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

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Message-ID: <e2d5936d-8490-5871-b3d4-b286d256832a@huawei.com>
Date:   Wed, 8 Dec 2021 18:30:26 +0800
From:   xiujianfeng <xiujianfeng@...wei.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
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


在 2021/12/8 12:28, Andrew Morton 写道:
> 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?
There maybe more call site for this function, but I just use bpf as an 
example.
>
>> ...
>>
>> --- 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.
I mean zeroing from member1 to member2(including position indicated by 
member1 and member2)
>
>> + *
>> + * @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"?
memset_range should include position indicated by member2 as well
>
>> + *
>> + */
>> +#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'?
if you want to zero out 'c' and 'd', you can use it like 
memset_range(a_ptr, c, d);
>
>
> .