Re: [PATCH -next 1/2] string.h: Introduce memset_range() for wiping members

[xiujianfeng]

在 2021/12/9 7:44, Andrew Morton 写道:
> On Wed, 8 Dec 2021 18:30:26 +0800 xiujianfeng <xiujianfeng@xxxxxxxxxx> wrote:
>
> > 在 2021/12/8 12:28, Andrew Morton 写道:
> > > On Wed, 8 Dec 2021 11:04:50 +0800 Xiu Jianfeng <xiujianfeng@xxxxxxxxxx> 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.
that is good, thank you for you advice :)
> > > > + *
> > > > + */
> > > > +#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.

#define offsetofend(TYPE, MEMBER) \
             (offsetof(TYPE, MEMBER)>+ sizeof_field(TYPE, MEMBER))

if I understand correctly, offsetofend(typeof(*(a), d) is 12, so it 
expands to

    memset(__ptr + 4, __val,

                  12 -

                  4);

Anyway,  I will drop this patch because of Kees's suggestion, thank you.

> .