[PATCH bpf-next 1/6] libbpf: Sanitize BTF_KIND_FUNC linkage

[Alexei Starovoitov <ast@xxxxxxxxxx>]

In case kernel doesn't support static/global/extern liknage of BTF_KIND_FUNC
sanitize BTF produced by llvm.

Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
---
 tools/include/uapi/linux/btf.h |  6 ++++++
 tools/lib/bpf/libbpf.c         | 35 +++++++++++++++++++++++++++++++++-
 2 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/tools/include/uapi/linux/btf.h b/tools/include/uapi/linux/btf.h
index 1a2898c482ee..5a667107ad2c 100644
--- a/tools/include/uapi/linux/btf.h
+++ b/tools/include/uapi/linux/btf.h
@@ -146,6 +146,12 @@ enum {
 	BTF_VAR_GLOBAL_EXTERN = 2,
 };
 
+enum btf_func_linkage {
+	BTF_FUNC_STATIC = 0,
+	BTF_FUNC_GLOBAL = 1,
+	BTF_FUNC_EXTERN = 2,
+};
+
 /* BTF_KIND_VAR is followed by a single "struct btf_var" to describe
  * additional information related to the variable such as its linkage.
  */
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 7513165b104f..f72b3ed6c34b 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -166,6 +166,8 @@ struct bpf_capabilities {
 	__u32 btf_datasec:1;
 	/* BPF_F_MMAPABLE is supported for arrays */
 	__u32 array_mmap:1;
+	/* static/global/extern is supported for BTF_KIND_FUNC */
+	__u32 btf_func_linkage:1;
 };
 
 enum reloc_type {
@@ -1817,13 +1819,14 @@ static bool section_have_execinstr(struct bpf_object *obj, int idx)
 
 static void bpf_object__sanitize_btf(struct bpf_object *obj)
 {
+	bool has_func_linkage = obj->caps.btf_func_linkage;
 	bool has_datasec = obj->caps.btf_datasec;
 	bool has_func = obj->caps.btf_func;
 	struct btf *btf = obj->btf;
 	struct btf_type *t;
 	int i, j, vlen;
 
-	if (!obj->btf || (has_func && has_datasec))
+	if (!obj->btf || (has_func && has_datasec && has_func_linkage))
 		return;
 
 	for (i = 1; i <= btf__get_nr_types(btf); i++) {
@@ -1871,6 +1874,9 @@ static void bpf_object__sanitize_btf(struct bpf_object *obj)
 		} else if (!has_func && btf_is_func(t)) {
 			/* replace FUNC with TYPEDEF */
 			t->info = BTF_INFO_ENC(BTF_KIND_TYPEDEF, 0, 0);
+		} else if (!has_func_linkage && btf_is_func(t)) {
+			/* replace BTF_FUNC_GLOBAL with BTF_FUNC_STATIC */
+			t->info = BTF_INFO_ENC(BTF_KIND_FUNC, 0, 0);
 		}
 	}
 }
@@ -2804,6 +2810,32 @@ static int bpf_object__probe_btf_func(struct bpf_object *obj)
 	return 0;
 }
 
+static int bpf_object__probe_btf_func_linkage(struct bpf_object *obj)
+{
+	static const char strs[] = "\0int\0x\0a";
+	/* static void x(int a) {} */
+	__u32 types[] = {
+		/* int */
+		BTF_TYPE_INT_ENC(1, BTF_INT_SIGNED, 0, 32, 4),  /* [1] */
+		/* FUNC_PROTO */                                /* [2] */
+		BTF_TYPE_ENC(0, BTF_INFO_ENC(BTF_KIND_FUNC_PROTO, 0, 1), 0),
+		BTF_PARAM_ENC(7, 1),
+		/* FUNC x BTF_FUNC_GLOBAL */                    /* [3] */
+		BTF_TYPE_ENC(5, BTF_INFO_ENC(BTF_KIND_FUNC, 0, 1), 2),
+	};
+	int btf_fd;
+
+	btf_fd = libbpf__load_raw_btf((char *)types, sizeof(types),
+				      strs, sizeof(strs));
+	if (btf_fd >= 0) {
+		obj->caps.btf_func_linkage = 1;
+		close(btf_fd);
+		return 1;
+	}
+
+	return 0;
+}
+
 static int bpf_object__probe_btf_datasec(struct bpf_object *obj)
 {
 	static const char strs[] = "\0x\0.data";
@@ -2859,6 +2891,7 @@ bpf_object__probe_caps(struct bpf_object *obj)
 		bpf_object__probe_name,
 		bpf_object__probe_global_data,
 		bpf_object__probe_btf_func,
+		bpf_object__probe_btf_func_linkage,
 		bpf_object__probe_btf_datasec,
 		bpf_object__probe_array_mmap,
 	};
-- 
2.23.0