Re: [PATCH v2 bpf-next 4/9] libbpf: ensure btf_dump__new() and btf_dump_opts are future-proof

[Jiri Olsa <jolsa@redhat.com>]

On Thu, Dec 23, 2021 at 11:23:13AM +0100, Jiri Olsa wrote:
> On Wed, Dec 22, 2021 at 02:14:35PM -0800, Andrii Nakryiko wrote:
> 
> SNIP
> 
> > > > +/* Choose either btf_dump__new() or btf_dump__new_deprecated() based on the
> > > > + * type of 4th argument. If it's btf_dump's print callback, use deprecated
> > > > + * API; otherwise, choose the new btf_dump__new(). ___libbpf_override()
> > > > + * doesn't work here because both variants have 4 input arguments.
> > > > + *
> > > > + * (void *) casts are necessary to avoid compilation warnings about type
> > > > + * mismatches, because even though __builtin_choose_expr() only ever evaluates
> > > > + * one side the other side still has to satisfy type constraints (this is
> > > > + * compiler implementation limitation which might be lifted eventually,
> > > > + * according to the documentation). So passing struct btf_ext in place of
> > > > + * btf_dump_printf_fn_t would be generating compilation warning.  Casting to
> > > > + * void * avoids this issue.
> > > > + *
> > > > + * Also, two type compatibility checks for a function and function pointer are
> > > > + * required because passing function reference into btf_dump__new() as
> > > > + * btf_dump__new(..., my_callback, ...) and as btf_dump__new(...,
> > > > + * &my_callback, ...) (not explicit ampersand in the latter case) actually
> > > > + * differs as far as __builtin_types_compatible_p() is concerned. Thus two
> > > > + * checks are combined to detect callback argument.
> > > > + *
> > > > + * The rest works just like in case of ___libbpf_override() usage with symbol
> > > > + * versioning.
> > > > + */
> > > > +#define btf_dump__new(a1, a2, a3, a4) __builtin_choose_expr(                         \
> > > > +     __builtin_types_compatible_p(typeof(a4), btf_dump_printf_fn_t) ||               \
> > > > +     __builtin_types_compatible_p(typeof(a4), void(void *, const char *, va_list)),  \
> > > > +     btf_dump__new_deprecated((void *)a1, (void *)a2, (void *)a3, (void *)a4),       \
> > > > +     btf_dump__new((void *)a1, (void *)a2, (void *)a3, (void *)a4))
> > >
> > > hi,
> > > this change breaks bpftrace g++ build that includes btf.h,
> > > because there's no typeof and __builtin_types_compatible_p in c++
> > >
> > > I guess there could be some c++ solution doing similar check,
> > > but I wonder we want to polute btf.h with that, I'll need to
> > > check on that
> > >
> > > I think I can add some detection code to bpftrace, to find out
> > > which version of btf_dump__new to use
> > >
> > > the build error can be generated with test_cpp.cpp below,
> > > so far I'm using __cplusplus ifdef in btf.h to workaround
> > > the issue
> > >
> > > thoughts?
> > 
> > This has been reported before, see [0]. I think the simplest solution
> > is what you did, just to opt-out on __cplusplus.
> > 
> > Please send the below as a proper two patches and I'll apply them and
> > sync to Github. Also let's add a comment explaining that
> > __builtin_types_compatible_p doesn't work with C++ compiler to make it
> > clear why we do this.
> 
> ok, will send

I see you've already provided the same patch in the github comment,
so I'll put your Signed-off-by in the patch as well

jirka

> 
> thanks,
> jirka
> 
> > 
> > In the [0] I was proposing the following comment, feel free to reuse it.
> > 
> > /* C++ compilers don't support __builtin_types_compatible_p(), so at least
> >  * don't screw up compilation for them and let C++ users pick btf_dump__new vs
> >  * btf_dump__new_deprecated explicitly.
> >  */
> > 
> >   [0] https://github.com/libbpf/libbpf/issues/283#issuecomment-986100727
> > 
> > >
> > > thanks,
> > > jirka
> > >
> > >
> > > ---
> > > diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
> > > index 742a2bf71c5e..bd2d77979571 100644
> > > --- a/tools/lib/bpf/btf.h
> > > +++ b/tools/lib/bpf/btf.h
> > > @@ -314,11 +314,13 @@ LIBBPF_API struct btf_dump *btf_dump__new_deprecated(const struct btf *btf,
> > >   * The rest works just like in case of ___libbpf_override() usage with symbol
> > >   * versioning.
> > >   */
> > > +#ifndef __cplusplus
> > >  #define btf_dump__new(a1, a2, a3, a4) __builtin_choose_expr(                           \
> > >         __builtin_types_compatible_p(typeof(a4), btf_dump_printf_fn_t) ||               \
> > >         __builtin_types_compatible_p(typeof(a4), void(void *, const char *, va_list)),  \
> > >         btf_dump__new_deprecated((void *)a1, (void *)a2, (void *)a3, (void *)a4),       \
> > >         btf_dump__new((void *)a1, (void *)a2, (void *)a3, (void *)a4))
> > > +#endif
> > >
> > >  LIBBPF_API void btf_dump__free(struct btf_dump *d);
> > >
> > > diff --git a/tools/testing/selftests/bpf/test_cpp.cpp b/tools/testing/selftests/bpf/test_cpp.cpp
> > > index a8d2e9a87fbf..e00201de2890 100644
> > > --- a/tools/testing/selftests/bpf/test_cpp.cpp
> > > +++ b/tools/testing/selftests/bpf/test_cpp.cpp
> > > @@ -7,9 +7,15 @@
> > >
> > >  /* do nothing, just make sure we can link successfully */
> > >
> > > +static void dump_printf(void *ctx, const char *fmt, va_list args)
> > > +{
> > > +}
> > > +
> > >  int main(int argc, char *argv[])
> > >  {
> > > +       struct btf_dump_opts opts = { };
> > >         struct test_core_extern *skel;
> > > +       struct btf *btf;
> > >
> > >         /* libbpf.h */
> > >         libbpf_set_print(NULL);
> > > @@ -18,7 +24,8 @@ int main(int argc, char *argv[])
> > >         bpf_prog_get_fd_by_id(0);
> > >
> > >         /* btf.h */
> > > -       btf__new(NULL, 0);
> > > +       btf = btf__new(NULL, 0);
> > > +       btf_dump__new(btf, dump_printf, nullptr, &opts);
> > >
> > >         /* BPF skeleton */
> > >         skel = test_core_extern__open_and_load();
> > >
> >