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

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Date:   Wed, 8 Dec 2021 15:44:37 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     xiujianfeng <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 18:30:26 +0800 xiujianfeng <xiujianfeng@...wei.com> wrote:

> 
> 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

In that case we could say "struct member where writing should stop
(inclusive)", to make it very clear.

> >
> >> + *
> >> + */
> >> +#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);

But I don't think that's what the code does!

it expands to

	memset(__ptr + offsetof(typeof(*(a)), c), __val,
	       offsetofend(typeof(*(a)), d) -
	       offsetof(typeof(*(a)), c));

which expands to

	memset(__ptr + 4, __val,
	       8 -
	       4);

and `d' will not be written to.