cmd/link: load symbols from .syso in external link mode

Fix linking with a package having a .syso file in external link mode,
that would otherwise cause an error before executing the external
linker because it can't find symbols that are exported in the said
.syso file.

Fixes #33139

Change-Id: Id3ee737fba1c6f1e37910593dfedf9c84486d398
Reviewed-on: https://go-review.googlesource.com/c/go/+/186417Reviewed-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 stub.
+# See issue 33139.
+[!gc] stop
+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
+	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)
@@ -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

@@ -2348,7 +2348,7 @@ func genasmsym(ctxt *Link, put func(*Link, *sym.Symbol, string, SymbolType, int6
 			}
 			put(ctxt, s, s.Name, BSSSym, Symaddr(s), s.Gotype)
 
-		case sym.SHOSTOBJ:
+		case sym.SHOSTOBJ, sym.SUNDEFEXT:
 			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/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) {