Commit 87dbdc23 authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by Daniel Borkmann
Browse files

libbpf: Don't require full struct enum64 in UAPI headers 

Drop the requirement for system-wide kernel UAPI headers to provide full
struct btf_enum64 definition. This is an unexpected requirement that
slipped in libbpf 1.0 and put unnecessary pressure ([0]) on users to have
a bleeding-edge kernel UAPI header from unreleased Linux 6.0.

To achieve this, we forward declare struct btf_enum64. But that's not
enough as there is btf_enum64_value() helper that expects to know the
layout of struct btf_enum64. So we get a bit creative with
reinterpreting memory layout as array of __u32 and accesing lo32/hi32
fields as array elements. Alternative way would be to have a local
pointer variable for anonymous struct with exactly the same layout as
struct btf_enum64, but that gets us into C++ compiler errors complaining
about invalid type casts. So play it safe, if ugly.

  [0] Closes: https://github.com/libbpf/libbpf/issues/562



Fixes: d90ec262 ("libbpf: Add enum64 support for btf_dump")
Reported-by: default avatarToke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarToke Høiland-Jørgensen <toke@toke.dk>
Link: https://lore.kernel.org/bpf/20220927042940.147185-1-andrii@kernel.org
parent 2702c789

tools/lib/bpf/btf.h

+24 −1
Original line number Diff line number Diff line
@@ -486,6 +486,8 @@ static inline struct btf_enum *btf_enum(const struct btf_type *t) 
	return (struct btf_enum *)(t + 1);

}



struct btf_enum64;



static inline struct btf_enum64 *btf_enum64(const struct btf_type *t)

{

	return (struct btf_enum64 *)(t + 1);
@@ -493,7 +495,28 @@ static inline struct btf_enum64 *btf_enum64(const struct btf_type *t) 


static inline __u64 btf_enum64_value(const struct btf_enum64 *e)

{

	return ((__u64)e->val_hi32 << 32) | e->val_lo32;

	/* struct btf_enum64 is introduced in Linux 6.0, which is very

	 * bleeding-edge. Here we are avoiding relying on struct btf_enum64

	 * definition coming from kernel UAPI headers to support wider range

	 * of system-wide kernel headers.

	 *

	 * Given this header can be also included from C++ applications, that

	 * further restricts C tricks we can use (like using compatible

	 * anonymous struct). So just treat struct btf_enum64 as

	 * a three-element array of u32 and access second (lo32) and third

	 * (hi32) elements directly.

	 *

	 * For reference, here is a struct btf_enum64 definition:

	 *

	 * const struct btf_enum64 {

	 *	__u32	name_off;

	 *	__u32	val_lo32;

	 *	__u32	val_hi32;

	 * };

	 */

	const __u32 *e64 = (const __u32 *)e;



	return ((__u64)e64[2] << 32) | e64[1];

}



static inline struct btf_member *btf_members(const struct btf_type *t)