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.
.
