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: d90ec262b35b ("libbpf: Add enum64 support for btf_dump")
Reported-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://lore.kernel.org/bpf/20220927042940.147185-1-andrii@kernel.org

diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index ae54314..8e6880d 100644 (file)
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -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)