elf_reader: permit multiple .data* sections

Mirror libbpf behavior. Quoting the respective commit message[1]:

> Having the ability to use multiple .data sections and putting
> dedicated global variables into separate .data.custom section also
> opens up some new and interesting possibilities (like dynamically
> sizing global arrays, without using BPF_MAP_TYPE_ARRAY map).

[1] libbpf/libbpf#274

Another motivation for dedicated data sections is a more granular access
control, such as making a subset of variables BPF_F_RDONLY_PROG
(userspace can alter but BPF can't). Also note that verifier doesn't
prevent overflowing .data buffers into adjacent variables as long as the
write is within the backing array map bounds.

Signed-off-by: Nick Zavaritsky <mejedi@gmail.com>

elf_reader.go

@@ -100,7 +100,7 @@ func LoadCollectionSpecFromReader(rd io.ReaderAt) (*CollectionSpec, error) {
 			sections[idx] = newElfSection(sec, mapSection)
 		case sec.Name == ".maps":
 			sections[idx] = newElfSection(sec, btfMapSection)
-		case sec.Name == ".bss" || sec.Name == ".data" || strings.HasPrefix(sec.Name, ".rodata"):
+		case sec.Name == ".bss" || strings.HasPrefix(sec.Name, ".data") || strings.HasPrefix(sec.Name, ".rodata"):
 			sections[idx] = newElfSection(sec, dataSection)
 		case sec.Type == elf.SHT_REL:
 			// Store relocations under the section index of the target

elf_reader_test.go

@@ -96,6 +96,14 @@ func TestLoadCollectionSpec(t *testing.T) {
 				ValueSize:  8,
 				MaxEntries: 1,
 			},
+			".data.custom": {
+				Name:       SanitizeName(".data.custom", -1),
+				Type:       Array,
+				KeySize:    4,
+				ValueSize:  4,
+				MaxEntries: 1,
+				Contents:   []MapKV{{Key: uint32(0), Value: make([]uint8, 4)}},
+			},
 		},
 		Programs: map[string]*ProgramSpec{
 			"xdp_prog": {

info_test.go

@@ -405,7 +405,7 @@ func TestProgInfoExtBTF(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	expectedLineInfoCount := 26
+	expectedLineInfoCount := 28
 	expectedFuncInfo := map[string]bool{
 		"xdp_prog":   false,
 		"static_fn":  false,

testdata/loader.c

@@ -101,12 +101,14 @@ volatile const unsigned int key3  = 2; // .rodata
 static volatile const uint32_t arg; // .rodata, populated by loader
 // custom .rodata section, populated by loader
 static volatile const uint32_t arg2 __section(".rodata.test");
+// custom .data section
+static volatile uint32_t arg3 __section(".data.custom");
 
 __section("xdp") int xdp_prog() {
 	map_lookup_elem(&hash_map, (void *)&key1);
 	map_lookup_elem(&hash_map2, (void *)&key2);
 	map_lookup_elem(&hash_map2, (void *)&key3);
-	return static_fn(arg) + global_fn(arg) + arg2;
+	return static_fn(arg) + global_fn(arg) + arg2 + arg3;
 }
 
 // This function has no relocations, and is thus parsed differently.