Merge pull request open-telemetry#11 from umanwizard/x86_tdata

Look at .tdata when computing TLS image size

interpreter/customlabels/customlabels.go

@@ -31,6 +31,13 @@ type data struct {
 
 var _ interpreter.Data = &data{}
 
+func roundUp(multiple, value uint64) uint64 {
+	if multiple == 0 {
+		return value
+	}
+	return (value + multiple - 1) / multiple * multiple
+}
+
 func Loader(_ interpreter.EbpfHandler, info *interpreter.LoaderInfo) (interpreter.Data, error) {
 	ef, err := info.GetELF()
 	if err != nil {
@@ -75,11 +82,29 @@ func Loader(_ interpreter.EbpfHandler, info *interpreter.LoaderInfo) (interprete
 		if ef.Machine == elf.EM_AARCH64 {
 			tlsAddr = libpf.Address(tlsSym.Address + 16)
 		} else if ef.Machine == elf.EM_X86_64 {
+			// Symbol addresses are relative to the start of the
+			// thread-local storage image, but the thread pointer points to the _end_
+			// of the image. So we need to find the size of the image in order to know where the
+			// beginning is.
+			//
+			// The image is just .tdata followed by .tbss,
+			// but we also have to respect the alignment.
 			tbss, err := ef.Tbss()
 			if err != nil {
 				return nil, err
 			}
-			tlsAddr = libpf.Address(int64(tlsSym.Address) - int64(tbss.Size))
+			tdata, err := ef.Tdata()
+			var tdataSize uint64
+			if err != nil {
+				// No Tdata is ok, it's the same as size 0
+				if err != pfelf.ErrNoTdata {
+					return nil, err
+				}
+			} else {
+				tdataSize = tdata.Size
+			}
+			imageSize := roundUp(tbss.Addralign, tdataSize) + tbss.Size
+			tlsAddr = libpf.Address(int64(tlsSym.Address) - int64(imageSize))
 		} else {
 			return nil, fmt.Errorf("unrecognized machine: %s", ef.Machine.String())
 		}

libpf/pfelf/file.go

@@ -57,6 +57,12 @@ var ErrSymbolNotFound = errors.New("symbol not found")
 // ErrNotELF is returned when the file is not an ELF
 var ErrNotELF = errors.New("not an ELF file")
 
+// ErrNoTbss is returned when the tbss section cannot be found
+var ErrNoTbss = errors.New("no thread-local uninitialized data section (tbss)")
+
+// ErrNoTdata is returned when the tdata section cannot be found
+var ErrNoTdata = errors.New("no thread-local initialized data section (tdata)")
+
 // File represents an open ELF file
 type File struct {
 	// closer is called internally when resources for this File are to be released
@@ -426,7 +432,20 @@ func (f *File) Tbss() (*Section, error) {
 			return &sec, nil
 		}
 	}
-	return nil, errors.New("no thread-local uninitialized data section (tbss)")
+	return nil, ErrNoTbss
+}
+
+// Tdata gets the thread-local initialized data section
+func (f *File) Tdata() (*Section, error) {
+	if err := f.LoadSections(); err != nil {
+		return nil, err
+	}
+	for _, sec := range f.Sections {
+		if sec.Type == elf.SHT_PROGBITS && sec.Flags&elf.SHF_TLS != 0 {
+			return &sec, nil
+		}
+	}
+	return nil, ErrNoTdata
 }
 
 // ReadVirtualMemory reads bytes from given virtual address