src/runtime/cgocall.go - go - Git at Google

 // Copyright 2009 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.

 // Cgo call and callback support.
 //
 // To call into the C function f from Go, the cgo-generated code calls
 // runtime.cgocall(_cgo_Cfunc_f, frame), where _cgo_Cfunc_f is a
 // gcc-compiled function written by cgo.
 //
 // runtime.cgocall (below) calls entersyscall so as not to block
 // other goroutines or the garbage collector, and then calls
 // runtime.asmcgocall(_cgo_Cfunc_f, frame).
 //
 // runtime.asmcgocall (in asm_$GOARCH.s) switches to the m->g0 stack
 // (assumed to be an operating system-allocated stack, so safe to run
 // gcc-compiled code on) and calls _cgo_Cfunc_f(frame).
 //
 // _cgo_Cfunc_f invokes the actual C function f with arguments
 // taken from the frame structure, records the results in the frame,
 // and returns to runtime.asmcgocall.
 //
 // After it regains control, runtime.asmcgocall switches back to the
 // original g (m->curg)'s stack and returns to runtime.cgocall.
 //
 // After it regains control, runtime.cgocall calls exitsyscall, which blocks
 // until this m can run Go code without violating the $GOMAXPROCS limit,
 // and then unlocks g from m.
 //
 // The above description skipped over the possibility of the gcc-compiled
 // function f calling back into Go. If that happens, we continue down
 // the rabbit hole during the execution of f.
 //
 // To make it possible for gcc-compiled C code to call a Go function p.GoF,
 // cgo writes a gcc-compiled function named GoF (not p.GoF, since gcc doesn't
 // know about packages).  The gcc-compiled C function f calls GoF.
 //
 // GoF initializes "frame", a structure containing all of its
 // arguments and slots for p.GoF's results. It calls
 // crosscall2(_cgoexp_GoF, frame, framesize, ctxt) using the gcc ABI.
 //
 // crosscall2 (in cgo/asm_$GOARCH.s) is a four-argument adapter from
 // the gcc function call ABI to the gc function call ABI. At this
 // point we're in the Go runtime, but we're still running on m.g0's
 // stack and outside the $GOMAXPROCS limit. crosscall2 calls
 // runtime.cgocallback(_cgoexp_GoF, frame, ctxt) using the gc ABI.
 // (crosscall2's framesize argument is no longer used, but there's one
 // case where SWIG calls crosscall2 directly and expects to pass this
 // argument. See _cgo_panic.)
 //
 // runtime.cgocallback (in asm_$GOARCH.s) switches from m.g0's stack
 // to the original g (m.curg)'s stack, on which it calls
 // runtime.cgocallbackg(_cgoexp_GoF, frame, ctxt). As part of the
 // stack switch, runtime.cgocallback saves the current SP as
 // m.g0.sched.sp, so that any use of m.g0's stack during the execution
 // of the callback will be done below the existing stack frames.
 // Before overwriting m.g0.sched.sp, it pushes the old value on the
 // m.g0 stack, so that it can be restored later.
 //
 // runtime.cgocallbackg (below) is now running on a real goroutine
 // stack (not an m.g0 stack).  First it calls runtime.exitsyscall, which will
 // block until the $GOMAXPROCS limit allows running this goroutine.
 // Once exitsyscall has returned, it is safe to do things like call the memory
 // allocator or invoke the Go callback function.  runtime.cgocallbackg
 // first defers a function to unwind m.g0.sched.sp, so that if p.GoF
 // panics, m.g0.sched.sp will be restored to its old value: the m.g0 stack
 // and the m.curg stack will be unwound in lock step.
 // Then it calls _cgoexp_GoF(frame).
 //
 // _cgoexp_GoF, which was generated by cmd/cgo, unpacks the arguments
 // from frame, calls p.GoF, writes the results back to frame, and
 // returns. Now we start unwinding this whole process.
 //
 // runtime.cgocallbackg pops but does not execute the deferred
 // function to unwind m.g0.sched.sp, calls runtime.entersyscall, and
 // returns to runtime.cgocallback.
 //
 // After it regains control, runtime.cgocallback switches back to
 // m.g0's stack (the pointer is still in m.g0.sched.sp), restores the old
 // m.g0.sched.sp value from the stack, and returns to crosscall2.
 //
 // crosscall2 restores the callee-save registers for gcc and returns
 // to GoF, which unpacks any result values and returns to f.

 package runtime

 import (
 	"internal/goarch"
 	"runtime/internal/atomic"
 	"runtime/internal/sys"
 	"unsafe"
 )

 // Addresses collected in a cgo backtrace when crashing.
 // Length must match arg.Max in x_cgo_callers in runtime/cgo/gcc_traceback.c.
 type cgoCallers [32]uintptr

 // argset matches runtime/cgo/linux_syscall.c:argset_t
 type argset struct {
 	args   unsafe.Pointer
 	retval uintptr
 }

 // wrapper for syscall package to call cgocall for libc (cgo) calls.
 //go:linkname syscall_cgocaller syscall.cgocaller
 //go:nosplit
 //go:uintptrescapes
 func syscall_cgocaller(fn unsafe.Pointer, args ...uintptr) uintptr {
 	as := argset{args: unsafe.Pointer(&args[0])}
 	cgocall(fn, unsafe.Pointer(&as))
 	return as.retval
 }

 var ncgocall uint64 // number of cgo calls in total for dead m

 // Call from Go to C.
 //
 // This must be nosplit because it's used for syscalls on some
 // platforms. Syscalls may have untyped arguments on the stack, so
 // it's not safe to grow or scan the stack.
 //
 //go:nosplit
 func cgocall(fn, arg unsafe.Pointer) int32 {
 	if !iscgo && GOOS != "solaris" && GOOS != "illumos" && GOOS != "windows" {
 		throw("cgocall unavailable")
 	}

 	if fn == nil {
 		throw("cgocall nil")
 	}

 	if raceenabled {
 		racereleasemerge(unsafe.Pointer(&racecgosync))
 	}

 	mp := getg().m
 	mp.ncgocall++
 	mp.ncgo++

 	// Reset traceback.
 	mp.cgoCallers[0] = 0

 	// Announce we are entering a system call
 	// so that the scheduler knows to create another
 	// M to run goroutines while we are in the
 	// foreign code.
 	//
 	// The call to asmcgocall is guaranteed not to
 	// grow the stack and does not allocate memory,
 	// so it is safe to call while "in a system call", outside
 	// the $GOMAXPROCS accounting.
 	//
 	// fn may call back into Go code, in which case we'll exit the
 	// "system call", run the Go code (which may grow the stack),
 	// and then re-enter the "system call" reusing the PC and SP
 	// saved by entersyscall here.
 	entersyscall()

 	// Tell asynchronous preemption that we're entering external
 	// code. We do this after entersyscall because this may block
 	// and cause an async preemption to fail, but at this point a
 	// sync preemption will succeed (though this is not a matter
 	// of correctness).
 	osPreemptExtEnter(mp)

 	mp.incgo = true
 	errno := asmcgocall(fn, arg)

 	// Update accounting before exitsyscall because exitsyscall may
 	// reschedule us on to a different M.
 	mp.incgo = false
 	mp.ncgo--

 	osPreemptExtExit(mp)

 	exitsyscall()

 	// Note that raceacquire must be called only after exitsyscall has
 	// wired this M to a P.
 	if raceenabled {
 		raceacquire(unsafe.Pointer(&racecgosync))
 	}

 	// From the garbage collector's perspective, time can move
 	// backwards in the sequence above. If there's a callback into
 	// Go code, GC will see this function at the call to
 	// asmcgocall. When the Go call later returns to C, the
 	// syscall PC/SP is rolled back and the GC sees this function
 	// back at the call to entersyscall. Normally, fn and arg
 	// would be live at entersyscall and dead at asmcgocall, so if
 	// time moved backwards, GC would see these arguments as dead
 	// and then live. Prevent these undead arguments from crashing
 	// GC by forcing them to stay live across this time warp.
 	KeepAlive(fn)
 	KeepAlive(arg)
 	KeepAlive(mp)

 	return errno
 }

 // Call from C back to Go. fn must point to an ABIInternal Go entry-point.
 //go:nosplit
 func cgocallbackg(fn, frame unsafe.Pointer, ctxt uintptr) {
 	gp := getg()
 	if gp != gp.m.curg {
 		println("runtime: bad g in cgocallback")
 		exit(2)
 	}

 	// The call from C is on gp.m's g0 stack, so we must ensure
 	// that we stay on that M. We have to do this before calling
 	// exitsyscall, since it would otherwise be free to move us to
 	// a different M. The call to unlockOSThread is in unwindm.
 	lockOSThread()

 	checkm := gp.m

 	// Save current syscall parameters, so m.syscall can be
 	// used again if callback decide to make syscall.
 	syscall := gp.m.syscall

 	// entersyscall saves the caller's SP to allow the GC to trace the Go
 	// stack. However, since we're returning to an earlier stack frame and
 	// need to pair with the entersyscall() call made by cgocall, we must
 	// save syscall* and let reentersyscall restore them.
 	savedsp := unsafe.Pointer(gp.syscallsp)
 	savedpc := gp.syscallpc
 	exitsyscall() // coming out of cgo call
 	gp.m.incgo = false

 	osPreemptExtExit(gp.m)

 	cgocallbackg1(fn, frame, ctxt) // will call unlockOSThread

 	// At this point unlockOSThread has been called.
 	// The following code must not change to a different m.
 	// This is enforced by checking incgo in the schedule function.

 	gp.m.incgo = true

 	if gp.m != checkm {
 		throw("m changed unexpectedly in cgocallbackg")
 	}

 	osPreemptExtEnter(gp.m)

 	// going back to cgo call
 	reentersyscall(savedpc, uintptr(savedsp))

 	gp.m.syscall = syscall
 }

 func cgocallbackg1(fn, frame unsafe.Pointer, ctxt uintptr) {
 	gp := getg()

 	// When we return, undo the call to lockOSThread in cgocallbackg.
 	// We must still stay on the same m.
 	defer unlockOSThread()

 	if gp.m.needextram || atomic.Load(&extraMWaiters) > 0 {
 		gp.m.needextram = false
 		systemstack(newextram)
 	}

 	if ctxt != 0 {
 		s := append(gp.cgoCtxt, ctxt)

 		// Now we need to set gp.cgoCtxt = s, but we could get
 		// a SIGPROF signal while manipulating the slice, and
 		// the SIGPROF handler could pick up gp.cgoCtxt while
 		// tracing up the stack.  We need to ensure that the
 		// handler always sees a valid slice, so set the
 		// values in an order such that it always does.
 		p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
 		atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0]))
 		p.cap = cap(s)
 		p.len = len(s)

 		defer func(gp *g) {
 			// Decrease the length of the slice by one, safely.
 			p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
 			p.len--
 		}(gp)
 	}

 	if gp.m.ncgo == 0 {
 		// The C call to Go came from a thread not currently running
 		// any Go. In the case of -buildmode=c-archive or c-shared,
 		// this call may be coming in before package initialization
 		// is complete. Wait until it is.
 		<-main_init_done
 	}

 	// Add entry to defer stack in case of panic.
 	restore := true
 	defer unwindm(&restore)

 	if raceenabled {
 		raceacquire(unsafe.Pointer(&racecgosync))
 	}

 	// Invoke callback. This function is generated by cmd/cgo and
 	// will unpack the argument frame and call the Go function.
 	var cb func(frame unsafe.Pointer)
 	cbFV := funcval{uintptr(fn)}
 	*(*unsafe.Pointer)(unsafe.Pointer(&cb)) = noescape(unsafe.Pointer(&cbFV))
 	cb(frame)

 	if raceenabled {
 		racereleasemerge(unsafe.Pointer(&racecgosync))
 	}

 	// Do not unwind m->g0->sched.sp.
 	// Our caller, cgocallback, will do that.
 	restore = false
 }

 func unwindm(restore *bool) {
 	if *restore {
 		// Restore sp saved by cgocallback during
 		// unwind of g's stack (see comment at top of file).
 		mp := acquirem()
 		sched := &mp.g0.sched
 		sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + alignUp(sys.MinFrameSize, sys.StackAlign)))

 		// Do the accounting that cgocall will not have a chance to do
 		// during an unwind.
 		//
 		// In the case where a Go call originates from C, ncgo is 0
 		// and there is no matching cgocall to end.
 		if mp.ncgo > 0 {
 			mp.incgo = false
 			mp.ncgo--
 			osPreemptExtExit(mp)
 		}

 		releasem(mp)
 	}
 }

 // called from assembly
 func badcgocallback() {
 	throw("misaligned stack in cgocallback")
 }

 // called from (incomplete) assembly
 func cgounimpl() {
 	throw("cgo not implemented")
 }

 var racecgosync uint64 // represents possible synchronization in C code

 // Pointer checking for cgo code.

 // We want to detect all cases where a program that does not use
 // unsafe makes a cgo call passing a Go pointer to memory that
 // contains a Go pointer. Here a Go pointer is defined as a pointer
 // to memory allocated by the Go runtime. Programs that use unsafe
 // can evade this restriction easily, so we don't try to catch them.
 // The cgo program will rewrite all possibly bad pointer arguments to
 // call cgoCheckPointer, where we can catch cases of a Go pointer
 // pointing to a Go pointer.

 // Complicating matters, taking the address of a slice or array
 // element permits the C program to access all elements of the slice
 // or array. In that case we will see a pointer to a single element,
 // but we need to check the entire data structure.

 // The cgoCheckPointer call takes additional arguments indicating that
 // it was called on an address expression. An additional argument of
 // true means that it only needs to check a single element. An
 // additional argument of a slice or array means that it needs to
 // check the entire slice/array, but nothing else. Otherwise, the
 // pointer could be anything, and we check the entire heap object,
 // which is conservative but safe.

 // When and if we implement a moving garbage collector,
 // cgoCheckPointer will pin the pointer for the duration of the cgo
 // call.  (This is necessary but not sufficient; the cgo program will
 // also have to change to pin Go pointers that cannot point to Go
 // pointers.)

 // cgoCheckPointer checks if the argument contains a Go pointer that
 // points to a Go pointer, and panics if it does.
 func cgoCheckPointer(ptr interface{}, arg interface{}) {
 	if debug.cgocheck == 0 {
 		return
 	}

 	ep := efaceOf(&ptr)
 	t := ep._type

 	top := true
 	if arg != nil && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) {
 		p := ep.data
 		if t.kind&kindDirectIface == 0 {
 			p = *(*unsafe.Pointer)(p)
 		}
 		if p == nil || !cgoIsGoPointer(p) {
 			return
 		}
 		aep := efaceOf(&arg)
 		switch aep._type.kind & kindMask {
 		case kindBool:
 			if t.kind&kindMask == kindUnsafePointer {
 				// We don't know the type of the element.
 				break
 			}
 			pt := (*ptrtype)(unsafe.Pointer(t))
 			cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail)
 			return
 		case kindSlice:
 			// Check the slice rather than the pointer.
 			ep = aep
 			t = ep._type
 		case kindArray:
 			// Check the array rather than the pointer.
 			// Pass top as false since we have a pointer
 			// to the array.
 			ep = aep
 			t = ep._type
 			top = false
 		default:
 			throw("can't happen")
 		}
 	}

 	cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail)
 }

 const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer"
 const cgoResultFail = "cgo result has Go pointer"

 // cgoCheckArg is the real work of cgoCheckPointer. The argument p
 // is either a pointer to the value (of type t), or the value itself,
 // depending on indir. The top parameter is whether we are at the top
 // level, where Go pointers are allowed.
 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) {
 	if t.ptrdata == 0 || p == nil {
 		// If the type has no pointers there is nothing to do.
 		return
 	}

 	switch t.kind & kindMask {
 	default:
 		throw("can't happen")
 	case kindArray:
 		at := (*arraytype)(unsafe.Pointer(t))
 		if !indir {
 			if at.len != 1 {
 				throw("can't happen")
 			}
 			cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg)
 			return
 		}
 		for i := uintptr(0); i < at.len; i++ {
 			cgoCheckArg(at.elem, p, true, top, msg)
 			p = add(p, at.elem.size)
 		}
 	case kindChan, kindMap:
 		// These types contain internal pointers that will
 		// always be allocated in the Go heap. It's never OK
 		// to pass them to C.
 		panic(errorString(msg))
 	case kindFunc:
 		if indir {
 			p = *(*unsafe.Pointer)(p)
 		}
 		if !cgoIsGoPointer(p) {
 			return
 		}
 		panic(errorString(msg))
 	case kindInterface:
 		it := *(**_type)(p)
 		if it == nil {
 			return
 		}
 		// A type known at compile time is OK since it's
 		// constant. A type not known at compile time will be
 		// in the heap and will not be OK.
 		if inheap(uintptr(unsafe.Pointer(it))) {
 			panic(errorString(msg))
 		}
 		p = *(*unsafe.Pointer)(add(p, goarch.PtrSize))
 		if !cgoIsGoPointer(p) {
 			return
 		}
 		if !top {
 			panic(errorString(msg))
 		}
 		cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg)
 	case kindSlice:
 		st := (*slicetype)(unsafe.Pointer(t))
 		s := (*slice)(p)
 		p = s.array
 		if p == nil || !cgoIsGoPointer(p) {
 			return
 		}
 		if !top {
 			panic(errorString(msg))
 		}
 		if st.elem.ptrdata == 0 {
 			return
 		}
 		for i := 0; i < s.cap; i++ {
 			cgoCheckArg(st.elem, p, true, false, msg)
 			p = add(p, st.elem.size)
 		}
 	case kindString:
 		ss := (*stringStruct)(p)
 		if !cgoIsGoPointer(ss.str) {
 			return
 		}
 		if !top {
 			panic(errorString(msg))
 		}
 	case kindStruct:
 		st := (*structtype)(unsafe.Pointer(t))
 		if !indir {
 			if len(st.fields) != 1 {
 				throw("can't happen")
 			}
 			cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg)
 			return
 		}
 		for _, f := range st.fields {
 			if f.typ.ptrdata == 0 {
 				continue
 			}
 			cgoCheckArg(f.typ, add(p, f.offset()), true, top, msg)
 		}
 	case kindPtr, kindUnsafePointer:
 		if indir {
 			p = *(*unsafe.Pointer)(p)
 			if p == nil {
 				return
 			}
 		}

 		if !cgoIsGoPointer(p) {
 			return
 		}
 		if !top {
 			panic(errorString(msg))
 		}

 		cgoCheckUnknownPointer(p, msg)
 	}
 }

 // cgoCheckUnknownPointer is called for an arbitrary pointer into Go
 // memory. It checks whether that Go memory contains any other
 // pointer into Go memory. If it does, we panic.
 // The return values are unused but useful to see in panic tracebacks.
 func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) {
 	if inheap(uintptr(p)) {
 		b, span, _ := findObject(uintptr(p), 0, 0)
 		base = b
 		if base == 0 {
 			return
 		}
 		hbits := heapBitsForAddr(base)
 		n := span.elemsize
 		for i = uintptr(0); i < n; i += goarch.PtrSize {
 			if !hbits.morePointers() {
 				// No more possible pointers.
 				break
 			}
 			if hbits.isPointer() && cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(base + i))) {
 				panic(errorString(msg))
 			}
 			hbits = hbits.next()
 		}

 		return
 	}

 	for _, datap := range activeModules() {
 		if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
 			// We have no way to know the size of the object.
 			// We have to assume that it might contain a pointer.
 			panic(errorString(msg))
 		}
 		// In the text or noptr sections, we know that the
 		// pointer does not point to a Go pointer.
 	}

 	return
 }

 // cgoIsGoPointer reports whether the pointer is a Go pointer--a
 // pointer to Go memory. We only care about Go memory that might
 // contain pointers.
 //go:nosplit
 //go:nowritebarrierrec
 func cgoIsGoPointer(p unsafe.Pointer) bool {
 	if p == nil {
 		return false
 	}

 	if inHeapOrStack(uintptr(p)) {
 		return true
 	}

 	for _, datap := range activeModules() {
 		if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
 			return true
 		}
 	}

 	return false
 }

 // cgoInRange reports whether p is between start and end.
 //go:nosplit
 //go:nowritebarrierrec
 func cgoInRange(p unsafe.Pointer, start, end uintptr) bool {
 	return start <= uintptr(p) && uintptr(p) < end
 }

 // cgoCheckResult is called to check the result parameter of an
 // exported Go function. It panics if the result is or contains a Go
 // pointer.
 func cgoCheckResult(val interface{}) {
 	if debug.cgocheck == 0 {
 		return
 	}

 	ep := efaceOf(&val)
 	t := ep._type
 	cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail)
 }
	// Copyright 2009 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.

	// Cgo call and callback support.
	//
	// To call into the C function f from Go, the cgo-generated code calls
	// runtime.cgocall(_cgo_Cfunc_f, frame), where _cgo_Cfunc_f is a
	// gcc-compiled function written by cgo.
	//
	// runtime.cgocall (below) calls entersyscall so as not to block
	// other goroutines or the garbage collector, and then calls
	// runtime.asmcgocall(_cgo_Cfunc_f, frame).
	//
	// runtime.asmcgocall (in asm_$GOARCH.s) switches to the m->g0 stack
	// (assumed to be an operating system-allocated stack, so safe to run
	// gcc-compiled code on) and calls _cgo_Cfunc_f(frame).
	//
	// _cgo_Cfunc_f invokes the actual C function f with arguments
	// taken from the frame structure, records the results in the frame,
	// and returns to runtime.asmcgocall.
	//
	// After it regains control, runtime.asmcgocall switches back to the
	// original g (m->curg)'s stack and returns to runtime.cgocall.
	//
	// After it regains control, runtime.cgocall calls exitsyscall, which blocks
	// until this m can run Go code without violating the $GOMAXPROCS limit,
	// and then unlocks g from m.
	//
	// The above description skipped over the possibility of the gcc-compiled
	// function f calling back into Go. If that happens, we continue down
	// the rabbit hole during the execution of f.
	//
	// To make it possible for gcc-compiled C code to call a Go function p.GoF,
	// cgo writes a gcc-compiled function named GoF (not p.GoF, since gcc doesn't
	// know about packages). The gcc-compiled C function f calls GoF.
	//
	// GoF initializes "frame", a structure containing all of its
	// arguments and slots for p.GoF's results. It calls
	// crosscall2(_cgoexp_GoF, frame, framesize, ctxt) using the gcc ABI.
	//
	// crosscall2 (in cgo/asm_$GOARCH.s) is a four-argument adapter from
	// the gcc function call ABI to the gc function call ABI. At this
	// point we're in the Go runtime, but we're still running on m.g0's
	// stack and outside the $GOMAXPROCS limit. crosscall2 calls
	// runtime.cgocallback(_cgoexp_GoF, frame, ctxt) using the gc ABI.
	// (crosscall2's framesize argument is no longer used, but there's one
	// case where SWIG calls crosscall2 directly and expects to pass this
	// argument. See _cgo_panic.)
	//
	// runtime.cgocallback (in asm_$GOARCH.s) switches from m.g0's stack
	// to the original g (m.curg)'s stack, on which it calls
	// runtime.cgocallbackg(_cgoexp_GoF, frame, ctxt). As part of the
	// stack switch, runtime.cgocallback saves the current SP as
	// m.g0.sched.sp, so that any use of m.g0's stack during the execution
	// of the callback will be done below the existing stack frames.
	// Before overwriting m.g0.sched.sp, it pushes the old value on the
	// m.g0 stack, so that it can be restored later.
	//
	// runtime.cgocallbackg (below) is now running on a real goroutine
	// stack (not an m.g0 stack). First it calls runtime.exitsyscall, which will
	// block until the $GOMAXPROCS limit allows running this goroutine.
	// Once exitsyscall has returned, it is safe to do things like call the memory
	// allocator or invoke the Go callback function. runtime.cgocallbackg
	// first defers a function to unwind m.g0.sched.sp, so that if p.GoF
	// panics, m.g0.sched.sp will be restored to its old value: the m.g0 stack
	// and the m.curg stack will be unwound in lock step.
	// Then it calls _cgoexp_GoF(frame).
	//
	// _cgoexp_GoF, which was generated by cmd/cgo, unpacks the arguments
	// from frame, calls p.GoF, writes the results back to frame, and
	// returns. Now we start unwinding this whole process.
	//
	// runtime.cgocallbackg pops but does not execute the deferred
	// function to unwind m.g0.sched.sp, calls runtime.entersyscall, and
	// returns to runtime.cgocallback.
	//
	// After it regains control, runtime.cgocallback switches back to
	// m.g0's stack (the pointer is still in m.g0.sched.sp), restores the old
	// m.g0.sched.sp value from the stack, and returns to crosscall2.
	//
	// crosscall2 restores the callee-save registers for gcc and returns
	// to GoF, which unpacks any result values and returns to f.

	package runtime

	import (
	"internal/goarch"
	"runtime/internal/atomic"
	"runtime/internal/sys"
	"unsafe"
	)

	// Addresses collected in a cgo backtrace when crashing.
	// Length must match arg.Max in x_cgo_callers in runtime/cgo/gcc_traceback.c.
	type cgoCallers [32]uintptr

	// argset matches runtime/cgo/linux_syscall.c:argset_t
	type argset struct {
	args unsafe.Pointer
	retval uintptr
	}

	// wrapper for syscall package to call cgocall for libc (cgo) calls.
	//go:linkname syscall_cgocaller syscall.cgocaller
	//go:nosplit
	//go:uintptrescapes
	func syscall_cgocaller(fn unsafe.Pointer, args ...uintptr) uintptr {
	as := argset{args: unsafe.Pointer(&args[0])}
	cgocall(fn, unsafe.Pointer(&as))
	return as.retval
	}

	var ncgocall uint64 // number of cgo calls in total for dead m

	// Call from Go to C.
	//
	// This must be nosplit because it's used for syscalls on some
	// platforms. Syscalls may have untyped arguments on the stack, so
	// it's not safe to grow or scan the stack.
	//
	//go:nosplit
	func cgocall(fn, arg unsafe.Pointer) int32 {
	if !iscgo && GOOS != "solaris" && GOOS != "illumos" && GOOS != "windows" {
	throw("cgocall unavailable")
	}

	if fn == nil {
	throw("cgocall nil")
	}

	if raceenabled {
	racereleasemerge(unsafe.Pointer(&racecgosync))
	}

	mp := getg().m
	mp.ncgocall++
	mp.ncgo++

	// Reset traceback.
	mp.cgoCallers[0] = 0

	// Announce we are entering a system call
	// so that the scheduler knows to create another
	// M to run goroutines while we are in the
	// foreign code.
	//
	// The call to asmcgocall is guaranteed not to
	// grow the stack and does not allocate memory,
	// so it is safe to call while "in a system call", outside
	// the $GOMAXPROCS accounting.
	//
	// fn may call back into Go code, in which case we'll exit the
	// "system call", run the Go code (which may grow the stack),
	// and then re-enter the "system call" reusing the PC and SP
	// saved by entersyscall here.
	entersyscall()

	// Tell asynchronous preemption that we're entering external
	// code. We do this after entersyscall because this may block
	// and cause an async preemption to fail, but at this point a
	// sync preemption will succeed (though this is not a matter
	// of correctness).
	osPreemptExtEnter(mp)

	mp.incgo = true
	errno := asmcgocall(fn, arg)

	// Update accounting before exitsyscall because exitsyscall may
	// reschedule us on to a different M.
	mp.incgo = false
	mp.ncgo--

	osPreemptExtExit(mp)

	exitsyscall()

	// Note that raceacquire must be called only after exitsyscall has
	// wired this M to a P.
	if raceenabled {
	raceacquire(unsafe.Pointer(&racecgosync))
	}

	// From the garbage collector's perspective, time can move
	// backwards in the sequence above. If there's a callback into
	// Go code, GC will see this function at the call to
	// asmcgocall. When the Go call later returns to C, the
	// syscall PC/SP is rolled back and the GC sees this function
	// back at the call to entersyscall. Normally, fn and arg
	// would be live at entersyscall and dead at asmcgocall, so if
	// time moved backwards, GC would see these arguments as dead
	// and then live. Prevent these undead arguments from crashing
	// GC by forcing them to stay live across this time warp.
	KeepAlive(fn)
	KeepAlive(arg)
	KeepAlive(mp)

	return errno
	}

	// Call from C back to Go. fn must point to an ABIInternal Go entry-point.
	//go:nosplit
	func cgocallbackg(fn, frame unsafe.Pointer, ctxt uintptr) {
	gp := getg()
	if gp != gp.m.curg {
	println("runtime: bad g in cgocallback")
	exit(2)
	}

	// The call from C is on gp.m's g0 stack, so we must ensure
	// that we stay on that M. We have to do this before calling
	// exitsyscall, since it would otherwise be free to move us to
	// a different M. The call to unlockOSThread is in unwindm.
	lockOSThread()

	checkm := gp.m

	// Save current syscall parameters, so m.syscall can be
	// used again if callback decide to make syscall.
	syscall := gp.m.syscall

	// entersyscall saves the caller's SP to allow the GC to trace the Go
	// stack. However, since we're returning to an earlier stack frame and
	// need to pair with the entersyscall() call made by cgocall, we must
	// save syscall* and let reentersyscall restore them.
	savedsp := unsafe.Pointer(gp.syscallsp)
	savedpc := gp.syscallpc
	exitsyscall() // coming out of cgo call
	gp.m.incgo = false

	osPreemptExtExit(gp.m)

	cgocallbackg1(fn, frame, ctxt) // will call unlockOSThread

	// At this point unlockOSThread has been called.
	// The following code must not change to a different m.
	// This is enforced by checking incgo in the schedule function.

	gp.m.incgo = true

	if gp.m != checkm {
	throw("m changed unexpectedly in cgocallbackg")
	}

	osPreemptExtEnter(gp.m)

	// going back to cgo call
	reentersyscall(savedpc, uintptr(savedsp))

	gp.m.syscall = syscall
	}

	func cgocallbackg1(fn, frame unsafe.Pointer, ctxt uintptr) {
	gp := getg()

	// When we return, undo the call to lockOSThread in cgocallbackg.
	// We must still stay on the same m.
	defer unlockOSThread()

	if gp.m.needextram \|\| atomic.Load(&extraMWaiters) > 0 {
	gp.m.needextram = false
	systemstack(newextram)
	}

	if ctxt != 0 {
	s := append(gp.cgoCtxt, ctxt)

	// Now we need to set gp.cgoCtxt = s, but we could get
	// a SIGPROF signal while manipulating the slice, and
	// the SIGPROF handler could pick up gp.cgoCtxt while
	// tracing up the stack. We need to ensure that the
	// handler always sees a valid slice, so set the
	// values in an order such that it always does.
	p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
	atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0]))
	p.cap = cap(s)
	p.len = len(s)

	defer func(gp *g) {
	// Decrease the length of the slice by one, safely.
	p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
	p.len--
	}(gp)
	}

	if gp.m.ncgo == 0 {
	// The C call to Go came from a thread not currently running
	// any Go. In the case of -buildmode=c-archive or c-shared,
	// this call may be coming in before package initialization
	// is complete. Wait until it is.
	<-main_init_done
	}

	// Add entry to defer stack in case of panic.
	restore := true
	defer unwindm(&restore)

	if raceenabled {
	raceacquire(unsafe.Pointer(&racecgosync))
	}

	// Invoke callback. This function is generated by cmd/cgo and
	// will unpack the argument frame and call the Go function.
	var cb func(frame unsafe.Pointer)
	cbFV := funcval{uintptr(fn)}
	(unsafe.Pointer)(unsafe.Pointer(&cb)) = noescape(unsafe.Pointer(&cbFV))
	cb(frame)

	if raceenabled {
	racereleasemerge(unsafe.Pointer(&racecgosync))
	}

	// Do not unwind m->g0->sched.sp.
	// Our caller, cgocallback, will do that.
	restore = false
	}

	func unwindm(restore *bool) {
	if *restore {
	// Restore sp saved by cgocallback during
	// unwind of g's stack (see comment at top of file).
	mp := acquirem()
	sched := &mp.g0.sched
	sched.sp = (uintptr)(unsafe.Pointer(sched.sp + alignUp(sys.MinFrameSize, sys.StackAlign)))

	// Do the accounting that cgocall will not have a chance to do
	// during an unwind.
	//
	// In the case where a Go call originates from C, ncgo is 0
	// and there is no matching cgocall to end.
	if mp.ncgo > 0 {
	mp.incgo = false
	mp.ncgo--
	osPreemptExtExit(mp)
	}

	releasem(mp)
	}
	}

	// called from assembly
	func badcgocallback() {
	throw("misaligned stack in cgocallback")
	}

	// called from (incomplete) assembly
	func cgounimpl() {
	throw("cgo not implemented")
	}

	var racecgosync uint64 // represents possible synchronization in C code

	// Pointer checking for cgo code.

	// We want to detect all cases where a program that does not use
	// unsafe makes a cgo call passing a Go pointer to memory that
	// contains a Go pointer. Here a Go pointer is defined as a pointer
	// to memory allocated by the Go runtime. Programs that use unsafe
	// can evade this restriction easily, so we don't try to catch them.
	// The cgo program will rewrite all possibly bad pointer arguments to
	// call cgoCheckPointer, where we can catch cases of a Go pointer
	// pointing to a Go pointer.

	// Complicating matters, taking the address of a slice or array
	// element permits the C program to access all elements of the slice
	// or array. In that case we will see a pointer to a single element,
	// but we need to check the entire data structure.

	// The cgoCheckPointer call takes additional arguments indicating that
	// it was called on an address expression. An additional argument of
	// true means that it only needs to check a single element. An
	// additional argument of a slice or array means that it needs to
	// check the entire slice/array, but nothing else. Otherwise, the
	// pointer could be anything, and we check the entire heap object,
	// which is conservative but safe.

	// When and if we implement a moving garbage collector,
	// cgoCheckPointer will pin the pointer for the duration of the cgo
	// call. (This is necessary but not sufficient; the cgo program will
	// also have to change to pin Go pointers that cannot point to Go
	// pointers.)

	// cgoCheckPointer checks if the argument contains a Go pointer that
	// points to a Go pointer, and panics if it does.
	func cgoCheckPointer(ptr interface{}, arg interface{}) {
	if debug.cgocheck == 0 {
	return
	}

	ep := efaceOf(&ptr)
	t := ep._type

	top := true
	if arg != nil && (t.kind&kindMask == kindPtr \|\| t.kind&kindMask == kindUnsafePointer) {
	p := ep.data
	if t.kind&kindDirectIface == 0 {
	p = (unsafe.Pointer)(p)
	}
	if p == nil \|\| !cgoIsGoPointer(p) {
	return
	}
	aep := efaceOf(&arg)
	switch aep._type.kind & kindMask {
	case kindBool:
	if t.kind&kindMask == kindUnsafePointer {
	// We don't know the type of the element.
	break
	}
	pt := (*ptrtype)(unsafe.Pointer(t))
	cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail)
	return
	case kindSlice:
	// Check the slice rather than the pointer.
	ep = aep
	t = ep._type
	case kindArray:
	// Check the array rather than the pointer.
	// Pass top as false since we have a pointer
	// to the array.
	ep = aep
	t = ep._type
	top = false
	default:
	throw("can't happen")
	}
	}

	cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail)
	}

	const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer"
	const cgoResultFail = "cgo result has Go pointer"

	// cgoCheckArg is the real work of cgoCheckPointer. The argument p
	// is either a pointer to the value (of type t), or the value itself,
	// depending on indir. The top parameter is whether we are at the top
	// level, where Go pointers are allowed.
	func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) {
	if t.ptrdata == 0 \|\| p == nil {
	// If the type has no pointers there is nothing to do.
	return
	}

	switch t.kind & kindMask {
	default:
	throw("can't happen")
	case kindArray:
	at := (*arraytype)(unsafe.Pointer(t))
	if !indir {
	if at.len != 1 {
	throw("can't happen")
	}
	cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg)
	return
	}
	for i := uintptr(0); i < at.len; i++ {
	cgoCheckArg(at.elem, p, true, top, msg)
	p = add(p, at.elem.size)
	}
	case kindChan, kindMap:
	// These types contain internal pointers that will
	// always be allocated in the Go heap. It's never OK
	// to pass them to C.
	panic(errorString(msg))
	case kindFunc:
	if indir {
	p = (unsafe.Pointer)(p)
	}
	if !cgoIsGoPointer(p) {
	return
	}
	panic(errorString(msg))
	case kindInterface:
	it := (*_type)(p)
	if it == nil {
	return
	}
	// A type known at compile time is OK since it's
	// constant. A type not known at compile time will be
	// in the heap and will not be OK.
	if inheap(uintptr(unsafe.Pointer(it))) {
	panic(errorString(msg))
	}
	p = (unsafe.Pointer)(add(p, goarch.PtrSize))
	if !cgoIsGoPointer(p) {
	return
	}
	if !top {
	panic(errorString(msg))
	}
	cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg)
	case kindSlice:
	st := (*slicetype)(unsafe.Pointer(t))
	s := (*slice)(p)
	p = s.array
	if p == nil \|\| !cgoIsGoPointer(p) {
	return
	}
	if !top {
	panic(errorString(msg))
	}
	if st.elem.ptrdata == 0 {
	return
	}
	for i := 0; i < s.cap; i++ {
	cgoCheckArg(st.elem, p, true, false, msg)
	p = add(p, st.elem.size)
	}
	case kindString:
	ss := (*stringStruct)(p)
	if !cgoIsGoPointer(ss.str) {
	return
	}
	if !top {
	panic(errorString(msg))
	}
	case kindStruct:
	st := (*structtype)(unsafe.Pointer(t))
	if !indir {
	if len(st.fields) != 1 {
	throw("can't happen")
	}
	cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg)
	return
	}
	for _, f := range st.fields {
	if f.typ.ptrdata == 0 {
	continue
	}
	cgoCheckArg(f.typ, add(p, f.offset()), true, top, msg)
	}
	case kindPtr, kindUnsafePointer:
	if indir {
	p = (unsafe.Pointer)(p)
	if p == nil {
	return
	}
	}

	if !cgoIsGoPointer(p) {
	return
	}
	if !top {
	panic(errorString(msg))
	}

	cgoCheckUnknownPointer(p, msg)
	}
	}

	// cgoCheckUnknownPointer is called for an arbitrary pointer into Go
	// memory. It checks whether that Go memory contains any other
	// pointer into Go memory. If it does, we panic.
	// The return values are unused but useful to see in panic tracebacks.
	func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) {
	if inheap(uintptr(p)) {
	b, span, _ := findObject(uintptr(p), 0, 0)
	base = b
	if base == 0 {
	return
	}
	hbits := heapBitsForAddr(base)
	n := span.elemsize
	for i = uintptr(0); i < n; i += goarch.PtrSize {
	if !hbits.morePointers() {
	// No more possible pointers.
	break
	}
	if hbits.isPointer() && cgoIsGoPointer((unsafe.Pointer)(unsafe.Pointer(base + i))) {
	panic(errorString(msg))
	}
	hbits = hbits.next()
	}

	return
	}

	for _, datap := range activeModules() {
	if cgoInRange(p, datap.data, datap.edata) \|\| cgoInRange(p, datap.bss, datap.ebss) {
	// We have no way to know the size of the object.
	// We have to assume that it might contain a pointer.
	panic(errorString(msg))
	}
	// In the text or noptr sections, we know that the
	// pointer does not point to a Go pointer.
	}

	return
	}

	// cgoIsGoPointer reports whether the pointer is a Go pointer--a
	// pointer to Go memory. We only care about Go memory that might
	// contain pointers.
	//go:nosplit
	//go:nowritebarrierrec
	func cgoIsGoPointer(p unsafe.Pointer) bool {
	if p == nil {
	return false
	}

	if inHeapOrStack(uintptr(p)) {
	return true
	}

	for _, datap := range activeModules() {
	if cgoInRange(p, datap.data, datap.edata) \|\| cgoInRange(p, datap.bss, datap.ebss) {
	return true
	}
	}

	return false
	}

	// cgoInRange reports whether p is between start and end.
	//go:nosplit
	//go:nowritebarrierrec
	func cgoInRange(p unsafe.Pointer, start, end uintptr) bool {
	return start <= uintptr(p) && uintptr(p) < end
	}

	// cgoCheckResult is called to check the result parameter of an
	// exported Go function. It panics if the result is or contains a Go
	// pointer.
	func cgoCheckResult(val interface{}) {
	if debug.cgocheck == 0 {
	return
	}

	ep := efaceOf(&val)
	t := ep._type
	cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail)
	}