cmd/link,cmd/compile: add -tls flag for TLS model control

Add -tls flag to control Thread Local Storage model selection with
options: auto, LE (Local Exec), IE (Initial Exec), GD (General Dynamic).
Default behavior selects GD for c-shared/c-archive builds on Unix
platforms to enable dlopen() compatibility with non-glibc systems.

Validates TLS model selection to prevent invalid combinations such as
LE with shared libraries. Updates all architectures (x86, ARM64, ARM,
PowerPC64, s390x, RISC-V, LoongArch64, MIPS) to use centralized TLS
model selection logic through ShouldUseTLSGD() and ShouldUseTLSLE()
helper functions.

Also fixes missing variable declaration in runtime/os_linux.go that
was causing build failures.

Fixes #13492

Co-Authored-By: Alexander Musman <[email protected]>

Author: jgowdy

src/cmd/compile/internal/base/flag.go

@@ -119,6 +119,7 @@ type CmdFlags struct {
 	Race               bool         "help:\"enable race detector\""
 	Shared             *bool        "help:\"generate code that can be linked into a shared library\"" // &Ctxt.Flag_shared, set below
 	SmallFrames        bool         "help:\"reduce the size limit for stack allocated objects\""      // small stacks, to diagnose GC latency; see golang.org/issue/27732
+	TLS                string       "help:\"set TLS model (auto, LE, IE, GD)\""
 	Spectre            string       "help:\"enable spectre mitigations in `list` (all, index, ret)\""
 	Std                bool         "help:\"compiling standard library\""
 	SymABIs            string       "help:\"read symbol ABIs from `file`\""
@@ -292,6 +293,11 @@ func ParseFlags() {
 		log.Fatalf("%s/%s does not support -shared", buildcfg.GOOS, buildcfg.GOARCH)
 	}
 	parseSpectre(Flag.Spectre) // left as string for RecordFlags
+	
+	// Parse TLS model before setting other flags that depend on it
+	if err := parseTLSModel(Flag.TLS); err != nil {
+		log.Fatalf("%v", err)
+	}
 
 	Ctxt.Flag_shared = Ctxt.Flag_dynlink || Ctxt.Flag_shared
 	Ctxt.Flag_optimize = Flag.N == 0
@@ -568,6 +574,27 @@ func readEmbedCfg(file string) {
 	}
 }
 
+// parseTLSModel parses the TLS model from the string s.
+func parseTLSModel(s string) error {
+	if s == "" {
+		s = "auto" // Default to auto
+	}
+	
+	switch s {
+	case "auto":
+		Ctxt.TLSModel = obj.TLSModelAuto
+	case "LE":
+		Ctxt.TLSModel = obj.TLSModelLE
+	case "IE":
+		Ctxt.TLSModel = obj.TLSModelIE
+	case "GD":
+		Ctxt.TLSModel = obj.TLSModelGD
+	default:
+		return fmt.Errorf("invalid TLS model %q; valid values are auto, LE, IE, GD", s)
+	}
+	return nil
+}
+
 // parseSpectre parses the spectre configuration from the string s.
 func parseSpectre(s string) {
 	for _, f := range strings.Split(s, ",") {

src/cmd/internal/obj/arm/asm5.go

@@ -871,7 +871,7 @@ func (c *ctxt5) aclass(a *obj.Addr) int {
 
 			c.instoffset = 0 // s.b. unused but just in case
 			if a.Sym.Type == objabi.STLSBSS {
-				if c.ctxt.Flag_shared && (c.ctxt.Headtype == objabi.Hlinux || c.ctxt.Headtype == objabi.Hfreebsd || c.ctxt.Headtype == objabi.Hopenbsd) {
+				if c.ctxt.ShouldUseTLSGD() {
 					// Use General Dynamic model for shared libraries
 					// to support non-glibc dlopen()
 					return C_TLS_GD

src/cmd/internal/obj/arm64/asm7.go

@@ -2103,7 +2103,7 @@ func (c *ctxt7) aclass(a *obj.Addr) int {
 				if a.Sym.Type == objabi.STLSBSS {
 					// For c-shared/c-archive on standards-compliant systems,
 					// use general dynamic model for dlopen compatibility.
-					if c.ctxt.Flag_shared && (c.ctxt.Headtype == objabi.Hlinux || c.ctxt.Headtype == objabi.Hfreebsd || c.ctxt.Headtype == objabi.Hopenbsd) {
+					if c.ctxt.ShouldUseTLSGD() {
 						return C_TLS_GD
 					} else {
 						return C_TLS_LE

src/cmd/internal/obj/link.go

@@ -45,6 +45,47 @@ import (
 	"sync/atomic"
 )
 
+// TLSModel represents different Thread Local Storage models
+type TLSModel int
+
+const (
+	TLSModelAuto TLSModel = iota // Automatic selection based on platform and build mode
+	TLSModelLE                   // Local Exec - fastest, static executables only
+	TLSModelIE                   // Initial Exec - fast, may not work with dlopen on non-glibc
+	TLSModelGD                   // General Dynamic - compatible with all dlopen scenarios
+)
+
+// ShouldUseTLSGD returns true if General Dynamic TLS model should be used
+func (ctxt *Link) ShouldUseTLSGD() bool {
+	switch ctxt.TLSModel {
+	case TLSModelGD:
+		return true
+	case TLSModelLE, TLSModelIE:
+		return false
+	case TLSModelAuto:
+		// Auto selection logic - matches our current implementation
+		return ctxt.Flag_shared && (ctxt.Headtype == objabi.Hlinux || 
+			ctxt.Headtype == objabi.Hfreebsd || ctxt.Headtype == objabi.Hopenbsd)
+	default:
+		return false
+	}
+}
+
+// ShouldUseTLSLE returns true if Local Exec TLS model should be used
+func (ctxt *Link) ShouldUseTLSLE() bool {
+	switch ctxt.TLSModel {
+	case TLSModelLE:
+		return true
+	case TLSModelGD, TLSModelIE:
+		return false
+	case TLSModelAuto:
+		// Auto selection: use LE for static executables on supported platforms
+		return !ctxt.Flag_shared && ctxt.Headtype != objabi.Hwindows && ctxt.Headtype != objabi.Hplan9
+	default:
+		return false
+	}
+}
+
 // An Addr is an argument to an instruction.
 // The general forms and their encodings are:
 //
@@ -1147,6 +1188,7 @@ type Link struct {
 	Flag_locationlists bool
 	Flag_noRefName     bool   // do not include referenced symbol names in object file
 	Retpoline          bool   // emit use of retpoline stubs for indirect jmp/call
+	TLSModel           TLSModel // TLS model selection
 	Flag_maymorestack  string // If not "", call this function before stack checks
 	Bso                *bufio.Writer
 	Pathname           string

src/cmd/internal/obj/loong64/asm.go

@@ -763,7 +763,7 @@ func (c *ctxt0) aclass(a *obj.Addr) int {
 			}
 			c.instoffset = a.Offset
 			if a.Sym.Type == objabi.STLSBSS {
-				if c.ctxt.Flag_shared && (c.ctxt.Headtype == objabi.Hlinux || c.ctxt.Headtype == objabi.Hfreebsd || c.ctxt.Headtype == objabi.Hopenbsd) {
+				if c.ctxt.ShouldUseTLSGD() {
 					return C_TLS_GD
 				} else {
 					return C_TLS_LE

src/cmd/internal/obj/mips/asm0.go

@@ -610,7 +610,7 @@ func (c *ctxt0) aclass(a *obj.Addr) int {
 			if a.Sym != nil { // use relocation
 				if a.Sym.Type == objabi.STLSBSS {
 					// For shared libraries, use general dynamic TLS model
-					if c.ctxt.Flag_shared && (c.ctxt.Headtype == objabi.Hlinux || c.ctxt.Headtype == objabi.Hfreebsd || c.ctxt.Headtype == objabi.Hopenbsd) {
+					if c.ctxt.ShouldUseTLSGD() {
 						return C_TLS_GD
 					}
 					return C_TLS

src/cmd/internal/obj/ppc64/asm9.go

@@ -910,7 +910,7 @@ func (c *ctxt9) aclass(a *obj.Addr) int {
 			} else if a.Sym.Type == objabi.STLSBSS {
 				// For shared libraries, use general dynamic TLS model
 				// to support non-glibc dlopen()
-				if c.ctxt.Flag_shared && (c.ctxt.Headtype == objabi.Hlinux || c.ctxt.Headtype == objabi.Hfreebsd || c.ctxt.Headtype == objabi.Hopenbsd) {
+				if c.ctxt.ShouldUseTLSGD() {
 					return C_TLS_GD
 				}
 				// Otherwise, use 8 byte local-exec TLS accesses.

src/cmd/internal/obj/riscv/obj.go

@@ -3904,7 +3904,7 @@ func assemble(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 				break
 			}
 			if addr.Sym.Type == objabi.STLSBSS {
-				if ctxt.Flag_shared && (ctxt.Headtype == objabi.Hlinux || ctxt.Headtype == objabi.Hfreebsd || ctxt.Headtype == objabi.Hopenbsd) {
+				if ctxt.ShouldUseTLSGD() {
 					rt = objabi.R_RISCV_TLS_GD
 				} else {
 					rt = objabi.R_RISCV_TLS_LE

src/cmd/internal/obj/s390x/asmz.go

@@ -582,7 +582,7 @@ func (c *ctxtz) aclass(a *obj.Addr) int {
 			}
 			c.instoffset = a.Offset
 			if a.Sym.Type == objabi.STLSBSS {
-				if c.ctxt.Flag_shared && (c.ctxt.Headtype == objabi.Hlinux || c.ctxt.Headtype == objabi.Hfreebsd || c.ctxt.Headtype == objabi.Hopenbsd) {
+				if c.ctxt.ShouldUseTLSGD() {
 					return C_TLS_GD // general dynamic model
 				}
 				return C_TLS_LE // local exec model

src/cmd/internal/obj/sym.go

@@ -57,6 +57,7 @@ func Linknew(arch *LinkArch) *Link {
 	}
 
 	ctxt.Flag_optimize = true
+	ctxt.TLSModel = TLSModelAuto // Default to automatic TLS model selection
 	return ctxt
 }