> On Apr 4, 2025, at 04:59, Bill Wendling <isanb...@gmail.com> wrote:
>
> On Thu, Apr 3, 2025 at 9:54 AM Kees Cook <k...@kernel.org> wrote:
>> On Wed, Apr 02, 2025 at 09:16:47PM +0000, Qing Zhao wrote:
>>> Hi, Bill,
>>>
>>> Thanks for the summary.
>>>
>>> I think that this is good.
>>>
>>> Two more questions:
>>>
>>> 1. Shall we keep the same name of the attribute counted_by for the 2nd new
>>> syntax?
>>> Or use a new name, such as, “counted_by_exp"?
>>
>> FWIW, the Linux kernel will likely need to create its own macros for all
>> of this stuff (just to do the compiler version testing), so there should
>> be no problem with different names (or same names) in the compiler API.
>>
>>> I don’t have strong preference here. As mentioned by Jacub in a
>>> previous email, these two syntaxes can be distinguished by the number
>>> of arguments of the attribute.
>>>
>>> So for GCC, there should be no issue with either old name of a new name.
>>> However, I am not sure about CLANG. (Considering the complication with
>>> APPLE’s implementation)
>
> I also don't have a strong opinion on whether we should add new
> '*_expr' attributes. It's pretty easy to determine a single identifier
> from a more complex expression, so it's probably okay to use the
> current name. (I think that's what Apple has been doing internally.)
>
>>> 2. The 2nd new syntax should resolve all the following new requests from
>>> Linux Kernel:
>>> 2.1 Refer to a field in the nested structure
>>> 2.2 Refer to globals or locals
>>> 2.3 Represent simple expression
>>> 2.4 Forward referencing
>>>
>>> Except not very sure on 2.1: refer to a field in the nested structure
>>>
>>> struct Y {
>>> int n;
>>> int other;
>>> }
>>>
>>> struct Z {
>>> struct Y y;
>>> int array[] __attribute__ ((counted_by(?y.n)));
>>> };
>>>
>>> in the above, what should be put instead of "?" to refer to the field "n"
>>> of the field "y" of the current object of this struct Z?
>>>
>>> Based on my understanding, with the new syntax,
>>>
>>> struct Z {
>>> struct Y y;
>>> int array[] __attribute__ ((counted_by(struct Y y, y.n)));
>>> };
>>>
>>> i.e, the compiler will lookup “y” inside the current structure, then
>>> resolving the member access operator “.” as an expression.
>>>
>>> Is this correct understanding?
>>
>> I had the same question, e.g. how is this supposed to be declared:
>>
>> struct Y {
>> int n;
>> int other;
>> }
>>
>> struct Z {
>> int *ints __attribute__ ((counted_by(y.n)));
>> struct Y y;
>> };
>>
> My initial thought is that you'd have something like this:
>
> struct Y {
> int n;
> };
>
> struct Z {
> int *ints __attrbiute__((counted_by(struct Y y; y.n)));
> struct Y y;
> };
>
> And it should "just work." I'm not sure if there's an issue with this though.
Yes, I think that “y.n” is an expression, not an identifier, so, forward
declaration is needed.
Which forward declaration, “y” is looked up inside the structure, . Is parsed
as a regular
member access operator. So, everything works smoothly.
Qing
>
> -bw
