Am Donnerstag, dem 18.09.2025 um 11:09 -0700 schrieb Kees Cook:
> On Thu, Sep 18, 2025 at 09:20:52AM +0200, Martin Uecker wrote:
> > Am Mittwoch, dem 17.09.2025 um 17:56 +0000 schrieb Qing Zhao:
> > > Hi,
> > >
> > > > On Sep 13, 2025, at 19:23, Kees Cook <[email protected]> wrote:
> > > >
> > > > To support the KCFI typeid and future type-based allocators,
> >
> > What I find problematic though is that this is not based on GNU / ISO C
> > rules but on stricter Linux kernel rules. I think such builtin should
> > have two versions.
> >
> > So maybe
> >
> > __builtin_typeinfo_hash_strict // strict
> > __builtin_typeinfo_hash_canonical // standard
> >
> > or similar, or maybe instead have a flag argument so that we can
> > other options which may turn out to be important in the future
> > (such as ignoring qualifiers or supporting newer languag features).
>
> Can you send me a patch to gcc/testsuite/gcc.dg/builtin-typeinfo.c
> that shows what differences you mean?
I can look at this in the next days.
> Because AFAICT, this C version
> matches the C++ typeinfo implementation.
> There isn't a need for these
> hashes to be comparable in a way that they could be used to, for
> example, reimplement __builtin_types_compatible_p. It's called
> "typeinfo" and that has a specific meaning currently...
I would want the hashes for types which are compatible
according to ISO C to be identical.
What I want avoid that this is used to implement some
run-time type checking (which is what KCFI does) and the run-time
check can fail even when a compile-time check according to the
usual rules of the language passes. Even if this ok for the
Linux kernel, I think this would be surprising in general.
Martin
>
> Given:
>
> typedef int arr10[10];
> typedef int arr_unknown[];
> typedef int *arr;
> typedef struct named { int a; int b; } named_t;
> typedef struct { int a; int b; } nameless_t;
> typedef void (*func_arr10)(int[10]);
> typedef void (*func_arr_unknown)(int[]);
> typedef void (*func_ptr)(int*);
> typedef void (*func_named(named_t*);
> typedef void (*func_nameless(nameless_t*);
>
> C++ typeinfo(...).name() shows:
>
> int[10]: A10_i
> int[]: A_i
> int *: Pi
> named_t: 5named
> nameless_t: 10nameless_t
> void(*)(int[10]): PFvPiE
> void(*)(int[]): PFvPiE
> void(*)(int*): PFvPiE
> void(*)(named_t*): PFvP5namedE
> void(*)(nameless_t*): PFvP10nameless_tE
>
> This __builtin_typeinfo_name(...) shows:
>
> int[10]: A10_i
> int[]: A_i
> int *: Pi
> __builtin_compatible_types_p(int[10], int[]): true
> __builtin_compatible_types_p(int[], int*): false
> named_t: 5named
> nameless_t: 10nameless_t
> void(*)(int[10]): PFvPiE
> void(*)(int[]): PFvPiE
> void(*)(int*): PFvPiE
> void(*)(named_t*): PFvP5namedE
> void(*)(nameless_t*): PFvP10nameless_tE
>
> What would you want the "Strict ISO C" builtin to do instead?
>
> -Kees
