Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [PATCH -next 1/2] string.h: Introduce memset_range() for wiping members | From | xiujianfeng <> | Date | Thu, 9 Dec 2021 14:29:26 +0800 |
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在 2021/12/9 7:44, Andrew Morton 写道: > On Wed, 8 Dec 2021 18:30:26 +0800 xiujianfeng <xiujianfeng@huawei.com> wrote: > >> 在 2021/12/8 12:28, Andrew Morton 写道: >>> On Wed, 8 Dec 2021 11:04:50 +0800 Xiu Jianfeng <xiujianfeng@huawei.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. 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.
> .
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