elf_reader: only read data from PROGBITS sections, ignore NOBITS

'Virtual' NOBITS sections like .bss always share an offset with another
ELF section, but reading from them is expected to yield only NUL bytes.

Prior to Go 1.18, opening a NOBITS section would yield a reader into
the overlapping 'real' section, dutifully returning its non-NUL bytes
and potentially reading past the end of the underlying file reader if
the size of the NOBITS section was large enough. This was addressed in
CL#375216.

As both an optimization (in case of large .bss sections) and a workaround
for this bug in Go versions 1.17 and earlier, only read data from PROGBITS
sections, which all known LLVM versions use correctly. Return an error if
a data section is not PROGBITS or NOBITS.

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

elf_reader.go

@@ -1027,22 +1027,33 @@ func (ec *elfCode) loadDataSections(maps map[string]*MapSpec) error {
 			continue
 		}
 
-		data, err := sec.Data()
-		if err != nil {
-			return fmt.Errorf("data section %s: can't get contents: %w", sec.Name, err)
-		}
-
-		if uint64(len(data)) > math.MaxUint32 {
-			return fmt.Errorf("data section %s: contents exceed maximum size", sec.Name)
-		}
-
 		mapSpec := &MapSpec{
 			Name:       SanitizeName(sec.Name, -1),
 			Type:       Array,
 			KeySize:    4,
-			ValueSize:  uint32(len(data)),
+			ValueSize:  uint32(sec.Size),
 			MaxEntries: 1,
-			Contents:   []MapKV{{uint32(0), data}},
+		}
+
+		switch sec.Type {
+		// Only open the section if we know there's actual data to be read.
+		case elf.SHT_PROGBITS:
+			data, err := sec.Data()
+			if err != nil {
+				return fmt.Errorf("data section %s: can't get contents: %w", sec.Name, err)
+			}
+
+			if uint64(len(data)) > math.MaxUint32 {
+				return fmt.Errorf("data section %s: contents exceed maximum size", sec.Name)
+			}
+			mapSpec.Contents = []MapKV{{uint32(0), data}}
+
+		case elf.SHT_NOBITS:
+			// NOBITS sections like .bss contain only zeroes, and since data sections
+			// are Arrays, the kernel already preallocates them. Skip reading zeroes
+			// from the ELF.
+		default:
+			return fmt.Errorf("data section %s: unknown section type %s", sec.Name, sec.Type)
 		}
 
 		// It is possible for a data section to exist without a corresponding BTF Datasec
@@ -1057,13 +1068,9 @@ func (ec *elfCode) loadDataSections(maps map[string]*MapSpec) error {
 			}
 		}
 
-		switch n := sec.Name; {
-		case strings.HasPrefix(n, ".rodata"):
+		if strings.HasPrefix(sec.Name, ".rodata") {
 			mapSpec.Flags = unix.BPF_F_RDONLY_PROG
 			mapSpec.Freeze = true
-		case n == ".bss":
-			// The kernel already zero-initializes the map
-			mapSpec.Contents = nil
 		}
 
 		maps[sec.Name] = mapSpec