linux-hardening - Re: [PATCH v3 1/7] typeinfo: Introduce KCFI typeinfo mangling API

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Message-ID: <202509171410.6D18849FF6@keescook>
Date: Wed, 17 Sep 2025 14:20:25 -0700
From: Kees Cook <kees@...nel.org>
To: Qing Zhao <qing.zhao@...cle.com>, Marco Elver <elver@...gle.com>
Cc: Andrew Pinski <pinskia@...il.com>, Jakub Jelinek <jakub@...hat.com>,
	Martin Uecker <uecker@...raz.at>,
	Richard Biener <rguenther@...e.de>,
	Joseph Myers <josmyers@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>, Jan Hubicka <hubicka@....cz>,
	Richard Earnshaw <richard.earnshaw@....com>,
	Richard Sandiford <richard.sandiford@....com>,
	Marcus Shawcroft <marcus.shawcroft@....com>,
	Kyrylo Tkachov <kyrylo.tkachov@....com>,
	Kito Cheng <kito.cheng@...il.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Andrew Waterman <andrew@...ive.com>,
	Jim Wilson <jim.wilson.gcc@...il.com>,
	Dan Li <ashimida.1990@...il.com>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Ramon de C Valle <rcvalle@...gle.com>,
	Joao Moreira <joao@...rdrivepizza.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	"gcc-patches@....gnu.org" <gcc-patches@....gnu.org>,
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH v3 1/7] typeinfo: Introduce KCFI typeinfo mangling API

On Wed, Sep 17, 2025 at 05:56:17PM +0000, Qing Zhao wrote:
> Hi, 
> 
> > On Sep 13, 2025, at 19:23, Kees Cook <kees@...nel.org> wrote:
> > 
> > To support the KCFI typeid and future type-based allocators,
> 
> Could you please explain a little bit more on the “future type-based allocators”?

Sure, here's a link to Marco Elver's work:
https://lore.kernel.org/lkml/20250825154505.1558444-1-elver@google.com/
The "alloc token" is a bit more complicated:
https://github.com/melver/llvm-project/blob/alloc-token/clang/docs/AllocToken.rst
but my proposed __builtin_typeinfo_hash would align with mode=2 in the
Clang proposal.

> And why these two new builtins are necessary for this purpose?

Andrew didn't want strings produced when they were unused so I needed a
second API for getting string names. In either case, the result needed
to be compile-time constant. Also, I wanted to mirror the existing C++
typeinfo API that has "hash" and "name" accessors separate.

> > which need
> > to convert unique types into unique 32-bit values, add a mangling system
> > based on the Itanium C++ mangling ABI, adapted for for C types.
> 
> There is a redundant “for” in the above last sentence. 

Oops! Fixed.

> > Introduce
> > __builtin_typeinfo_hash for the hash, and __builtin_typeinfo_name for
> > testing and debugging (to see the human-readable mangling form).
> 
> In addition to the testing and debugging purpose, are  there any use cases for
> these two new compiler provided builtins? 

For _hash, yes, using it for type-based compile-time constant values
provides a building block for having type-aware logic in Linux,
especially with the allocator. For _name, it's a nice way to get at the
typeinfo for getting a stable string (right now there is no way to use
typeof() output for any reporting).

-- 
Kees Cook