cmd/link: pass-through undefined call targets in external link mode

Allows Go asm calls referencing a function in a .syso file to be
passed through to the external linker, that would have otherwise
raised a "relocation target X not defined" error in cmd/link.

Fixes #33139

Change-Id: I2a8eb6063ebcd05fac96f141acf7652cf9189766
Reviewed-on: https://go-review.googlesource.com/c/go/+/198798
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/testdata/script/link_syso_issue33139.txt

+# Test that we can use the external linker with a host syso file that is
+# embedded in a package, that is referenced by a Go assembly function.
+# See issue 33139.
+[!gc] skip
+[!exec:cc] skip
+
+# External linking is not supported on linux/ppc64.
+# See: https://github.com/golang/go/issues/8912
+[linux] [ppc64] skip
+
+# External linking is not supported on darwin/386 (10.14+).
+# See: https://github.com/golang/go/issues/31751
+[darwin] [386] skip
+
+cc -c -o syso/objTestImpl.syso syso/src/objTestImpl.c
+go build -ldflags='-linkmode=external' ./cmd/main.go
+
+-- syso/objTest.s --
+#include "textflag.h"
+
+TEXT ·ObjTest(SB), NOSPLIT, $0
+	// We do not actually execute this function in the test above, thus
+	// there is no stack frame setup here.
+	// We only care about Go build and/or link errors when referencing
+	// the objTestImpl symbol in the syso file.
+	JMP objTestImpl(SB)
+
+-- syso/pkg.go --
+package syso
+
+func ObjTest()
+
+-- syso/src/objTestImpl.c --
+void objTestImpl() { /* Empty */ }
+
+-- cmd/main.go --
+package main
+
+import "syso"
+
+func main() {
+	syso.ObjTest()
+}

src/cmd/link/internal/arm/asm.go

@@ -613,7 +613,7 @@ func archreloc(ctxt *ld.Link, r *sym.Reloc, s *sym.Symbol, val int64) (int64, bo
 				rs = rs.Outer
 			}
 
-			if rs.Type != sym.SHOSTOBJ && rs.Type != sym.SDYNIMPORT && rs.Sect == nil {
+			if rs.Type != sym.SHOSTOBJ && rs.Type != sym.SDYNIMPORT && rs.Type != sym.SUNDEFEXT && rs.Sect == nil {
 				ld.Errorf(s, "missing section for %s", rs.Name)
 			}
 			r.Xsym = rs

src/cmd/link/internal/ld/data.go

@@ -96,7 +96,7 @@ func trampoline(ctxt *Link, s *sym.Symbol) {
 		if !r.Type.IsDirectJump() {
 			continue
 		}
-		if Symaddr(r.Sym) == 0 && r.Sym.Type != sym.SDYNIMPORT {
+		if Symaddr(r.Sym) == 0 && (r.Sym.Type != sym.SDYNIMPORT && r.Sym.Type != sym.SUNDEFEXT) {
 			if r.Sym.File != s.File {
 				if !isRuntimeDepPkg(s.File) || !isRuntimeDepPkg(r.Sym.File) {
 					ctxt.ErrorUnresolved(s, r)
@@ -298,7 +298,7 @@ func relocsym(ctxt *Link, s *sym.Symbol) {
 					rs = rs.Outer
 				}
 
-				if rs.Type != sym.SHOSTOBJ && rs.Type != sym.SDYNIMPORT && rs.Sect == nil {
+				if rs.Type != sym.SHOSTOBJ && rs.Type != sym.SDYNIMPORT && rs.Type != sym.SUNDEFEXT && rs.Sect == nil {
 					Errorf(s, "missing section for relocation target %s", rs.Name)
 				}
 				r.Xsym = rs
@@ -418,6 +418,17 @@ func relocsym(ctxt *Link, s *sym.Symbol) {
 			}
 			fallthrough
 		case objabi.R_CALL, objabi.R_PCREL:
+			if ctxt.LinkMode == LinkExternal && r.Sym != nil && r.Sym.Type == sym.SUNDEFEXT {
+				// pass through to the external linker.
+				r.Done = false
+				r.Xadd = 0
+				if ctxt.IsELF {
+					r.Xadd -= int64(r.Siz)
+				}
+				r.Xsym = r.Sym
+				o = 0
+				break
+			}
 			if ctxt.LinkMode == LinkExternal && r.Sym != nil && r.Sym.Type != sym.SCONST && (r.Sym.Sect != s.Sect || r.Type == objabi.R_GOTPCREL) {
 				r.Done = false
 

src/cmd/link/internal/ld/go.go

@@ -334,6 +334,24 @@ func fieldtrack(ctxt *Link) {
 }
 
 func (ctxt *Link) addexport() {
+	// Track undefined external symbols during external link.
+	if ctxt.LinkMode == LinkExternal {
+		for _, s := range ctxt.Syms.Allsym {
+			if !s.Attr.Reachable() || s.Attr.Special() || s.Attr.SubSymbol() {
+				continue
+			}
+			if s.Type != sym.STEXT {
+				continue
+			}
+			for i := range s.R {
+				r := &s.R[i]
+				if r.Sym != nil && r.Sym.Type == sym.Sxxx {
+					r.Sym.Type = sym.SUNDEFEXT
+				}
+			}
+		}
+	}
+
 	// TODO(aix)
 	if ctxt.HeadType == objabi.Hdarwin || ctxt.HeadType == objabi.Haix {
 		return

src/cmd/link/internal/ld/lib.go

@@ -2369,6 +2369,11 @@ func genasmsym(ctxt *Link, put func(*Link, *sym.Symbol, string, SymbolType, int6
 			}
 			put(ctxt, s, s.Name, BSSSym, Symaddr(s), s.Gotype)
 
+		case sym.SUNDEFEXT:
+			if ctxt.HeadType == objabi.Hwindows || ctxt.HeadType == objabi.Haix || ctxt.IsELF {
+				put(ctxt, s, s.Name, UndefinedSym, s.Value, nil)
+			}
+
 		case sym.SHOSTOBJ:
 			if ctxt.HeadType == objabi.Hwindows || ctxt.IsELF {
 				put(ctxt, s, s.Name, UndefinedSym, s.Value, nil)

src/cmd/link/internal/ld/macho.go

@@ -809,7 +809,7 @@ func machogenasmsym(ctxt *Link) {
 			}
 		}
 
-		if s.Type == sym.SDYNIMPORT || s.Type == sym.SHOSTOBJ {
+		if s.Type == sym.SDYNIMPORT || s.Type == sym.SHOSTOBJ || s.Type == sym.SUNDEFEXT {
 			if s.Attr.Reachable() {
 				addsym(ctxt, s, "", DataSym, 0, nil)
 			}
@@ -886,7 +886,7 @@ func machosymtab(ctxt *Link) {
 		// replace "·" as ".", because DTrace cannot handle it.
 		Addstring(symstr, strings.Replace(s.Extname(), "·", ".", -1))
 
-		if s.Type == sym.SDYNIMPORT || s.Type == sym.SHOSTOBJ {
+		if s.Type == sym.SDYNIMPORT || s.Type == sym.SHOSTOBJ || s.Type == sym.SUNDEFEXT {
 			symtab.AddUint8(0x01)                             // type N_EXT, external symbol
 			symtab.AddUint8(0)                                // no section
 			symtab.AddUint16(ctxt.Arch, 0)                    // desc

src/cmd/link/internal/ld/pe.go

@@ -685,7 +685,7 @@ func (f *peFile) writeSymbols(ctxt *Link) {
 		// Only windows/386 requires underscore prefix on external symbols.
 		if ctxt.Arch.Family == sys.I386 &&
 			ctxt.LinkMode == LinkExternal &&
-			(s.Type == sym.SHOSTOBJ || s.Attr.CgoExport()) {
+			(s.Type == sym.SHOSTOBJ || s.Type == sym.SUNDEFEXT || s.Attr.CgoExport()) {
 			s.Name = "_" + s.Name
 		}
 

src/cmd/link/internal/ld/symtab.go

@@ -110,7 +110,7 @@ func putelfsym(ctxt *Link, x *sym.Symbol, s string, t SymbolType, addr int64, go
 	}
 
 	var elfshnum int
-	if xo.Type == sym.SDYNIMPORT || xo.Type == sym.SHOSTOBJ {
+	if xo.Type == sym.SDYNIMPORT || xo.Type == sym.SHOSTOBJ || xo.Type == sym.SUNDEFEXT {
 		elfshnum = SHN_UNDEF
 	} else {
 		if xo.Sect == nil {

src/cmd/link/internal/ld/xcoff.go

@@ -955,7 +955,7 @@ func putaixsym(ctxt *Link, x *sym.Symbol, str string, t SymbolType, addr int64,
 		syms = append(syms, a4)
 
 	case UndefinedSym:
-		if x.Type != sym.SDYNIMPORT && x.Type != sym.SHOSTOBJ {
+		if x.Type != sym.SDYNIMPORT && x.Type != sym.SHOSTOBJ && x.Type != sym.SUNDEFEXT {
 			return
 		}
 		s := &XcoffSymEnt64{

src/cmd/link/internal/ppc64/asm.go

@@ -801,7 +801,7 @@ func archreloc(ctxt *ld.Link, r *sym.Reloc, s *sym.Symbol, val int64) (int64, bo
 				rs = rs.Outer
 			}
 
-			if rs.Type != sym.SHOSTOBJ && rs.Type != sym.SDYNIMPORT && rs.Sect == nil {
+			if rs.Type != sym.SHOSTOBJ && rs.Type != sym.SDYNIMPORT && rs.Type != sym.SUNDEFEXT && rs.Sect == nil {
 				ld.Errorf(s, "missing section for %s", rs.Name)
 			}
 			r.Xsym = rs

src/cmd/link/internal/sym/symkind.go

@@ -104,6 +104,7 @@ const (
 	SCONST
 	SDYNIMPORT
 	SHOSTOBJ
+	SUNDEFEXT // Undefined symbol for resolution by external linker
 
 	// Sections for debugging information
 	SDWARFSECT

src/cmd/link/internal/sym/symkind_string.go

-// Code generated by "stringer -type=SymKind"; DO NOT EDIT.
+// Code generated by "stringer -type=SymKind symkind.go"; DO NOT EDIT.
 
 package sym
 
@@ -54,17 +54,18 @@ func _() {
 	_ = x[SCONST-43]
 	_ = x[SDYNIMPORT-44]
 	_ = x[SHOSTOBJ-45]
-	_ = x[SDWARFSECT-46]
-	_ = x[SDWARFINFO-47]
-	_ = x[SDWARFRANGE-48]
-	_ = x[SDWARFLOC-49]
-	_ = x[SDWARFLINES-50]
-	_ = x[SABIALIAS-51]
+	_ = x[SUNDEFEXT-46]
+	_ = x[SDWARFSECT-47]
+	_ = x[SDWARFINFO-48]
+	_ = x[SDWARFRANGE-49]
+	_ = x[SDWARFLOC-50]
+	_ = x[SDWARFLINES-51]
+	_ = x[SABIALIAS-52]
 }
 
-const _SymKind_name = "SxxxSTEXTSELFRXSECTSTYPESSTRINGSGOSTRINGSGOFUNCSGCBITSSRODATASFUNCTABSELFROSECTSMACHOPLTSTYPERELROSSTRINGRELROSGOSTRINGRELROSGOFUNCRELROSGCBITSRELROSRODATARELROSFUNCTABRELROSTYPELINKSITABLINKSSYMTABSPCLNTABSFirstWritableSBUILDINFOSELFSECTSMACHOSMACHOGOTSWINDOWSSELFGOTSNOPTRDATASINITARRSDATASXCOFFTOCSBSSSNOPTRBSSSTLSBSSSXREFSMACHOSYMSTRSMACHOSYMTABSMACHOINDIRECTPLTSMACHOINDIRECTGOTSFILEPATHSCONSTSDYNIMPORTSHOSTOBJSDWARFSECTSDWARFINFOSDWARFRANGESDWARFLOCSDWARFLINESSABIALIAS"
+const _SymKind_name = "SxxxSTEXTSELFRXSECTSTYPESSTRINGSGOSTRINGSGOFUNCSGCBITSSRODATASFUNCTABSELFROSECTSMACHOPLTSTYPERELROSSTRINGRELROSGOSTRINGRELROSGOFUNCRELROSGCBITSRELROSRODATARELROSFUNCTABRELROSTYPELINKSITABLINKSSYMTABSPCLNTABSFirstWritableSBUILDINFOSELFSECTSMACHOSMACHOGOTSWINDOWSSELFGOTSNOPTRDATASINITARRSDATASXCOFFTOCSBSSSNOPTRBSSSTLSBSSSXREFSMACHOSYMSTRSMACHOSYMTABSMACHOINDIRECTPLTSMACHOINDIRECTGOTSFILEPATHSCONSTSDYNIMPORTSHOSTOBJSUNDEFEXTSDWARFSECTSDWARFINFOSDWARFRANGESDWARFLOCSDWARFLINESSABIALIAS"
 
-var _SymKind_index = [...]uint16{0, 4, 9, 19, 24, 31, 40, 47, 54, 61, 69, 79, 88, 98, 110, 124, 136, 148, 160, 173, 182, 191, 198, 206, 220, 230, 238, 244, 253, 261, 268, 278, 286, 291, 300, 304, 313, 320, 325, 337, 349, 366, 383, 392, 398, 408, 416, 426, 436, 447, 456, 467, 476}
+var _SymKind_index = [...]uint16{0, 4, 9, 19, 24, 31, 40, 47, 54, 61, 69, 79, 88, 98, 110, 124, 136, 148, 160, 173, 182, 191, 198, 206, 220, 230, 238, 244, 253, 261, 268, 278, 286, 291, 300, 304, 313, 320, 325, 337, 349, 366, 383, 392, 398, 408, 416, 425, 435, 445, 456, 465, 476, 485}
 
 func (i SymKind) String() string {
 	if i >= SymKind(len(_SymKind_index)-1) {