elf_reader: support value BTF on ringbuf and perf array maps

While reviewing #1610, I noticed we were generating VariableSpecs for variables
named 'unused' in our examples. There had to be a better way to get their BTF
info into the ELF.

I thoroughly checked all map types present in Linux 6.12 for any validation code
on attr->value_size, and only RingBuf and Arena maps require value_size to be
zero at all times. Special-case PerfEventArray so type annotations aren't limited
to 4 bytes.

Signed-off-by: Timo Beckers <timo@isovalent.com>

elf_reader.go

@@ -1013,6 +1013,13 @@ func mapSpecFromBTF(es *elfSection, vs *btf.VarSecinfo, def *btf.Struct, spec *b
 		}
 	}
 
+	// Some maps don't support value sizes, but annotating their map definitions
+	// with __type macros can still be useful, especially to let bpf2go generate
+	// type definitions for them.
+	if value != nil && !mapType.canHaveValueSize() {
+		valueSize = 0
+	}
+
 	return &MapSpec{
 		Name:       SanitizeName(name, -1),
 		Type:       MapType(mapType),

elf_reader_test.go

@@ -247,6 +247,9 @@ func TestLoadCollectionSpec(t *testing.T) {
 		}
 		qt.Assert(t, qt.ContentEquals(mErr.Constants, []string{"totallyBogus", "totallyBogus2"}))
 
+		qt.Assert(t, qt.Equals(have.Maps["perf_event_array"].ValueSize, 0))
+		qt.Assert(t, qt.IsNotNil(have.Maps["perf_event_array"].Value))
+
 		if diff := cmp.Diff(coll, have, cmpOpts...); diff != "" {
 			t.Errorf("MapSpec mismatch (-want +got):\n%s", diff)
 		}

testdata/loader.c

@@ -58,9 +58,15 @@ struct {
 		});
 } btf_outer_map_anon __section(".maps");
 
+struct perf_event {
+	uint64_t foo;
+	uint64_t bar;
+};
+
 struct {
 	__uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
 	__uint(max_entries, 4096);
+	__type(value, struct perf_event);
 } perf_event_array __section(".maps");
 
 struct bpf_map_def array_of_hash_map __section("maps") = {

types.go

@@ -127,6 +127,21 @@ func (mt MapType) canStoreProgram() bool {
 	return mt == ProgramArray
 }
 
+// canHaveValueSize returns true if the map type supports setting a value size.
+func (mt MapType) canHaveValueSize() bool {
+	switch mt {
+	case RingBuf, Arena:
+		return false
+
+	// Special-case perf events since they require a value size of either 0 or 4
+	// for historical reasons. Let the library fix this up later.
+	case PerfEventArray:
+		return false
+	}
+
+	return true
+}
+
 // ProgramType of the eBPF program
 type ProgramType uint32