internal: add safe ELF file wrapper

Fuzzing the ELF loader isn't very succesful since it keeps crashing
in debug/elf. Most of the time the culprit is a call to elf.Section.Data().
This method allocates a buffer with a size taken from the ELF, which
can lead to outlandishly large allocations. It's not clear how to validate
elf.Section.Size since the code that creates the section doesn't know
the total length of the ELF.

Instead, add a wrapper that catches panics due to ELF parsing, and turns
them into errors. This isn't a fool proof solution since the runtime
can still kill the process due to an OOM, but hopefully we will still
crash less overall.

See golang/go#33121

btf.go

@@ -53,7 +53,7 @@ type btfHeader struct {
 //
 // Returns a nil Spec and no error if no BTF was present.
 func LoadSpecFromReader(rd io.ReaderAt) (*Spec, error) {
-	file, err := elf.NewFile(rd)
+	file, err := internal.NewSafeELFFile(rd)
 	if err != nil {
 		return nil, err
 	}
@@ -109,7 +109,7 @@ func LoadSpecFromReader(rd io.ReaderAt) (*Spec, error) {
 	return spec, nil
 }
 
-func findBtfSections(file *elf.File) (*elf.Section, *elf.Section, map[string]uint32, error) {
+func findBtfSections(file *internal.SafeELFFile) (*elf.Section, *elf.Section, map[string]uint32, error) {
 	var (
 		btfSection    *elf.Section
 		btfExtSection *elf.Section
@@ -138,7 +138,7 @@ func findBtfSections(file *elf.File) (*elf.Section, *elf.Section, map[string]uin
 }
 
 func loadSpecFromVmlinux(rd io.ReaderAt) (*Spec, error) {
-	file, err := elf.NewFile(rd)
+	file, err := internal.NewSafeELFFile(rd)
 	if err != nil {
 		return nil, err
 	}