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Date:   Wed, 13 Dec 2023 11:35:06 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Wang <wanglikun@lixiang.com>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
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        Dan Li <ashimida.1990@gmail.com>
Subject: Re: [RFC/RFT,V2] CFI: Add support for gcc CFI in aarch64
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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

-- 
Kees Cook