From: Andrii Nakryiko <andrii@kernel.org>
To: <bpf@vger.kernel.org>, <ast@kernel.org>, <daniel@iogearbox.net>
Cc: andrii@kernel.org, kernel-team@fb.com,
"Toke Høiland-Jørgensen" <toke@toke.dk>
Subject: [PATCH bpf-next 1/2] libbpf: don't require full struct enum64 in UAPI headers
Date: Mon, 26 Sep 2022 21:29:39 -0700 [thread overview]
Message-ID: <20220927042940.147185-1-andrii@kernel.org> (raw)
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
Reported-by: Toke Høiland-Jørgensen <toke@toke.dk>
Fixes: d90ec262b35b ("libbpf: Add enum64 support for btf_dump")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
tools/lib/bpf/btf.h | 25 ++++++++++++++++++++++++-
1 file changed, 24 insertions(+), 1 deletion(-)
diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index ae543144ee30..8e6880d91c84 100644
--- 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)
--
2.30.2
next reply other threads:[~2022-09-27 4:30 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-27 4:29 Andrii Nakryiko [this message]
2022-09-27 4:29 ` [PATCH bpf-next 2/2] selftests/bpf: enforce C++11 mode for test_cpp test Andrii Nakryiko
2022-09-27 13:28 ` Daniel Borkmann
2022-09-27 11:43 ` [PATCH bpf-next 1/2] libbpf: don't require full struct enum64 in UAPI headers Toke Høiland-Jørgensen
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