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

On Wed, 8 Dec 2021 18:30:26 +0800 xiujianfeng [off-list ref] wrote:

在 2021/12/8 12:28, Andrew Morton 写道:

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On Wed, 8 Dec 2021 11:04:50 +0800 Xiu Jianfeng [off-list ref] wrote:

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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.

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...

--- 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)

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+ *
+ * @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.

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+ *
+ */
+#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.