libbpf: Add BTF_KIND_FLOAT support

The logic follows that of BTF_KIND_INT most of the time. Sanitization
replaces BTF_KIND_FLOATs with equally-sized empty BTF_KIND_STRUCTs on
older kernels, for example, the following:

    [4] FLOAT 'float' size=4

becomes the following:

    [4] STRUCT '(anon)' size=4 vlen=0

With dwarves patch [1] and this patch, the older kernels, which were
failing with the floating-point-related errors, will now start working
correctly.

[1] https://github.com/iii-i/dwarves/commit/btf-kind-float-v2

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210226202256.116518-4-iii@linux.ibm.com

1 files changed, 28 insertions, 1 deletions

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 62d9ed56b081..2f351d3ad3e7 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -178,6 +178,8 @@ enum kern_feature_id {
 	FEAT_PROG_BIND_MAP,
 	/* Kernel support for module BTFs */
 	FEAT_MODULE_BTF,
+	/* BTF_KIND_FLOAT support */
+	FEAT_BTF_FLOAT,
 	__FEAT_CNT,
 };
 
@@ -1946,6 +1948,7 @@ static const char *btf_kind_str(const struct btf_type *t)
 	case BTF_KIND_FUNC_PROTO: return "func_proto";
 	case BTF_KIND_VAR: return "var";
 	case BTF_KIND_DATASEC: return "datasec";
+	case BTF_KIND_FLOAT: return "float";
 	default: return "unknown";
 	}
 }
@@ -2395,15 +2398,17 @@ static bool btf_needs_sanitization(struct bpf_object *obj)
 {
 	bool has_func_global = kernel_supports(FEAT_BTF_GLOBAL_FUNC);
 	bool has_datasec = kernel_supports(FEAT_BTF_DATASEC);
+	bool has_float = kernel_supports(FEAT_BTF_FLOAT);
 	bool has_func = kernel_supports(FEAT_BTF_FUNC);
 
-	return !has_func || !has_datasec || !has_func_global;
+	return !has_func || !has_datasec || !has_func_global || !has_float;
 }
 
 static void bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
 {
 	bool has_func_global = kernel_supports(FEAT_BTF_GLOBAL_FUNC);
 	bool has_datasec = kernel_supports(FEAT_BTF_DATASEC);
+	bool has_float = kernel_supports(FEAT_BTF_FLOAT);
 	bool has_func = kernel_supports(FEAT_BTF_FUNC);
 	struct btf_type *t;
 	int i, j, vlen;
@@ -2456,6 +2461,13 @@ static void bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *btf)
 		} else if (!has_func_global && btf_is_func(t)) {
 			/* replace BTF_FUNC_GLOBAL with BTF_FUNC_STATIC */
 			t->info = BTF_INFO_ENC(BTF_KIND_FUNC, 0, 0);
+		} else if (!has_float && btf_is_float(t)) {
+			/* replace FLOAT with an equally-sized empty STRUCT;
+			 * since C compilers do not accept e.g. "float" as a
+			 * valid struct name, make it anonymous
+			 */
+			t->name_off = 0;
+			t->info = BTF_INFO_ENC(BTF_KIND_STRUCT, 0, 0);
 		}
 	}
 }
@@ -3927,6 +3939,18 @@ static int probe_kern_btf_datasec(void)
 					     strs, sizeof(strs)));
 }
 
+static int probe_kern_btf_float(void)
+{
+	static const char strs[] = "\0float";
+	__u32 types[] = {
+		/* float */
+		BTF_TYPE_FLOAT_ENC(1, 4),
+	};
+
+	return probe_fd(libbpf__load_raw_btf((char *)types, sizeof(types),
+					     strs, sizeof(strs)));
+}
+
 static int probe_kern_array_mmap(void)
 {
 	struct bpf_create_map_attr attr = {
@@ -4106,6 +4130,9 @@ static struct kern_feature_desc {
 	[FEAT_MODULE_BTF] = {
 		"module BTF support", probe_module_btf,
 	},
+	[FEAT_BTF_FLOAT] = {
+		"BTF_KIND_FLOAT support", probe_kern_btf_float,
+	},
 };
 
 static bool kernel_supports(enum kern_feature_id feat_id)