btf: only take ELF file handle in loadSpecFromELF

Move symbol scanning into loadSpecFromELF() to prevent passing in an empty
list. This removes the flexibility for the caller to skip processing symbols
(e.g. when parsing vmlinux, which doesn't need datasec fixups), but there
should be no assumption of whether LoadSpecFromReader will receive vmlinux
or a program ELF.

As a future optimization, symbol parsing could be deferred until the BTF parser
actually comes across a datasec that needs fixing up.

Signed-off-by: Timo Beckers <[email protected]>

Author: ti-mo

internal/btf/btf.go

@@ -90,6 +90,17 @@ func LoadSpecFromReader(rd io.ReaderAt) (*Spec, error) {
 	}
 	defer file.Close()
 
+	return loadSpecFromELF(file)
+}
+
+// variableOffsets extracts all symbols offsets from an ELF and indexes them by
+// section and variable name.
+//
+// References to variables in BTF data sections carry unsigned 32-bit offsets.
+// Some ELF symbols (e.g. in vmlinux) may point to virtual memory that is well
+// beyond this range. Since these symbols cannot be described by BTF info,
+// ignore them here.
+func variableOffsets(file *internal.SafeELFFile) (map[variable]uint32, error) {
 	symbols, err := file.Symbols()
 	if err != nil {
 		return nil, fmt.Errorf("can't read symbols: %v", err)
@@ -115,10 +126,10 @@ func LoadSpecFromReader(rd io.ReaderAt) (*Spec, error) {
 		variableOffsets[variable{secName, symbol.Name}] = uint32(symbol.Value)
 	}
 
-	return loadSpecFromELF(file, variableOffsets)
+	return variableOffsets, nil
 }
 
-func loadSpecFromELF(file *internal.SafeELFFile, variableOffsets map[variable]uint32) (*Spec, error) {
+func loadSpecFromELF(file *internal.SafeELFFile) (*Spec, error) {
 	var (
 		btfSection    *elf.Section
 		btfExtSection *elf.Section
@@ -148,7 +159,12 @@ func loadSpecFromELF(file *internal.SafeELFFile, variableOffsets map[variable]ui
 		return nil, fmt.Errorf("btf: %w", ErrNotFound)
 	}
 
-	spec, err := loadRawSpec(btfSection.Open(), file.ByteOrder, sectionSizes, variableOffsets)
+	vars, err := variableOffsets(file)
+	if err != nil {
+		return nil, err
+	}
+
+	spec, err := loadRawSpec(btfSection.Open(), file.ByteOrder, sectionSizes, vars)
 	if err != nil {
 		return nil, err
 	}
@@ -339,7 +355,7 @@ func loadKernelSpec() (*Spec, error) {
 		}
 		defer file.Close()
 
-		return loadSpecFromELF(file, nil)
+		return loadSpecFromELF(file)
 	}
 
 	return nil, fmt.Errorf("no BTF for kernel version %s: %w", release, internal.ErrNotSupported)