cmd/link: support weak binding on darwin

Symbols loaded from host files can have the N_WEAK_REF bit set,
which is used to instruct the loader to not fail if that symbol
can't be resolved.

The Go internal linker should honor this information by setting the
BIND_SYMBOL_FLAGS_WEAK_IMPORT flag in the corresponding bind table
entry.

Fixes #76023

Cq-Include-Trybots: luci.golang.try:gotip-darwin-amd64-longtest,gotip-darwin-amd64_12,gotip-darwin-arm64_12,gotip-darwin-arm64_15,gotip-darwin-arm64-longtest,gotip-darwin-amd64_14
Change-Id: Id2cef247ec7a9cb08455844f3c30ff874772bb7b
Reviewed-on: https://go-review.googlesource.com/c/go/+/713760
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/cgo/internal/test/cgo_darwin_test.go

@@ -0,0 +1,11 @@
+// Copyright 2025 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.
+
+//go:build cgo && darwin
+
+package cgotest
+
+import "testing"
+
+func TestIssue76023(t *testing.T) { issue76023(t) }

src/cmd/cgo/internal/test/issue76023.go

@@ -0,0 +1,27 @@
+// Copyright 2025 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.
+
+//go:build darwin
+
+package cgotest
+
+/*
+#cgo LDFLAGS: -Wl,-undefined,dynamic_lookup
+
+extern void __gotest_cgo_null_api(void) __attribute__((weak_import));
+
+int issue76023(void) {
+    if (__gotest_cgo_null_api) return 1;
+    return 0;
+}
+*/
+import "C"
+import "testing"
+
+func issue76023(t *testing.T) {
+	r := C.issue76023()
+	if r != 0 {
+		t.Error("found __gotest_cgo_null_api")
+	}
+}

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

@@ -180,6 +180,8 @@ const (
 	BIND_SPECIAL_DYLIB_FLAT_LOOKUP     = -2
 	BIND_SPECIAL_DYLIB_WEAK_LOOKUP     = -3
 
+	BIND_SYMBOL_FLAGS_WEAK_IMPORT = 0x1
+
 	BIND_OPCODE_MASK                                         = 0xF0
 	BIND_IMMEDIATE_MASK                                      = 0x0F
 	BIND_OPCODE_DONE                                         = 0x00
@@ -1429,7 +1431,11 @@ func machoDyldInfo(ctxt *Link) {
 			bind.AddUint8(BIND_OPCODE_SET_DYLIB_SPECIAL_IMM | uint8(d)&0xf)
 		}
 
-		bind.AddUint8(BIND_OPCODE_SET_SYMBOL_TRAILING_FLAGS_IMM)
+		flags := uint8(0)
+		if ldr.SymWeakBinding(r.targ) {
+			flags |= BIND_SYMBOL_FLAGS_WEAK_IMPORT
+		}
+		bind.AddUint8(BIND_OPCODE_SET_SYMBOL_TRAILING_FLAGS_IMM | flags)
 		// target symbol name as a C string, with _ prefix
 		bind.AddUint8('_')
 		bind.Addstring(ldr.SymExtname(r.targ))

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

@@ -241,6 +241,7 @@ type Loader struct {
 	plt         map[Sym]int32       // stores dynimport for pe objects
 	got         map[Sym]int32       // stores got for pe objects
 	dynid       map[Sym]int32       // stores Dynid for symbol
+	weakBinding map[Sym]bool        // stores whether a symbol has a weak binding
 
 	relocVariant map[relocId]sym.RelocVariant // stores variant relocs
 
@@ -326,6 +327,7 @@ func NewLoader(flags uint32, reporter *ErrorReporter) *Loader {
 		plt:                  make(map[Sym]int32),
 		got:                  make(map[Sym]int32),
 		dynid:                make(map[Sym]int32),
+		weakBinding:          make(map[Sym]bool),
 		attrCgoExportDynamic: make(map[Sym]struct{}),
 		attrCgoExportStatic:  make(map[Sym]struct{}),
 		deferReturnTramp:     make(map[Sym]bool),
@@ -1447,6 +1449,18 @@ func (l *Loader) SetSymExtname(i Sym, value string) {
 	}
 }
 
+func (l *Loader) SymWeakBinding(i Sym) bool {
+	return l.weakBinding[i]
+}
+
+func (l *Loader) SetSymWeakBinding(i Sym, v bool) {
+	// reject bad symbols
+	if i >= Sym(len(l.objSyms)) || i == 0 {
+		panic("bad symbol index in SetSymWeakBinding")
+	}
+	l.weakBinding[i] = v
+}
+
 // SymElfType returns the previously recorded ELF type for a symbol
 // (used only for symbols read from shared libraries by ldshlibsyms).
 // It is not set for symbols defined by the packages being linked or

src/cmd/link/internal/loadmacho/ldmacho.go

@@ -613,6 +613,9 @@ func Load(l *loader.Loader, arch *sys.Arch, localSymVersion int, f *bio.Reader,
 		}
 		if machsym.desc&(N_WEAK_REF|N_WEAK_DEF) != 0 {
 			l.SetAttrDuplicateOK(s, true)
+			if machsym.desc&N_WEAK_REF != 0 {
+				l.SetSymWeakBinding(s, true)
+			}
 		}
 		machsym.sym = s
 		if machsym.sectnum == 0 { // undefined