> On Jun 3, 2025, at 4:15 AM, Frederick Virchanza Gotham via Std-Proposals <std-proposals_at_[hidden]> wrote:
>
> On Tue, Jun 3, 2025 at 11:37 AM Oliver Hunt wrote:
>>
>> Here's what you need:
>>
>> https://godbolt.org/z/aaMjfvbro
>>
>> No. That does not work.
>>
>> I do not know why you are arguing with me. What you are trying to do does _not_ work.
>>
>> I gave you multiple godbolt links showing you the required, type specific, codegen required to load the vtable pointer.
>
>
>
> Specifically, you provided me with assembler that loads the vtable
> pointer in such a way that it does pointer authentication checks
> because you used compiler options such as
> "-fptrauth-vtable-pointer-address-discrimination", meaning that the
> machine code emitted from the compiler used instructions such as
> 'autd'. It is the introduction of these instructions (such as 'autd')
> into the machine code that crashed the program.
>
> Your code did way more stuff than it was supposed to do. All it had to do was:
>
> (1) Get the vtable pointer from the object, conveniently always
> located at [base + 0x00]
> (2) Dereference the vtable pointer and substract 8 to yield the
> address of the type_info
>

No, because that’s what the vtable authentication schema *is* on darwin.

The reason those flags are passed explicitly in the godbolt example is because godbolt does not target darwin.

> and:
>
> (1) Get the vtable pointer from the object, conveniently always
> located at [base + 0x00]
> (2) Dereference the vtable pointer and substract 16 to yield the
> address of the "distance to most derived" offset.

And this faults immediately because you failed to authenticate the load, and so the load is invalid.

> (3) Add the offset to the address of the sub-object.

You don’t get this far.

> Your code did way more than that. Sabotage is a strong word but it's
> borderline applicable here.
>

My code literally did what the Darwin ABI is. Please stop arguing with reality.

As a followup: don’t you *ever* accuse me of “sabotage” when I am literally describing the ABI for a platform.

I am at this point going to consider your behavior and attitude to not simply be bad faith, but outright abusive, and will be raising this through the appropriate channels.

—Oliver