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Date: Wed, 13 Dec 2023 11:35:06 -0800
From: Kees Cook <keescook@...omium.org>
To: Wang <wanglikun@...iang.com>
Cc: Sami Tolvanen <samitolvanen@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Aaron Tomlin <atomlin@...hat.com>,
	Alexander Potapenko <glider@...gle.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Alexandru Elisei <alexandru.elisei@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anshuman Khandual <anshuman.khandual@....com>,
	Ard Biesheuvel <ardb@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Arnd Bergmann <arnd@...db.de>, Boqun Feng <boqun.feng@...il.com>,
	Borislav Petkov <bp@...en8.de>, Borislav Petkov <bp@...e.de>,
	Brian Gerst <brgerst@...il.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Changbin Du <changbin.du@...el.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Frederic Weisbecker <frederic@...nel.org>, gcc-patches@....gnu.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Jiri Olsa <jolsa@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>,
	Juergen Gross <jgross@...e.com>,
	Kalesh Singh <kaleshsingh@...gle.com>,
	Kumar Kartikeya Dwivedi <memxor@...il.com>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Marco Elver <elver@...gle.com>, Mark Brown <broonie@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Michael Roth <michael.roth@....com>,
	Michal Marek <michal.lkml@...kovi.net>,
	Miguel Ojeda <ojeda@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Nicolas Schier <nicolas@...sle.eu>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Richard Sandiford <richard.sandiford@....com>,
	Song Liu <song@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Tom Rix <trix@...hat.com>, Uros Bizjak <ubizjak@...il.com>,
	Will Deacon <will@...nel.org>, x86@...nel.org,
	Yuntao Wang <ytcoode@...il.com>, Yu Zhao <yuzhao@...gle.com>,
	Zhen Lei <thunder.leizhen@...wei.com>, linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	llvm@...ts.linux.dev, linux-hardening@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-modules@...r.kernel.org,
	linux-perf-users@...r.kernel.org, Dan Li <ashimida.1990@...il.com>
Subject: Re: [RFC/RFT,V2] CFI: Add support for gcc CFI in aarch64

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@...gle.com> wrote:
> >> On Mon, Mar 27, 2023 at 2:30 AM Peter Zijlstra <peterz@...radead.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

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