Extract DTV info from __tls_get_addr, add to LibcInfo (open-telemetry#929)

Co-authored-by: Timo Teräs <timo.teras@iki.fi>

Author: dalehamel

interpreter/perl/instance.go

@@ -74,7 +74,21 @@ func hashCOPKey(k copKey) uint32 {
 
 func (i *perlInstance) UpdateLibcInfo(ebpf interpreter.EbpfHandler, pid libpf.PID,
 	libcInfo libc.LibcInfo) error {
+	// Perl requires TSDInfo to access thread state. If stateInTSD is true,
+	// we need valid TSDInfo to proceed. If it's false, we can proceed without it.
+	// Since UpdateLibcInfo may be called multiple times as LibcInfo is collected
+	// from multiple DSOs, we should only insert proc data when we have what we need.
 	d := i.d
+	if d.stateInTSD && !libcInfo.HasTSDInfo() {
+		// We need TSDInfo but don't have it yet, wait for another call
+		return nil
+	}
+
+	// If we've already inserted proc info, don't do it again
+	if i.procInfoInserted {
+		return nil
+	}
+
 	stateInTSD := uint8(0)
 	if d.stateInTSD {
 		stateInTSD = 1

interpreter/python/python.go

@@ -377,6 +377,20 @@ func (p *pythonInstance) GetAndResetMetrics() ([]metrics.Metric, error) {
 func (p *pythonInstance) UpdateLibcInfo(ebpf interpreter.EbpfHandler, pid libpf.PID,
 	libcInfo libc.LibcInfo) error {
 	d := p.d
+
+	// If we don't have a static TLS offset (Python < 3.13 or extraction failed),
+	// we need TSDInfo to access thread state via pthread_getspecific.
+	// Since UpdateLibcInfo may be called multiple times as LibcInfo is collected
+	// from multiple DSOs, wait until we have TSDInfo before inserting proc data.
+	if d.staticTLSOffset == 0 && !libcInfo.HasTSDInfo() {
+		return nil
+	}
+
+	// Prevent duplicate inserts
+	if p.procInfoInserted {
+		return nil
+	}
+
 	vm := &d.vmStructs
 
 	cdata := support.PyProcInfo{

libc/libc.go

@@ -13,11 +13,73 @@ import (
 )
 
 type TSDInfo = support.TSDInfo
+type DTVInfo = support.DTVInfo
 
 // LibcInfo contains introspection information extracted from the C-library
 type LibcInfo struct {
 	// TSDInfo is the TSDInfo extracted for this C-library
 	TSDInfo TSDInfo
+	// DTVInfo contains DTV (Dynamic Thread Vector) introspection data for accessing
+	// TLS variables when TLS descriptors are not available
+	DTVInfo DTVInfo
+}
+
+// IsEqual checks if two LibcInfo instances are equal
+func (l LibcInfo) IsEqual(other LibcInfo) bool {
+	return l.TSDInfo == other.TSDInfo && l.DTVInfo == other.DTVInfo
+}
+
+// Merge fills in empty fields of the receiver with corresponding values from other.
+// Fields already populated in the receiver are not overwritten.
+func (l *LibcInfo) Merge(other LibcInfo) {
+	// If other has TSDInfo and this instance does not, take it
+	if l.TSDInfo == (TSDInfo{}) {
+		l.TSDInfo = other.TSDInfo
+	}
+
+	// If other has DTVInfo and this instance does not, take it
+	if l.DTVInfo == (DTVInfo{}) {
+		l.DTVInfo = other.DTVInfo
+	}
+}
+
+// HasTSDInfo returns true if the LibcInfo contains valid TSD information.
+// TSDInfo is considered valid when the Multiplier field is non-zero.
+func (l LibcInfo) HasTSDInfo() bool {
+	return l.TSDInfo.Multiplier != 0
+}
+
+// HasDTVInfo returns true if the LibcInfo contains valid DTV information.
+// DTVInfo is considered valid when the Multiplier field is non-zero.
+func (l LibcInfo) HasDTVInfo() bool {
+	return l.DTVInfo.Multiplier != 0
+}
+
+var (
+	// regex for the libc
+	libcRegex = regexp.MustCompile(`.*/(ld-musl|ld-linux|libc|libpthread)([-.].*)?\.so`)
+)
+
+// IsPotentialLibcDSO determines if the DSO filename potentially contains libc code
+func IsPotentialLibcDSO(filename string) bool {
+	return libcRegex.MatchString(filename)
+}
+
+func ExtractLibcInfo(ef *pfelf.File) (*LibcInfo, error) {
+	tsdinfo, err := extractTSDInfo(ef)
+	if err != nil {
+		return nil, err
+	}
+
+	dtvinfo, err := extractDTVInfo(ef)
+	if err != nil {
+		return &LibcInfo{}, err
+	}
+
+	return &LibcInfo{
+		TSDInfo: tsdinfo,
+		DTVInfo: dtvinfo,
+	}, nil
 }
 
 // This code analyzes the C-library provided POSIX defined function which is used
@@ -65,27 +127,6 @@ type LibcInfo struct {
 //
 // Reading the value is basically "return self->specific_1stblock[key].data;"
 
-var (
-	// regex for the libc
-	libcRegex = regexp.MustCompile(`.*/(ld-musl|libc|libpthread)([-.].*)?\.so`)
-)
-
-// IsPotentialTSDDSO determines if the DSO filename potentially contains pthread code
-func IsPotentialTSDDSO(filename string) bool {
-	return libcRegex.MatchString(filename)
-}
-
-func ExtractLibcInfo(ef *pfelf.File) (*LibcInfo, error) {
-	tsdinfo, err := extractTSDInfo(ef)
-	if err != nil {
-		return nil, err
-	}
-
-	return &LibcInfo{
-		TSDInfo: tsdinfo,
-	}, nil
-}
-
 // extractTSDInfo extracts the introspection data for pthread thread specific data.
 func extractTSDInfo(ef *pfelf.File) (TSDInfo, error) {
 	_, code, err := ef.SymbolData("__pthread_getspecific", 2048)
@@ -113,3 +154,31 @@ func extractTSDInfo(ef *pfelf.File) (TSDInfo, error) {
 	}
 	return info, nil
 }
+
+// extractDTVInfo extracts the introspection data for the DTV to access TLS vars
+func extractDTVInfo(ef *pfelf.File) (DTVInfo, error) {
+	var info DTVInfo
+	_, code, err := ef.SymbolData("__tls_get_addr", 2048)
+	if err != nil {
+		// If the symbol is not exported, this is not a critical error.
+		// Callers can check HasDTVInfo() to determine if DTV data is available.
+		return info, nil
+	}
+
+	if len(code) < 8 {
+		return info, fmt.Errorf("__tls_get_addr function size is %d", len(code))
+	}
+
+	switch ef.Machine {
+	case elf.EM_AARCH64:
+		info, err = extractDTVInfoARM(code)
+	case elf.EM_X86_64:
+		info, err = extractDTVInfoX86(code)
+	default:
+		return info, fmt.Errorf("unsupported arch %s", ef.Machine.String())
+	}
+	if err != nil {
+		return info, fmt.Errorf("failed to extract DTV data: %s", err)
+	}
+	return info, nil
+}

libc/libc_aarch64.go

@@ -268,3 +268,146 @@ func extractTSDInfoARM(code []byte) (TSDInfo, error) {
 		Indirect:   indirect,
 	}, nil
 }
+
+func extractDTVInfoARM(code []byte) (DTVInfo, error) {
+	// Track register states similar to extractTSDInfoARM
+	var regs [32]regState
+
+	dtvOffset := int16(0)
+	entryWidth := uint32(0)
+	resetReg := int(-1)
+
+	// Scan entire function
+	for offs := 0; offs < len(code); offs += 4 {
+		if offs+4 > len(code) {
+			break
+		}
+
+		if resetReg >= 0 {
+			// Reset register state if something unsupported happens on it
+			regs[resetReg] = regState{status: Unspec}
+		}
+
+		inst, err := aa.Decode(code[offs:])
+		if err != nil {
+			continue
+		}
+		if inst.Op == aa.RET {
+			break
+		}
+
+		destReg, ok := arm.Xreg2num(inst.Args[0])
+		if !ok {
+			continue
+		}
+
+		resetReg = destReg
+		switch inst.Op {
+		case aa.MOV:
+			// Track register moves
+			srcReg, ok := arm.Xreg2num(inst.Args[1])
+			if !ok {
+				continue
+			}
+			regs[destReg] = regs[srcReg]
+
+		case aa.MRS:
+			// MRS X1, S3_3_C13_C0_2 (tpidr_el0)
+			if inst.Args[1].String() == "S3_3_C13_C0_2" {
+				regs[destReg] = regState{
+					status:     TSDBase, // Reuse TSDBase to mean thread pointer
+					multiplier: 1,
+				}
+			}
+
+		case aa.LDUR:
+			// LDUR X1, [X1,#-8]
+			m, ok := inst.Args[1].(aa.MemImmediate)
+			if !ok {
+				continue
+			}
+			srcReg, ok := arm.Xreg2num(m.Base)
+			if !ok {
+				continue
+			}
+			if regs[srcReg].status == TSDBase {
+				imm, ok := arm.DecodeImmediate(m)
+				if !ok {
+					continue
+				}
+				// This is loading the DTV pointer from thread pointer
+				dtvOffset = int16(imm & 0xFFFF)
+				regs[destReg] = regState{
+					status:     TSDElementBase, // DTV pointer
+					offset:     imm,
+					multiplier: 1,
+				}
+			} else {
+				continue
+			}
+
+		case aa.LDR:
+			if len(inst.Args) < 2 {
+				continue
+			}
+			switch m := inst.Args[1].(type) {
+			case aa.MemImmediate:
+				// ldr x1, [x1, #0] or ldr x1, [x1]
+				srcReg, ok := arm.Xreg2num(m.Base)
+				if !ok {
+					continue
+				}
+				if regs[srcReg].status == TSDBase {
+					// Loading DTV pointer from thread pointer
+					imm, ok := arm.DecodeImmediate(m)
+					if !ok {
+						imm = 0
+					}
+					dtvOffset = int16(imm & 0xFFFF)
+					regs[destReg] = regState{
+						status:     TSDElementBase, // DTV pointer
+						offset:     imm,
+						multiplier: 1,
+					}
+				} else {
+					continue
+				}
+
+			case aa.MemExtend:
+				// ldr x1, [x1, x2, lsl #3]
+				srcReg, ok := arm.Xreg2num(m.Base)
+				if !ok {
+					continue
+				}
+				if regs[srcReg].status == TSDElementBase {
+					// This is indexing into the DTV array
+					if m.Amount > 0 {
+						entryWidth = uint32(1 << m.Amount)
+					}
+				}
+			}
+
+		case aa.LSL:
+			// lsl x3, x3, #4
+			if len(inst.Args) >= 3 {
+				if imm, ok := inst.Args[2].(aa.Imm); ok {
+					entryWidth = uint32(1 << imm.Imm)
+				}
+			}
+
+		case aa.CMP, aa.CBZ, aa.CMN:
+			// Opcode with no affect on first argument.
+			// Noop to exit switch without default continue.
+
+		default:
+			continue
+		}
+		resetReg = -1
+	}
+
+	return DTVInfo{
+		Offset:     dtvOffset,
+		Multiplier: uint8(entryWidth),
+		Indirect:   1,
+	}, nil
+}