Re: [RFC/RFT,V2] CFI: Add support for gcc CFI in aarch64

["Wang" <wanglikun@lixiang.com>]

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On 2023/12/14 03:35, Kees Cook wrote:
> On Wed, Dec 13, 2023 at 05:01:07PM +0800, Wang wrote:
>> On 2023/12/13 16:48, Dan Li wrote:
>>> + Likun
>>>
>>> On Tue, 28 Mar 2023 at 06:18, Sami Tolvanen <samitolvanen@google.com> wrote:
>>>> On Mon, Mar 27, 2023 at 2:30 AM Peter Zijlstra <peterz@infradead.org> wrote:
>>>>> On Sat, Mar 25, 2023 at 01:54:16AM -0700, Dan Li wrote:
>>>>>
>>>>>> In the compiler part[4], most of the content is the same as Sami's
>>>>>> implementation[3], except for some minor differences, mainly including:
>>>>>>
>>>>>> 1. The function typeid is calculated differently and it is difficult
>>>>>> to be consistent.
>>>>> This means there is an effective ABI break between the compilers, which
>>>>> is sad :-( Is there really nothing to be done about this?
>>>> I agree, this would be unfortunate, and would also be a compatibility
>>>> issue with rustc where there's ongoing work to support
>>>> clang-compatible CFI type hashes:
>>>>
>>>> https://github.com/rust-lang/rust/pull/105452
>>>>
>>>> Sami
>>
>>
>> Hi Peter and Sami
>>
>> I am Dan Li's colleague, and I will take over and continue the work of CFI.
> 
> Welcome; this is great news! :) Thanks for picking up the work.
> 
>>
>> Regarding the issue of gcc cfi type id being compatible with clang, we
>> have analyzed and verified:
>>
>> 1. clang uses Mangling defined in Itanium C++ ABI to encode the function
>> prototype, and uses the encoding result as input to generate cfi type id;
>> 2. Currently, gcc only implements mangling for the C++ compiler, and the
>> function prototype coding generated by these interfaces is compatible
>> with clang, but gcc's c compiler does not support mangling.;
>>
>> Adding mangling to gcc's c compiler is a huge and difficult task，because
>> we have to refactor the mangling of C++, splitting it into basic
>> mangling and language specific mangling, and adding support for the c
>> language which requires a deep understanding of the compiler and
>> language processing parts.
>>
>> And for the kernel cfi, I suggest separating type compatibility from CFI
>> basic functions. Type compatibility is independent from CFI basic
>> funcitons and should be dealt with under another topic. Should we focus
>> on the main issus of cfi, and  let it work first on linux kernel, and
>> left the compatible issue to be solved later?
> 
> If you mean keeping the hashes identical between Clang/LLVM and GCC,
> I think this is going to be a requirement due to adding Rust to the
> build environment (which uses the LLVM mangling and hashing).
> 
> FWIW, I think the subset of type mangling needed isn't the entirely C++
> language spec, so it shouldn't be hard to add this to GCC.
> 
> -Kees
> 

Thanks Kees, I will first try to implement a simple interface based on 
mangle to generate cfi type id.

Likun Wang

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