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 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(); > > >
