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

[Kees Cook <kees@kernel.org>]

On Wed, Sep 17, 2025 at 05:56:17PM +0000, Qing Zhao wrote:
> Hi, 
> 
> > On Sep 13, 2025, at 19:23, Kees Cook <kees@kernel.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