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

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Message-ID: <YbDne/nYsVai5pCV@localhost.localdomain>
Date:   Wed, 8 Dec 2021 20:12:27 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Xiu Jianfeng <xiujianfeng@...wei.com>
Cc:     akpm@...ux-foundation.org, keescook@...omium.org,
        laniel_francis@...vacyrequired.com,
        andriy.shevchenko@...ux.intel.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, Dec 08, 2021 at 11:04:50AM +0800, Xiu Jianfeng 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.
> 
> Signed-off-by: Xiu Jianfeng <xiujianfeng@...wei.com>
> ---
>  include/linux/string.h | 20 ++++++++++++++++++++
>  lib/memcpy_kunit.c     | 12 ++++++++++++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/include/linux/string.h b/include/linux/string.h
> index b6572aeca2f5..7f19863253f2 100644
> --- 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.
> + *
> + * @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
> + *
> + */
> +#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));			\
> +})

"u8*" should be "void*" as kernel legitimises pointer arithmetic on void*
and there is no dereference.

__val is redundant, just toss "v" into memset(), it will do the right
thing. In fact, toss "__ptr" as well, it is simply unnecessary.

All previous memsets are the same...