cmd/link: handle dynamic import variables on Darwin

Currently, on darwin, we only support cgo_dynamic_import for
functions, but not variables, as we don't need it before.
mach_task_self_ is a variable defined in the system library, which
can be used to e.g. access the process's memory mappings via the
mach API. The C header defines a macro mach_task_self(), which
refers to the variable. To use mach_task_self_ (in pure-Go
programs) we need to access it in Go.

This CL handles cgo_dynamic_import for variables in the linker,
loading its address via the GOT. (Currently only on Darwin, as
we only need it there.)

For #50891.

Change-Id: Idf64fa88ba2f2381443a1ed0b42b14b581843493
Reviewed-on: https://go-review.googlesource.com/c/go/+/501855
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

4 files changed, 69 insertions, 0 deletions

diff --git a/src/cmd/link/testdata/dynimportvar/asm/a.go b/src/cmd/link/testdata/dynimportvar/asm/a.go
new file mode 100644
index 0000000000..8d9299972b
--- /dev/null
+++ b/src/cmd/link/testdata/dynimportvar/asm/a.go
@@ -0,0 +1,15 @@
+// Copyright 2023 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.
+
+// This is a separate package because we cannot have Go
+// assembly code and cgo code in the same package.
+
+//go:build darwin
+
+package asm
+
+//go:cgo_import_dynamic libc_mach_task_self_ mach_task_self_ "/usr/lib/libSystem.B.dylib"
+
+// load mach_task_self_ from assembly code
+func Mach_task_self() uint32
diff --git a/src/cmd/link/testdata/dynimportvar/asm/a_amd64.s b/src/cmd/link/testdata/dynimportvar/asm/a_amd64.s
new file mode 100644
index 0000000000..93547e32f1
--- /dev/null
+++ b/src/cmd/link/testdata/dynimportvar/asm/a_amd64.s
@@ -0,0 +1,11 @@
+// Copyright 2023 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
+
+TEXT ·Mach_task_self(SB),0,$0-4
+	MOVQ	$libc_mach_task_self_(SB), AX
+	MOVQ	(AX), AX
+	MOVL	AX, ret+0(FP)
+	RET
diff --git a/src/cmd/link/testdata/dynimportvar/asm/a_arm64.s b/src/cmd/link/testdata/dynimportvar/asm/a_arm64.s
new file mode 100644
index 0000000000..bd3c9d71f5
--- /dev/null
+++ b/src/cmd/link/testdata/dynimportvar/asm/a_arm64.s
@@ -0,0 +1,11 @@
+// Copyright 2023 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
+
+TEXT ·Mach_task_self(SB),0,$0-4
+	MOVD	$libc_mach_task_self_(SB), R0
+	MOVD	(R0), R0
+	MOVW	R0, ret+0(FP)
+	RET
diff --git a/src/cmd/link/testdata/dynimportvar/main.go b/src/cmd/link/testdata/dynimportvar/main.go
new file mode 100644
index 0000000000..658d340532
--- /dev/null
+++ b/src/cmd/link/testdata/dynimportvar/main.go
@@ -0,0 +1,32 @@
+// Copyright 2023 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.
+
+// Test that we can access dynamically imported variables.
+// We ues mach_task_self_ from darwin's system library.
+// Check that loading the variable from C and Go gets the
+// same result.
+
+//go:build darwin
+
+package main
+
+/*
+#include <mach/mach_init.h>
+
+unsigned int Mach_task_self(void) {
+	return mach_task_self();
+}
+*/
+import "C"
+
+import "cmd/link/testdata/dynimportvar/asm"
+
+func main() {
+	c := uint32(C.Mach_task_self())
+	a := asm.Mach_task_self()
+	if a != c {
+		println("got", a, "want", c)
+		panic("FAIL")
+	}
+}