[release-branch.go1.18] 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.

Fixes #54502

Change-Id: I6c93f5378f50e5edf30d5121402a48214abb1ce2
GitHub-Last-Rev: 085bbc55dbc95d55456cc2acc46a0d02416c2848
GitHub-Pull-Request: golang/go#54321
Reviewed-on: https://go-review.googlesource.com/c/go/+/422154
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit 2340d371796f95acdcc399e60f9ecefec2add8df)
Reviewed-on: https://go-review.googlesource.com/c/go/+/424536
Run-TryBot: Cherry Mui <cherryyz@google.com>

2 files changed, 81 insertions, 7 deletions

diff --git a/src/cmd/link/internal/ld/dwarf_test.go b/src/cmd/link/internal/ld/dwarf_test.go
index 2f9bf25d10..c1f60f0289 100644
--- a/src/cmd/link/internal/ld/dwarf_test.go
+++ b/src/cmd/link/internal/ld/dwarf_test.go
@@ -1843,3 +1843,82 @@ 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")
+	}
+}
diff --git a/src/cmd/link/internal/loader/loader.go b/src/cmd/link/internal/loader/loader.go
index 371a84ab2e..2a9435b96d 100644
--- a/src/cmd/link/internal/loader/loader.go
+++ b/src/cmd/link/internal/loader/loader.go
@@ -1630,13 +1630,8 @@ 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() {