.

Is this likely to have more than a single call site?

> ...
>
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -291,6 +291,26 @@ void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count,
>  	       sizeof(*(obj)) - offsetof(typeof(*(obj)), member));	\
>  })
>  
> +/**
> + * memset_range - Set a value ranging from member1 to member2, boundary included.

I'm not sure what "boundary included" means.

> + *
> + * @obj: Address of target struct instance
> + * @v: Byte value to repeatedly write
> + * @member1: struct member to start writing at
> + * @member2: struct member where writing should stop

Perhaps "struct member before which writing should stop"?

> + *
> + */
> +#define memset_range(obj, v, member_1, member_2)			\
> +({									\
> +	u8 *__ptr = (u8 *)(obj);					\
> +	typeof(v) __val = (v);						\
> +	BUILD_BUG_ON(offsetof(typeof(*(obj)), member_1) >		\
> +		     offsetof(typeof(*(obj)), member_2));		\
> +	memset(__ptr + offsetof(typeof(*(obj)), member_1), __val,	\
> +	       offsetofend(typeof(*(obj)), member_2) -			\
> +	       offsetof(typeof(*(obj)), member_1));			\
> +})

struct a {
	int b;
	int c;
	int d;
};

How do I zero out `c' and `d'?