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

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[ Upstream commit 87dbdc230d162bf9ee1ac77c8ade178b6b1e199e ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Andrii Nakryiko 2022-09-26 21:29:39 -07:00 committed by Greg Kroah-Hartman
parent a2edd3ae56
commit e0832a9786
1 changed files with 24 additions and 1 deletions
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25
tools/lib/bpf/btf.h
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@ -487,6 +487,8 @@ 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);
@ -494,7 +496,28 @@ 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)