runtime/cgo: catch EXC_BAD_ACCESS on darwin/arm

The Go builders (and standard development cycle) for programs on iOS
require running the programs under lldb. Unfortunately lldb intercepts
SIGSEGV and will not give it back.

https://llvm.org/bugs/show_bug.cgi?id=22868

We get around this by never letting lldb see the SIGSEGV. On darwin,
Unix signals are emulated on top of mach exceptions. The debugger
registers a task-level mach exception handler. We register a
thread-level exception handler which acts as a faux signal handler.
The thread-level handler gets precedence over the task-level handler,
so we can turn the exception EXC_BAD_ACCESS into a panic before lldb
can see it.

Fixes #10043

Change-Id: I64d7c310dfa7ecf60eb1e59f094966520d473335
Reviewed-on: https://go-review.googlesource.com/7072
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>

src/runtime/cgo/gcc_darwin_arm.c

@@ -84,6 +84,8 @@ threadentry(void *v)
 	ts = *(ThreadStart*)v;
 	free(v);
 
+	darwin_arm_init_thread_exception_port();
+
 	crosscall_arm1(ts.fn, setg_gcc, (void*)ts.g);
 	return nil;
 }
@@ -145,5 +147,7 @@ x_cgo_init(G *g, void (*setg)(void*), void **tlsg, void **tlsbase)
 	// yes, tlsbase from mrc might not be correctly aligned.
 	inittls(tlsg, (void**)((uintptr)tlsbase & ~3));
 
+	darwin_arm_init_mach_exception_handler();
+	darwin_arm_init_thread_exception_port();
 	init_working_dir();
 }

src/runtime/cgo/gcc_signal_darwin_armx.c

@@ -0,0 +1,185 @@
+// Copyright 2015 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.
+
+// Emulation of the Unix signal SIGSEGV.
+//
+// On iOS, Go tests and apps under development are run by lldb.
+// The debugger uses a task-level exception handler to intercept signals.
+// Despite having a 'handle' mechanism like gdb, lldb will not allow a
+// SIGSEGV to pass to the running program. For Go, this means we cannot
+// generate a panic, which cannot be recovered, and so tests fail.
+//
+// We work around this by registering a thread-level mach exception handler
+// and intercepting EXC_BAD_ACCESS. The kernel offers thread handlers a
+// chance to resolve exceptions before the task handler, so we can generate
+// the panic and avoid lldb's SIGSEGV handler.
+//
+// If you want to debug a segfault under lldb, compile the standard library
+// with the build tag lldb:
+//
+//	go test -tags lldb -installsuffix lldb
+
+// +build darwin,arm,!lldb
+
+// TODO(crawshaw): darwin,arm64,!lldb
+
+#include <limits.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <signal.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#include <mach/arm/thread_status.h>
+#include <mach/exception_types.h>
+#include <mach/mach.h>
+#include <mach/mach_init.h>
+#include <mach/mach_port.h>
+#include <mach/thread_act.h>
+#include <mach/thread_status.h>
+
+#include "libcgo.h"
+
+uintptr_t x_cgo_panicmem;
+
+static pthread_mutex_t mach_exception_handler_port_set_mu;
+static mach_port_t mach_exception_handler_port_set = MACH_PORT_NULL;
+
+kern_return_t
+catch_exception_raise(
+	mach_port_t exception_port,
+	mach_port_t thread,
+	mach_port_t task,
+	exception_type_t exception,
+	exception_data_t code_vector,
+	mach_msg_type_number_t code_count)
+{
+	kern_return_t ret;
+	arm_unified_thread_state_t thread_state;
+	mach_msg_type_number_t state_count = ARM_UNIFIED_THREAD_STATE_COUNT;
+
+	// Returning KERN_SUCCESS intercepts the exception.
+	//
+	// Returning KERN_FAILURE lets the exception fall through to the
+	// next handler, which is the standard signal emulation code
+	// registered on the task port.
+
+	if (exception != EXC_BAD_ACCESS) {
+		return KERN_FAILURE;
+	}
+
+	ret = thread_get_state(thread, ARM_UNIFIED_THREAD_STATE, (thread_state_t)&thread_state, &state_count);
+	if (ret) {
+		fprintf(stderr, "runtime/cgo: thread_get_state failed: %d\n", ret);
+		abort();
+	}
+
+	// Bounce call to sigpanic through asm that makes it look like
+	// we call sigpanic directly from the faulting code.
+	thread_state.ts_32.__r[1] = thread_state.ts_32.__lr;
+	thread_state.ts_32.__r[2] = thread_state.ts_32.__pc;
+	thread_state.ts_32.__pc = x_cgo_panicmem;
+
+	ret = thread_set_state(thread, ARM_UNIFIED_THREAD_STATE, (thread_state_t)&thread_state, state_count);
+	if (ret) {
+		fprintf(stderr, "runtime/cgo: thread_set_state failed: %d\n", ret);
+		abort();
+	}
+
+	return KERN_SUCCESS;
+}
+
+void
+darwin_arm_init_thread_exception_port()
+{
+	// Called by each new OS thread to bind its EXC_BAD_ACCESS exception
+	// to mach_exception_handler_port_set.
+	int ret;
+	mach_port_t port = MACH_PORT_NULL;
+
+	ret = mach_port_allocate(mach_task_self(), MACH_PORT_RIGHT_RECEIVE, &port);
+	if (ret) {
+		fprintf(stderr, "runtime/cgo: mach_port_allocate failed: %d\n", ret);
+		abort();
+	}
+	ret = mach_port_insert_right(
+		mach_task_self(),
+		port,
+		port,
+		MACH_MSG_TYPE_MAKE_SEND);
+	if (ret) {
+		fprintf(stderr, "runtime/cgo: mach_port_insert_right failed: %d\n", ret);
+		abort();
+	}
+
+	ret = thread_set_exception_ports(
+		mach_thread_self(),
+		EXC_MASK_BAD_ACCESS,
+		port,
+		EXCEPTION_DEFAULT,
+		THREAD_STATE_NONE);
+	if (ret) {
+		fprintf(stderr, "runtime/cgo: thread_set_exception_ports failed: %d\n", ret);
+		abort();
+	}
+
+	ret = pthread_mutex_lock(&mach_exception_handler_port_set_mu);
+	if (ret) {
+		fprintf(stderr, "runtime/cgo: pthread_mutex_lock failed: %d\n", ret);
+		abort();
+	}
+	ret = mach_port_move_member(
+		mach_task_self(),
+		port,
+		mach_exception_handler_port_set);
+	if (ret) {
+		fprintf(stderr, "runtime/cgo: mach_port_move_member failed: %d\n", ret);
+		abort();
+	}
+	ret = pthread_mutex_unlock(&mach_exception_handler_port_set_mu);
+	if (ret) {
+		fprintf(stderr, "runtime/cgo: pthread_mutex_unlock failed: %d\n", ret);
+		abort();
+	}
+}
+
+static void*
+mach_exception_handler(void *port)
+{
+	// Calls catch_exception_raise.
+	extern boolean_t exc_server();
+	mach_msg_server(exc_server, 2048, (mach_port_t)port, 0);
+	abort(); // never returns
+}
+
+void
+darwin_arm_init_mach_exception_handler()
+{
+	pthread_mutex_init(&mach_exception_handler_port_set_mu, NULL);
+
+	// Called once per process to initialize a mach port server, listening
+	// for EXC_BAD_ACCESS thread exceptions.
+	int ret;
+	pthread_t thr = NULL;
+	pthread_attr_t attr;
+
+	ret = mach_port_allocate(
+		mach_task_self(),
+		MACH_PORT_RIGHT_PORT_SET,
+		&mach_exception_handler_port_set);
+	if (ret) {
+		fprintf(stderr, "runtime/cgo: mach_port_allocate failed for port_set: %d\n", ret);
+		abort();
+	}
+
+	// Start a thread to handle exceptions.
+	pthread_attr_init(&attr);
+	pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
+	ret = pthread_create(&thr, &attr, mach_exception_handler, (void*)mach_exception_handler_port_set);
+	if (ret) {
+		fprintf(stderr, "runtime/cgo: pthread_create failed: %d\n", ret);
+		abort();
+	}
+	pthread_attr_destroy(&attr);
+}

src/runtime/cgo/gcc_signal_darwin_lldb.c

@@ -0,0 +1,14 @@
+// Copyright 2015 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.
+
+// +build darwin
+// +build arm arm64
+// +build lldb
+
+#include <stdint.h>
+
+uintptr_t x_cgo_panicmem;
+
+void darwin_arm_init_thread_exception_port() {}
+void darwin_arm_init_mach_exception_handler() {}

src/runtime/cgo/libcgo.h

@@ -63,3 +63,13 @@ void crosscall_386(void (*fn)(void));
  * Prints error then calls abort. For linux and android.
  */
 void fatalf(const char* format, ...);
+
+/*
+ * Registers the current mach thread port for EXC_BAD_ACCESS processing.
+ */
+void darwin_arm_init_thread_exception_port(void);
+
+/*
+ * Starts a mach message server processing EXC_BAD_ACCESS.
+ */
+void darwin_arm_init_mach_exception_handler(void);

src/runtime/cgo/signal_darwin_arm.s

@@ -0,0 +1,49 @@
+// Copyright 2015 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.
+
+#include "textflag.h"
+
+// panicmem is the entrypoint for SIGSEGV as intercepted via a
+// mach thread port as EXC_BAD_ACCESS. As the segfault may have happened
+// in C code, we first need to load_g then call panicmem.
+//
+//	R1 - LR at moment of fault
+//	R2 - PC at moment of fault
+TEXT ·panicmem(SB),NOSPLIT,$-4
+	// If in external C code, we need to load the g register.
+	BL  runtime·load_g(SB)
+	CMP $0, g
+	BNE ongothread
+
+	// On a foreign thread. We call badsignal, which will, if all
+	// goes according to plan, not return.
+	SUB  $4, R13
+	MOVW $11, R1
+	MOVW $11, R2
+	MOVM.DB.W [R1,R2], (R13)
+	// TODO: badsignal should not return, but it does. Issue #10139.
+	//BL runtime·badsignal(SB)
+	MOVW $139, R1
+	MOVW R1, 4(R13)
+	B    runtime·exit(SB)
+
+ongothread:
+	// Trigger a SIGSEGV panic.
+	//
+	// The goal is to arrange the stack so it looks like the runtime
+	// function sigpanic was called from the PC that faulted. It has
+	// to be sigpanic, as the stack unwinding code in traceback.go
+	// looks explicitly for it.
+	//
+	// To do this we call into runtime·setsigsegv, which sets the
+	// appropriate state inside the g object. We give it the faulting
+	// PC on the stack, then put it in the LR before calling sigpanic.
+	MOVM.DB.W [R1,R2], (R13)
+	BL runtime·setsigsegv(SB)
+	MOVM.IA.W (R13), [R1,R2]
+
+	SUB $4, R13
+	MOVW R1, 0(R13)
+	MOVW R2, R14
+	B runtime·sigpanic(SB)

src/runtime/cgo/signal_darwin_armx.go

@@ -0,0 +1,31 @@
+// Copyright 2015 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.
+
+// +build darwin
+// +build arm arm64
+
+package cgo
+
+import "unsafe"
+
+//go:cgo_import_static x_cgo_panicmem
+//go:linkname x_cgo_panicmem x_cgo_panicmem
+var x_cgo_panicmem uintptr
+
+// TODO(crawshaw): move this into x_cgo_init, it will not run until
+// runtime has finished loading, which may be after its use.
+func init() {
+	x_cgo_panicmem = funcPC(panicmem)
+}
+
+func funcPC(f interface{}) uintptr {
+	var ptrSize = unsafe.Sizeof(uintptr(0))
+	return **(**uintptr)(add(unsafe.Pointer(&f), ptrSize))
+}
+
+func add(p unsafe.Pointer, x uintptr) unsafe.Pointer {
+	return unsafe.Pointer(uintptr(p) + x)
+}
+
+func panicmem()

src/runtime/sigpanic_unix.go

@@ -41,3 +41,13 @@ func sigpanic() {
 	}
 	panic(errorString(sigtable[g.sig].name))
 }
+
+// setsigsegv is used on darwin/arm{,64} to fake a segmentation fault.
+//go:nosplit
+func setsigsegv(pc uintptr) {
+	g := getg()
+	g.sig = _SIGSEGV
+	g.sigpc = pc
+	g.sigcode0 = _SEGV_MAPERR
+	g.sigcode1 = 0 // TODO: emulate si_addr
+}