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 14:29:30 +0800 |
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在 2021/12/9 13:17, Kees Cook 写道: > On Wed, Dec 08, 2021 at 03:44:37PM -0800, Andrew Morton wrote: >> 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. >> >>>>> + * >>>>> + */ >>>>> +#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. > Please don't add memset_range(): just use a struct_group() to capture > the range and use memset() against the new substruct. This will allow > for the range to be documented where it is defined in the struct (rather > than deep in some code), keep any changes centralized instead of spread > around in memset_range() calls, protect against accidental struct member > reordering breaking things, and lets the compiler be able to examine > the range explicitly and do all the correct bounds checking: > > struct a { > int b; > struct_group(range, > int c; > int d; > ); > int e; > }; > > memset(&instance->range, 0, sizeof(instance->range)); > > memset_from/after() were added because of the very common case of "wipe > from here to end", which stays tied to a single member, and addressed > cases where struct_group() couldn't help (e.g. trailing padding). got it, thank you, I will drop this patch.
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