> On May 2, 2025, at 01:58, Jan Hendrik Farr <ker...@jfarr.cc> wrote:
>
> On 02 01:28:28, Alan Huang wrote:
>>
>> Thanks,
>> Alan
>>
>>
>>
>>> On May 2, 2025, at 01:22, Jan Hendrik Farr <ker...@jfarr.cc> wrote:
>>>
>>>>
>>>> I wonder if the __counted_by(x_name_len) in struct bch_xattr is needed,
>>>> since there is also a value after x_name.
>>>
>>> Wait a minute. Are you saying that the value with length x_val_len
>>> is behind the name (of length x_name_len) at the end of the struct.
>>> So essentially the flexible array member x_name has a length of
>>> x_name_len + x_val_len and contains both the name and value?
>>
>> Yes.
>
> I assume you can't easily change the struct such that there exists a member
> that contains the result of x_val_len + x_name_len, correct?
>
> In that case the only available course of action at this time is to
> remove the __counted_by, because it is incorrect.
>
> In addition I would recommend changing the name of x_name to something
> like x_name_and_val or similar. It's very misleading to call it x_name
> when it also contains the value.
>
>>
>>>
>>> If that's the case:
>>>
>>> 1. that's not at all clear from the struct definition
>>> 2. __counted_by(x_name_len) is not correct in that case
>>>
>>
>> Both clang and gcc say:
>>
>> • p->array has at least p->count number of elements available all the
>> time.
>>
>> Note the at least here. Though I think the counted_by is misleading here.
>>
>
> Here's how clang defines __bdos language extension [1]. Also note the
> attribute reference for __counted_by [2]. It assumes that the flexible array
> member contains exactly the amount of elements that are specified.
>
> I guess your quote from the gcc docs is misleading, as gcc's behavior
> is like clang's.
>
> The kernel uses the type & 2 == 0 case.
>
> So let's say you have a simple struct like so:
>
> struct foo{
> int val_len;
> char val[] __counted_by(val_len);
> }
>
> If val_len is 10 then foo->val[10] will be considered out of bounds.
> Even if you did a malloc for enough space.
OK, thanks, I’ll revert that.
>
> [1]
> https://github.com/llvm/llvm-project/blob/3b88805ca20018ae202afd3aea39f4fa856a8c64/clang/docs/LanguageExtensions.rst?plain=1#L5502-L5507
> [2]
> https://clang.llvm.org/docs/AttributeReference.html#counted-by-counted-by-or-null-sized-by-sized-by-or-null
>
>
> Best Regards
> Jan
