Re: [PATCH bpf] libbpf: define BTF_KIND_* constants in btf.h to avoid compilation errors

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Tue, Jan 18, 2022 at 03:13:27PM +0100, Toke Høiland-Jørgensen escreveu:
> The btf.h header included with libbpf contains inline helper functions to
> check for various BTF kinds. These helpers directly reference the
> BTF_KIND_* constants defined in the kernel header, and because the header
> file is included in user applications, this happens in the user application
> compile units.
> 
> This presents a problem if a user application is compiled on a system with
> older kernel headers because the constants are not available. To avoid
> this, add #defines of the constants directly in btf.h before using them.
> 
> Since the kernel header moved to an enum for BTF_KIND_*, the #defines can
> shadow the enum values without any errors, so we only need #ifndef guards
> for the constants that predates the conversion to enum. We group these so
> there's only one guard for groups of values that were added together.
> 
>   [0] Closes: https://github.com/libbpf/libbpf/issues/436

The coexistence of enums with the defines (in turn #ifndef guarded) as
something I hadn't considered, clever.

Should fix lots of build errors in my test containers :-)

FWIW:

Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
 
> Fixes: 223f903e9c83 ("bpf: Rename BTF_KIND_TAG to BTF_KIND_DECL_TAG")
> Fixes: 5b84bd10363e ("libbpf: Add support for BTF_KIND_TAG")
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> ---
>  tools/lib/bpf/btf.h | 22 +++++++++++++++++++++-
>  1 file changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
> index 061839f04525..51862fdee850 100644
> --- a/tools/lib/bpf/btf.h
> +++ b/tools/lib/bpf/btf.h
> @@ -375,8 +375,28 @@ btf_dump__dump_type_data(struct btf_dump *d, __u32 id,
>  			 const struct btf_dump_type_data_opts *opts);
>  
>  /*
> - * A set of helpers for easier BTF types handling
> + * A set of helpers for easier BTF types handling.
> + *
> + * The inline functions below rely on constants from the kernel headers which
> + * may not be available for applications including this header file. To avoid
> + * compilation errors, we define all the constants here that were added after
> + * the initial introduction of the BTF_KIND* constants.
>   */
> +#ifndef BTF_KIND_FUNC
> +#define BTF_KIND_FUNC		12	/* Function	*/
> +#define BTF_KIND_FUNC_PROTO	13	/* Function Proto	*/
> +#endif
> +#ifndef BTF_KIND_VAR
> +#define BTF_KIND_VAR		14	/* Variable	*/
> +#define BTF_KIND_DATASEC	15	/* Section	*/
> +#endif
> +#ifndef BTF_KIND_FLOAT
> +#define BTF_KIND_FLOAT		16	/* Floating point	*/
> +#endif
> +/* The kernel header switched to enums, so these two were never #defined */
> +#define BTF_KIND_DECL_TAG	17	/* Decl Tag */
> +#define BTF_KIND_TYPE_TAG	18	/* Type Tag */
> +
>  static inline __u16 btf_kind(const struct btf_type *t)
>  {
>  	return BTF_INFO_KIND(t->info);
> -- 
> 2.34.1

-- 

- Arnaldo