cmd/link: Fix trampolines breaking DWARF line info

When trampolines are needed (e.g. Darwin ARM64), the DWARF LPT (Line
Program Table - see DWARF section 6.1) generation fails because the
replacement symbols are marked as external symbols and skipped during
the DWARF LPT generation phase.

This PR is still subject to extensive testing but has rectified the
issue for the very large binaries we link at Uber.

Fixes golang#54320

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

@@ -1842,3 +1842,82 @@ func main() {
 		}
 	}
 }
+func TestIssue54320(t *testing.T) {
+	// Check that when trampolines are used, the DWARF LPT is correctly
+	// emitted in the final binary
+	testenv.MustHaveGoBuild(t)
+
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; no DWARF symbol table in executables")
+	}
+
+	t.Parallel()
+
+	const prog = `
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Printf("Hello world\n");
+}
+`
+
+	dir := t.TempDir()
+	f := gobuild(t, dir, prog, "-ldflags=-debugtramp=2")
+	defer f.Close()
+
+	d, err := f.DWARF()
+	if err != nil {
+		t.Fatalf("error reading DWARF: %v", err)
+	}
+
+	rdr := d.Reader()
+	found := false
+	var entry *dwarf.Entry
+	for entry, err = rdr.Next(); entry != nil; entry, err = rdr.Next() {
+		if err != nil {
+			t.Fatalf("error reading DWARF: %v", err)
+		}
+		if entry.Tag != dwarf.TagCompileUnit {
+			continue
+		}
+		name, _ := entry.Val(dwarf.AttrName).(string)
+		if name == "main" {
+			found = true
+			break
+		}
+		rdr.SkipChildren()
+	}
+
+	if !found {
+		t.Fatalf("could not find main compile unit")
+	}
+	lr, err := d.LineReader(entry)
+	if err != nil {
+		t.Fatalf("error obtaining linereader: %v", err)
+	}
+
+	var le dwarf.LineEntry
+	found = false
+	for {
+		if err := lr.Next(&le); err != nil {
+			if err == io.EOF {
+				break
+			}
+			t.Fatalf("error reading linentry: %v", err)
+		}
+		// check LE contains an entry to test.go
+		if le.File == nil {
+			continue
+		}
+		file := filepath.Base(le.File.Name)
+		if file == "test.go" {
+			found = true
+			break
+		}
+	}
+	if !found {
+		t.Errorf("no LPT entries for test.go")
+	}
+}

src/cmd/link/internal/loader/loader.go

@@ -1610,13 +1610,8 @@ func (l *Loader) GetFuncDwarfAuxSyms(fnSymIdx Sym) (auxDwarfInfo, auxDwarfLoc, a
 	if l.SymType(fnSymIdx) != sym.STEXT {
 		log.Fatalf("error: non-function sym %d/%s t=%s passed to GetFuncDwarfAuxSyms", fnSymIdx, l.SymName(fnSymIdx), l.SymType(fnSymIdx).String())
 	}
-	if l.IsExternal(fnSymIdx) {
-		// Current expectation is that any external function will
-		// not have auxsyms.
-		return
-	}
-	r, li := l.toLocal(fnSymIdx)
-	auxs := r.Auxs(li)
+	r, auxs := l.auxs(fnSymIdx)
+
 	for i := range auxs {
 		a := &auxs[i]
 		switch a.Type() {