cmd/link: don't decode type symbol in shared library in deadcode

In the linker's deadcode pass we decode type symbols for
interface satisfaction analysis. When linking against Go shared
libraries, the type symbol may come from a shared library, so it
doesn't have data in the current module being linked, so we cannot
decode it. We already have code to skip DYNIMPORT symbols. However,
this doesn't actually work, because at that point the type symbols'
names haven't been mangled, whereas they may be mangled in the
shared library. So the symbol definition (in shared library) and
reference (in current module) haven't been connected.

Skip decoding type symbols of type Sxxx (along with DYNIMPORT)
when linkShared.

Note: we cannot skip all type symbols, as we still need to mark
unexported methods defined in the current module.

Fixes #44031.

Change-Id: I833d19a060c94edbd6fc448172358f9a7d760657
Reviewed-on: https://go-review.googlesource.com/c/go/+/288496
Trust: Cherry Zhang <cherryyz@google.com>
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

misc/cgo/testshared/shared_test.go

@@ -1063,3 +1063,11 @@ func TestGCData(t *testing.T) {
 	goCmd(t, "build", "-linkshared", "./gcdata/main")
 	runWithEnv(t, "running gcdata/main", []string{"GODEBUG=clobberfree=1"}, "./main")
 }
+
+// Test that we don't decode type symbols from shared libraries (which has no data,
+// causing panic). See issue 44031.
+func TestIssue44031(t *testing.T) {
+	goCmd(t, "install", "-buildmode=shared", "-linkshared", "./issue44031/a")
+	goCmd(t, "install", "-buildmode=shared", "-linkshared", "./issue44031/b")
+	goCmd(t, "run", "-linkshared", "./issue44031/main")
+}

misc/cgo/testshared/testdata/issue44031/a/a.go

@@ -0,0 +1,9 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package a
+
+type ATypeWithALoooooongName interface { // a long name, so the type descriptor symbol name is mangled
+	M()
+}

misc/cgo/testshared/testdata/issue44031/b/b.go

@@ -0,0 +1,17 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package b
+
+import "testshared/issue44031/a"
+
+type T int
+
+func (T) M() {}
+
+var i = a.ATypeWithALoooooongName(T(0))
+
+func F() {
+	i.M()
+}

misc/cgo/testshared/testdata/issue44031/main/main.go

@@ -0,0 +1,20 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "testshared/issue44031/b"
+
+type t int
+
+func (t) m() {}
+
+type i interface{ m() } // test that unexported method is correctly marked
+
+var v interface{} = t(0)
+
+func main() {
+	b.F()
+	v.(i).m()
+}

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

@@ -165,13 +165,17 @@ func (d *deadcodePass) flood() {
 				// R_USEIFACEMETHOD is a marker relocation that marks an interface
 				// method as used.
 				rs := r.Sym()
-				if d.ldr.SymType(rs) != sym.SDYNIMPORT { // don't decode DYNIMPORT symbol (we'll mark all exported methods anyway)
-					m := d.decodeIfaceMethod(d.ldr, d.ctxt.Arch, rs, r.Add())
-					if d.ctxt.Debugvlog > 1 {
-						d.ctxt.Logf("reached iface method: %v\n", m)
-					}
-					d.ifaceMethod[m] = true
+				if d.ctxt.linkShared && (d.ldr.SymType(rs) == sym.SDYNIMPORT || d.ldr.SymType(rs) == sym.Sxxx) {
+					// Don't decode symbol from shared library (we'll mark all exported methods anyway).
+					// We check for both SDYNIMPORT and Sxxx because name-mangled symbols haven't
+					// been resolved at this point.
+					continue
 				}
+				m := d.decodeIfaceMethod(d.ldr, d.ctxt.Arch, rs, r.Add())
+				if d.ctxt.Debugvlog > 1 {
+					d.ctxt.Logf("reached iface method: %v\n", m)
+				}
+				d.ifaceMethod[m] = true
 				continue
 			}
 			rs := r.Sym()