Re: [PATCH v6 2/4] Compiler Attributes: Add __overloadable for Clang

Nick Desaulniers
2022-02-04T00:59:16Z

On Thu, Feb 3, 2022 at 4:26 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Thu, Feb 03, 2022 at 02:11:48PM -0800, Nick Desaulniers wrote:
> > On Thu, Feb 3, 2022 at 1:04 PM Kees Cook <keescook@chromium.org> wrote:
> > >
> > > On Thu, Feb 03, 2022 at 12:26:15PM -0800, Nick Desaulniers wrote:
> > > > On Thu, Feb 3, 2022 at 9:33 AM Kees Cook <keescook@chromium.org> wrote:
> > > > >
> > > > > must be marked as being overloadable (i.e. different prototypes).
> > > > > This allows the __pass_object_size versions to take precedence.
> > > >
> > > > Is this because of the `const` additions to the function signatures?
> > >
> > > That might be an issue, but the *real* issue is the implicit mutation of
> > > the function into an inline with _additional_ arguments. i.e.
> > >
> > > char *strcpy(char * POS p, const char * POS q)
> > >
> > > is really
> > >
> > > char *strcpy(char * const p, const char * const q, size_t __size_of_p, size_t __size_of_q)
> > >
> > > (i.e. what I was doing with macros, but all internally and still an
> > > extern inline)
> >
> > What do you mean "is really"? 4/4 doesn't change the number of
> > parameters in strcpy explicitly in the definition AFAICT.
>
> It really does change the number of parameters. See the IR difference:
>
> $ cat example.c
> #ifdef USE_POS
> # define POS __attribute__((pass_object_size(1)))
> #else
> # define POS
> #endif
>
> int func(void * const POS);
>
> struct foo
> {
> int a;
> char *b;
> };
>
> void usage(struct foo *example)
> {
> func(example);
> }
>
> $ IR="-O2 -Xclang -disable-llvm-passes -emit-llvm -S"
> $ clang example.c $IR -o normal.ll
> $ clang -DUSE_POS example.c $IR -o pos.ll
> $ diff -u normal.ll pos.ll
> --- normal.ll 2022-02-03 16:23:39.734065036 -0800
> +++ pos.ll 2022-02-03 16:23:49.518083451 -0800
> @@ -11,14 +11,19 @@
> store %struct.foo* %0, %struct.foo** %2, align 8, !tbaa !3
> %3 = load %struct.foo*, %struct.foo** %2, align 8, !tbaa !3
> %4 = bitcast %struct.foo* %3 to i8*
> - %5 = call i32 @func(i8* noundef %4)
> + %5 = call i64 @llvm.objectsize.i64.p0i8(i8* %4, i1 false, i1 true, i1 false)
> + %6 = call i32 @func(i8* noundef %4, i64 noundef %5)
> ret void
> }
> ...

Woah, that's fancy.

>
> This is basically doing internally exactly what I was doing in v4 and
> earlier with macros (passing in the caller's view of __bos(arg, 1)).

Yeah. Ok now it all makes sense to me.

With the s/inline extern/extern inline/:
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
--
Thanks,
~Nick Desaulniers