1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
5 // Cgo call and callback support.
7 // To call into the C function f from Go, the cgo-generated code calls
8 // runtime.cgocall(_cgo_Cfunc_f, frame), where _cgo_Cfunc_f is a
9 // gcc-compiled function written by cgo.
11 // runtime.cgocall (below) calls entersyscall so as not to block
12 // other goroutines or the garbage collector, and then calls
13 // runtime.asmcgocall(_cgo_Cfunc_f, frame).
15 // runtime.asmcgocall (in asm_$GOARCH.s) switches to the m->g0 stack
16 // (assumed to be an operating system-allocated stack, so safe to run
17 // gcc-compiled code on) and calls _cgo_Cfunc_f(frame).
19 // _cgo_Cfunc_f invokes the actual C function f with arguments
20 // taken from the frame structure, records the results in the frame,
21 // and returns to runtime.asmcgocall.
23 // After it regains control, runtime.asmcgocall switches back to the
24 // original g (m->curg)'s stack and returns to runtime.cgocall.
26 // After it regains control, runtime.cgocall calls exitsyscall, which blocks
27 // until this m can run Go code without violating the $GOMAXPROCS limit,
28 // and then unlocks g from m.
30 // The above description skipped over the possibility of the gcc-compiled
31 // function f calling back into Go. If that happens, we continue down
32 // the rabbit hole during the execution of f.
34 // To make it possible for gcc-compiled C code to call a Go function p.GoF,
35 // cgo writes a gcc-compiled function named GoF (not p.GoF, since gcc doesn't
36 // know about packages). The gcc-compiled C function f calls GoF.
38 // GoF initializes "frame", a structure containing all of its
39 // arguments and slots for p.GoF's results. It calls
40 // crosscall2(_cgoexp_GoF, frame, framesize, ctxt) using the gcc ABI.
42 // crosscall2 (in cgo/asm_$GOARCH.s) is a four-argument adapter from
43 // the gcc function call ABI to the gc function call ABI. At this
44 // point we're in the Go runtime, but we're still running on m.g0's
45 // stack and outside the $GOMAXPROCS limit. crosscall2 calls
46 // runtime.cgocallback(_cgoexp_GoF, frame, ctxt) using the gc ABI.
47 // (crosscall2's framesize argument is no longer used, but there's one
48 // case where SWIG calls crosscall2 directly and expects to pass this
49 // argument. See _cgo_panic.)
51 // runtime.cgocallback (in asm_$GOARCH.s) switches from m.g0's stack
52 // to the original g (m.curg)'s stack, on which it calls
53 // runtime.cgocallbackg(_cgoexp_GoF, frame, ctxt). As part of the
54 // stack switch, runtime.cgocallback saves the current SP as
55 // m.g0.sched.sp, so that any use of m.g0's stack during the execution
56 // of the callback will be done below the existing stack frames.
57 // Before overwriting m.g0.sched.sp, it pushes the old value on the
58 // m.g0 stack, so that it can be restored later.
60 // runtime.cgocallbackg (below) is now running on a real goroutine
61 // stack (not an m.g0 stack). First it calls runtime.exitsyscall, which will
62 // block until the $GOMAXPROCS limit allows running this goroutine.
63 // Once exitsyscall has returned, it is safe to do things like call the memory
64 // allocator or invoke the Go callback function. runtime.cgocallbackg
65 // first defers a function to unwind m.g0.sched.sp, so that if p.GoF
66 // panics, m.g0.sched.sp will be restored to its old value: the m.g0 stack
67 // and the m.curg stack will be unwound in lock step.
68 // Then it calls _cgoexp_GoF(frame).
70 // _cgoexp_GoF, which was generated by cmd/cgo, unpacks the arguments
71 // from frame, calls p.GoF, writes the results back to frame, and
72 // returns. Now we start unwinding this whole process.
74 // runtime.cgocallbackg pops but does not execute the deferred
75 // function to unwind m.g0.sched.sp, calls runtime.entersyscall, and
76 // returns to runtime.cgocallback.
78 // After it regains control, runtime.cgocallback switches back to
79 // m.g0's stack (the pointer is still in m.g0.sched.sp), restores the old
80 // m.g0.sched.sp value from the stack, and returns to crosscall2.
82 // crosscall2 restores the callee-save registers for gcc and returns
83 // to GoF, which unpacks any result values and returns to f.
89 "runtime/internal/atomic"
90 "runtime/internal/sys"
94 // Addresses collected in a cgo backtrace when crashing.
95 // Length must match arg.Max in x_cgo_callers in runtime/cgo/gcc_traceback.c.
96 type cgoCallers [32]uintptr
98 // argset matches runtime/cgo/linux_syscall.c:argset_t
104 // wrapper for syscall package to call cgocall for libc (cgo) calls.
105 //go:linkname syscall_cgocaller syscall.cgocaller
108 func syscall_cgocaller(fn unsafe.Pointer, args ...uintptr) uintptr {
109 as := argset{args: unsafe.Pointer(&args[0])}
110 cgocall(fn, unsafe.Pointer(&as))
114 var ncgocall uint64 // number of cgo calls in total for dead m
116 // Call from Go to C.
118 // This must be nosplit because it's used for syscalls on some
119 // platforms. Syscalls may have untyped arguments on the stack, so
120 // it's not safe to grow or scan the stack.
123 func cgocall(fn, arg unsafe.Pointer) int32 {
124 if !iscgo && GOOS != "solaris" && GOOS != "illumos" && GOOS != "windows" {
125 throw("cgocall unavailable")
133 racereleasemerge(unsafe.Pointer(&racecgosync))
143 // Announce we are entering a system call
144 // so that the scheduler knows to create another
145 // M to run goroutines while we are in the
148 // The call to asmcgocall is guaranteed not to
149 // grow the stack and does not allocate memory,
150 // so it is safe to call while "in a system call", outside
151 // the $GOMAXPROCS accounting.
153 // fn may call back into Go code, in which case we'll exit the
154 // "system call", run the Go code (which may grow the stack),
155 // and then re-enter the "system call" reusing the PC and SP
156 // saved by entersyscall here.
159 // Tell asynchronous preemption that we're entering external
160 // code. We do this after entersyscall because this may block
161 // and cause an async preemption to fail, but at this point a
162 // sync preemption will succeed (though this is not a matter
164 osPreemptExtEnter(mp)
167 errno := asmcgocall(fn, arg)
169 // Update accounting before exitsyscall because exitsyscall may
170 // reschedule us on to a different M.
178 // Note that raceacquire must be called only after exitsyscall has
179 // wired this M to a P.
181 raceacquire(unsafe.Pointer(&racecgosync))
184 // From the garbage collector's perspective, time can move
185 // backwards in the sequence above. If there's a callback into
186 // Go code, GC will see this function at the call to
187 // asmcgocall. When the Go call later returns to C, the
188 // syscall PC/SP is rolled back and the GC sees this function
189 // back at the call to entersyscall. Normally, fn and arg
190 // would be live at entersyscall and dead at asmcgocall, so if
191 // time moved backwards, GC would see these arguments as dead
192 // and then live. Prevent these undead arguments from crashing
193 // GC by forcing them to stay live across this time warp.
201 // Call from C back to Go. fn must point to an ABIInternal Go entry-point.
203 func cgocallbackg(fn, frame unsafe.Pointer, ctxt uintptr) {
206 println("runtime: bad g in cgocallback")
210 // The call from C is on gp.m's g0 stack, so we must ensure
211 // that we stay on that M. We have to do this before calling
212 // exitsyscall, since it would otherwise be free to move us to
213 // a different M. The call to unlockOSThread is in unwindm.
216 // Save current syscall parameters, so m.syscall can be
217 // used again if callback decide to make syscall.
218 syscall := gp.m.syscall
220 // entersyscall saves the caller's SP to allow the GC to trace the Go
221 // stack. However, since we're returning to an earlier stack frame and
222 // need to pair with the entersyscall() call made by cgocall, we must
223 // save syscall* and let reentersyscall restore them.
224 savedsp := unsafe.Pointer(gp.syscallsp)
225 savedpc := gp.syscallpc
226 exitsyscall() // coming out of cgo call
229 osPreemptExtExit(gp.m)
231 cgocallbackg1(fn, frame, ctxt)
233 // At this point unlockOSThread has been called.
234 // The following code must not change to a different m.
235 // This is enforced by checking incgo in the schedule function.
237 osPreemptExtEnter(gp.m)
240 // going back to cgo call
241 reentersyscall(savedpc, uintptr(savedsp))
243 gp.m.syscall = syscall
246 func cgocallbackg1(fn, frame unsafe.Pointer, ctxt uintptr) {
248 if gp.m.needextram || atomic.Load(&extraMWaiters) > 0 {
249 gp.m.needextram = false
250 systemstack(newextram)
254 s := append(gp.cgoCtxt, ctxt)
256 // Now we need to set gp.cgoCtxt = s, but we could get
257 // a SIGPROF signal while manipulating the slice, and
258 // the SIGPROF handler could pick up gp.cgoCtxt while
259 // tracing up the stack. We need to ensure that the
260 // handler always sees a valid slice, so set the
261 // values in an order such that it always does.
262 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
263 atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0]))
268 // Decrease the length of the slice by one, safely.
269 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
275 // The C call to Go came from a thread not currently running
276 // any Go. In the case of -buildmode=c-archive or c-shared,
277 // this call may be coming in before package initialization
278 // is complete. Wait until it is.
282 // Add entry to defer stack in case of panic.
284 defer unwindm(&restore)
287 raceacquire(unsafe.Pointer(&racecgosync))
290 // Invoke callback. This function is generated by cmd/cgo and
291 // will unpack the argument frame and call the Go function.
292 var cb func(frame unsafe.Pointer)
293 cbFV := funcval{uintptr(fn)}
294 *(*unsafe.Pointer)(unsafe.Pointer(&cb)) = noescape(unsafe.Pointer(&cbFV))
298 racereleasemerge(unsafe.Pointer(&racecgosync))
301 // Do not unwind m->g0->sched.sp.
302 // Our caller, cgocallback, will do that.
306 func unwindm(restore *bool) {
308 // Restore sp saved by cgocallback during
309 // unwind of g's stack (see comment at top of file).
311 sched := &mp.g0.sched
312 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + alignUp(sys.MinFrameSize, sys.StackAlign)))
314 // Do the accounting that cgocall will not have a chance to do
317 // In the case where a Go call originates from C, ncgo is 0
318 // and there is no matching cgocall to end.
328 // Undo the call to lockOSThread in cgocallbackg.
329 // We must still stay on the same m.
333 // called from assembly
334 func badcgocallback() {
335 throw("misaligned stack in cgocallback")
338 // called from (incomplete) assembly
340 throw("cgo not implemented")
343 var racecgosync uint64 // represents possible synchronization in C code
345 // Pointer checking for cgo code.
347 // We want to detect all cases where a program that does not use
348 // unsafe makes a cgo call passing a Go pointer to memory that
349 // contains a Go pointer. Here a Go pointer is defined as a pointer
350 // to memory allocated by the Go runtime. Programs that use unsafe
351 // can evade this restriction easily, so we don't try to catch them.
352 // The cgo program will rewrite all possibly bad pointer arguments to
353 // call cgoCheckPointer, where we can catch cases of a Go pointer
354 // pointing to a Go pointer.
356 // Complicating matters, taking the address of a slice or array
357 // element permits the C program to access all elements of the slice
358 // or array. In that case we will see a pointer to a single element,
359 // but we need to check the entire data structure.
361 // The cgoCheckPointer call takes additional arguments indicating that
362 // it was called on an address expression. An additional argument of
363 // true means that it only needs to check a single element. An
364 // additional argument of a slice or array means that it needs to
365 // check the entire slice/array, but nothing else. Otherwise, the
366 // pointer could be anything, and we check the entire heap object,
367 // which is conservative but safe.
369 // When and if we implement a moving garbage collector,
370 // cgoCheckPointer will pin the pointer for the duration of the cgo
371 // call. (This is necessary but not sufficient; the cgo program will
372 // also have to change to pin Go pointers that cannot point to Go
375 // cgoCheckPointer checks if the argument contains a Go pointer that
376 // points to a Go pointer, and panics if it does.
377 func cgoCheckPointer(ptr interface{}, arg interface{}) {
378 if debug.cgocheck == 0 {
386 if arg != nil && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) {
388 if t.kind&kindDirectIface == 0 {
389 p = *(*unsafe.Pointer)(p)
391 if p == nil || !cgoIsGoPointer(p) {
395 switch aep._type.kind & kindMask {
397 if t.kind&kindMask == kindUnsafePointer {
398 // We don't know the type of the element.
401 pt := (*ptrtype)(unsafe.Pointer(t))
402 cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail)
405 // Check the slice rather than the pointer.
409 // Check the array rather than the pointer.
410 // Pass top as false since we have a pointer
416 throw("can't happen")
420 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail)
423 const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer"
424 const cgoResultFail = "cgo result has Go pointer"
426 // cgoCheckArg is the real work of cgoCheckPointer. The argument p
427 // is either a pointer to the value (of type t), or the value itself,
428 // depending on indir. The top parameter is whether we are at the top
429 // level, where Go pointers are allowed.
430 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) {
431 if t.ptrdata == 0 || p == nil {
432 // If the type has no pointers there is nothing to do.
436 switch t.kind & kindMask {
438 throw("can't happen")
440 at := (*arraytype)(unsafe.Pointer(t))
443 throw("can't happen")
445 cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg)
448 for i := uintptr(0); i < at.len; i++ {
449 cgoCheckArg(at.elem, p, true, top, msg)
450 p = add(p, at.elem.size)
452 case kindChan, kindMap:
453 // These types contain internal pointers that will
454 // always be allocated in the Go heap. It's never OK
455 // to pass them to C.
456 panic(errorString(msg))
459 p = *(*unsafe.Pointer)(p)
461 if !cgoIsGoPointer(p) {
464 panic(errorString(msg))
470 // A type known at compile time is OK since it's
471 // constant. A type not known at compile time will be
472 // in the heap and will not be OK.
473 if inheap(uintptr(unsafe.Pointer(it))) {
474 panic(errorString(msg))
476 p = *(*unsafe.Pointer)(add(p, goarch.PtrSize))
477 if !cgoIsGoPointer(p) {
481 panic(errorString(msg))
483 cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg)
485 st := (*slicetype)(unsafe.Pointer(t))
488 if p == nil || !cgoIsGoPointer(p) {
492 panic(errorString(msg))
494 if st.elem.ptrdata == 0 {
497 for i := 0; i < s.cap; i++ {
498 cgoCheckArg(st.elem, p, true, false, msg)
499 p = add(p, st.elem.size)
502 ss := (*stringStruct)(p)
503 if !cgoIsGoPointer(ss.str) {
507 panic(errorString(msg))
510 st := (*structtype)(unsafe.Pointer(t))
512 if len(st.fields) != 1 {
513 throw("can't happen")
515 cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg)
518 for _, f := range st.fields {
519 if f.typ.ptrdata == 0 {
522 cgoCheckArg(f.typ, add(p, f.offset()), true, top, msg)
524 case kindPtr, kindUnsafePointer:
526 p = *(*unsafe.Pointer)(p)
532 if !cgoIsGoPointer(p) {
536 panic(errorString(msg))
539 cgoCheckUnknownPointer(p, msg)
543 // cgoCheckUnknownPointer is called for an arbitrary pointer into Go
544 // memory. It checks whether that Go memory contains any other
545 // pointer into Go memory. If it does, we panic.
546 // The return values are unused but useful to see in panic tracebacks.
547 func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) {
548 if inheap(uintptr(p)) {
549 b, span, _ := findObject(uintptr(p), 0, 0)
554 hbits := heapBitsForAddr(base)
556 for i = uintptr(0); i < n; i += goarch.PtrSize {
557 if !hbits.morePointers() {
558 // No more possible pointers.
561 if hbits.isPointer() && cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(base + i))) {
562 panic(errorString(msg))
570 for _, datap := range activeModules() {
571 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
572 // We have no way to know the size of the object.
573 // We have to assume that it might contain a pointer.
574 panic(errorString(msg))
576 // In the text or noptr sections, we know that the
577 // pointer does not point to a Go pointer.
583 // cgoIsGoPointer reports whether the pointer is a Go pointer--a
584 // pointer to Go memory. We only care about Go memory that might
587 //go:nowritebarrierrec
588 func cgoIsGoPointer(p unsafe.Pointer) bool {
593 if inHeapOrStack(uintptr(p)) {
597 for _, datap := range activeModules() {
598 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
606 // cgoInRange reports whether p is between start and end.
608 //go:nowritebarrierrec
609 func cgoInRange(p unsafe.Pointer, start, end uintptr) bool {
610 return start <= uintptr(p) && uintptr(p) < end
613 // cgoCheckResult is called to check the result parameter of an
614 // exported Go function. It panics if the result is or contains a Go
616 func cgoCheckResult(val interface{}) {
617 if debug.cgocheck == 0 {
623 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail)