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.
88 "runtime/internal/atomic"
89 "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 // Call from Go to C.
116 // This must be nosplit because it's used for syscalls on some
117 // platforms. Syscalls may have untyped arguments on the stack, so
118 // it's not safe to grow or scan the stack.
121 func cgocall(fn, arg unsafe.Pointer) int32 {
122 if !iscgo && GOOS != "solaris" && GOOS != "illumos" && GOOS != "windows" {
123 throw("cgocall unavailable")
131 racereleasemerge(unsafe.Pointer(&racecgosync))
141 // Announce we are entering a system call
142 // so that the scheduler knows to create another
143 // M to run goroutines while we are in the
146 // The call to asmcgocall is guaranteed not to
147 // grow the stack and does not allocate memory,
148 // so it is safe to call while "in a system call", outside
149 // the $GOMAXPROCS accounting.
151 // fn may call back into Go code, in which case we'll exit the
152 // "system call", run the Go code (which may grow the stack),
153 // and then re-enter the "system call" reusing the PC and SP
154 // saved by entersyscall here.
157 // Tell asynchronous preemption that we're entering external
158 // code. We do this after entersyscall because this may block
159 // and cause an async preemption to fail, but at this point a
160 // sync preemption will succeed (though this is not a matter
162 osPreemptExtEnter(mp)
165 errno := asmcgocall(fn, arg)
167 // Update accounting before exitsyscall because exitsyscall may
168 // reschedule us on to a different M.
176 // Note that raceacquire must be called only after exitsyscall has
177 // wired this M to a P.
179 raceacquire(unsafe.Pointer(&racecgosync))
182 // From the garbage collector's perspective, time can move
183 // backwards in the sequence above. If there's a callback into
184 // Go code, GC will see this function at the call to
185 // asmcgocall. When the Go call later returns to C, the
186 // syscall PC/SP is rolled back and the GC sees this function
187 // back at the call to entersyscall. Normally, fn and arg
188 // would be live at entersyscall and dead at asmcgocall, so if
189 // time moved backwards, GC would see these arguments as dead
190 // and then live. Prevent these undead arguments from crashing
191 // GC by forcing them to stay live across this time warp.
199 // Call from C back to Go. fn must point to an ABIInternal Go entry-point.
201 func cgocallbackg(fn, frame unsafe.Pointer, ctxt uintptr) {
204 println("runtime: bad g in cgocallback")
208 // The call from C is on gp.m's g0 stack, so we must ensure
209 // that we stay on that M. We have to do this before calling
210 // exitsyscall, since it would otherwise be free to move us to
211 // a different M. The call to unlockOSThread is in unwindm.
214 // Save current syscall parameters, so m.syscall can be
215 // used again if callback decide to make syscall.
216 syscall := gp.m.syscall
218 // entersyscall saves the caller's SP to allow the GC to trace the Go
219 // stack. However, since we're returning to an earlier stack frame and
220 // need to pair with the entersyscall() call made by cgocall, we must
221 // save syscall* and let reentersyscall restore them.
222 savedsp := unsafe.Pointer(gp.syscallsp)
223 savedpc := gp.syscallpc
224 exitsyscall() // coming out of cgo call
227 osPreemptExtExit(gp.m)
229 cgocallbackg1(fn, frame, ctxt)
231 // At this point unlockOSThread has been called.
232 // The following code must not change to a different m.
233 // This is enforced by checking incgo in the schedule function.
235 osPreemptExtEnter(gp.m)
238 // going back to cgo call
239 reentersyscall(savedpc, uintptr(savedsp))
241 gp.m.syscall = syscall
244 func cgocallbackg1(fn, frame unsafe.Pointer, ctxt uintptr) {
246 if gp.m.needextram || atomic.Load(&extraMWaiters) > 0 {
247 gp.m.needextram = false
248 systemstack(newextram)
252 s := append(gp.cgoCtxt, ctxt)
254 // Now we need to set gp.cgoCtxt = s, but we could get
255 // a SIGPROF signal while manipulating the slice, and
256 // the SIGPROF handler could pick up gp.cgoCtxt while
257 // tracing up the stack. We need to ensure that the
258 // handler always sees a valid slice, so set the
259 // values in an order such that it always does.
260 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
261 atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0]))
266 // Decrease the length of the slice by one, safely.
267 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
273 // The C call to Go came from a thread not currently running
274 // any Go. In the case of -buildmode=c-archive or c-shared,
275 // this call may be coming in before package initialization
276 // is complete. Wait until it is.
280 // Add entry to defer stack in case of panic.
282 defer unwindm(&restore)
285 raceacquire(unsafe.Pointer(&racecgosync))
288 // Invoke callback. This function is generated by cmd/cgo and
289 // will unpack the argument frame and call the Go function.
290 var cb func(frame unsafe.Pointer)
291 cbFV := funcval{uintptr(fn)}
292 *(*unsafe.Pointer)(unsafe.Pointer(&cb)) = noescape(unsafe.Pointer(&cbFV))
296 racereleasemerge(unsafe.Pointer(&racecgosync))
299 // Do not unwind m->g0->sched.sp.
300 // Our caller, cgocallback, will do that.
304 func unwindm(restore *bool) {
306 // Restore sp saved by cgocallback during
307 // unwind of g's stack (see comment at top of file).
309 sched := &mp.g0.sched
310 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + alignUp(sys.MinFrameSize, sys.StackAlign)))
312 // Do the accounting that cgocall will not have a chance to do
315 // In the case where a Go call originates from C, ncgo is 0
316 // and there is no matching cgocall to end.
326 // Undo the call to lockOSThread in cgocallbackg.
327 // We must still stay on the same m.
331 // called from assembly
332 func badcgocallback() {
333 throw("misaligned stack in cgocallback")
336 // called from (incomplete) assembly
338 throw("cgo not implemented")
341 var racecgosync uint64 // represents possible synchronization in C code
343 // Pointer checking for cgo code.
345 // We want to detect all cases where a program that does not use
346 // unsafe makes a cgo call passing a Go pointer to memory that
347 // contains a Go pointer. Here a Go pointer is defined as a pointer
348 // to memory allocated by the Go runtime. Programs that use unsafe
349 // can evade this restriction easily, so we don't try to catch them.
350 // The cgo program will rewrite all possibly bad pointer arguments to
351 // call cgoCheckPointer, where we can catch cases of a Go pointer
352 // pointing to a Go pointer.
354 // Complicating matters, taking the address of a slice or array
355 // element permits the C program to access all elements of the slice
356 // or array. In that case we will see a pointer to a single element,
357 // but we need to check the entire data structure.
359 // The cgoCheckPointer call takes additional arguments indicating that
360 // it was called on an address expression. An additional argument of
361 // true means that it only needs to check a single element. An
362 // additional argument of a slice or array means that it needs to
363 // check the entire slice/array, but nothing else. Otherwise, the
364 // pointer could be anything, and we check the entire heap object,
365 // which is conservative but safe.
367 // When and if we implement a moving garbage collector,
368 // cgoCheckPointer will pin the pointer for the duration of the cgo
369 // call. (This is necessary but not sufficient; the cgo program will
370 // also have to change to pin Go pointers that cannot point to Go
373 // cgoCheckPointer checks if the argument contains a Go pointer that
374 // points to a Go pointer, and panics if it does.
375 func cgoCheckPointer(ptr interface{}, arg interface{}) {
376 if debug.cgocheck == 0 {
384 if arg != nil && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) {
386 if t.kind&kindDirectIface == 0 {
387 p = *(*unsafe.Pointer)(p)
389 if p == nil || !cgoIsGoPointer(p) {
393 switch aep._type.kind & kindMask {
395 if t.kind&kindMask == kindUnsafePointer {
396 // We don't know the type of the element.
399 pt := (*ptrtype)(unsafe.Pointer(t))
400 cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail)
403 // Check the slice rather than the pointer.
407 // Check the array rather than the pointer.
408 // Pass top as false since we have a pointer
414 throw("can't happen")
418 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail)
421 const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer"
422 const cgoResultFail = "cgo result has Go pointer"
424 // cgoCheckArg is the real work of cgoCheckPointer. The argument p
425 // is either a pointer to the value (of type t), or the value itself,
426 // depending on indir. The top parameter is whether we are at the top
427 // level, where Go pointers are allowed.
428 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) {
429 if t.ptrdata == 0 || p == nil {
430 // If the type has no pointers there is nothing to do.
434 switch t.kind & kindMask {
436 throw("can't happen")
438 at := (*arraytype)(unsafe.Pointer(t))
441 throw("can't happen")
443 cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg)
446 for i := uintptr(0); i < at.len; i++ {
447 cgoCheckArg(at.elem, p, true, top, msg)
448 p = add(p, at.elem.size)
450 case kindChan, kindMap:
451 // These types contain internal pointers that will
452 // always be allocated in the Go heap. It's never OK
453 // to pass them to C.
454 panic(errorString(msg))
457 p = *(*unsafe.Pointer)(p)
459 if !cgoIsGoPointer(p) {
462 panic(errorString(msg))
468 // A type known at compile time is OK since it's
469 // constant. A type not known at compile time will be
470 // in the heap and will not be OK.
471 if inheap(uintptr(unsafe.Pointer(it))) {
472 panic(errorString(msg))
474 p = *(*unsafe.Pointer)(add(p, goarch.PtrSize))
475 if !cgoIsGoPointer(p) {
479 panic(errorString(msg))
481 cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg)
483 st := (*slicetype)(unsafe.Pointer(t))
486 if p == nil || !cgoIsGoPointer(p) {
490 panic(errorString(msg))
492 if st.elem.ptrdata == 0 {
495 for i := 0; i < s.cap; i++ {
496 cgoCheckArg(st.elem, p, true, false, msg)
497 p = add(p, st.elem.size)
500 ss := (*stringStruct)(p)
501 if !cgoIsGoPointer(ss.str) {
505 panic(errorString(msg))
508 st := (*structtype)(unsafe.Pointer(t))
510 if len(st.fields) != 1 {
511 throw("can't happen")
513 cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg)
516 for _, f := range st.fields {
517 if f.typ.ptrdata == 0 {
520 cgoCheckArg(f.typ, add(p, f.offset()), true, top, msg)
522 case kindPtr, kindUnsafePointer:
524 p = *(*unsafe.Pointer)(p)
530 if !cgoIsGoPointer(p) {
534 panic(errorString(msg))
537 cgoCheckUnknownPointer(p, msg)
541 // cgoCheckUnknownPointer is called for an arbitrary pointer into Go
542 // memory. It checks whether that Go memory contains any other
543 // pointer into Go memory. If it does, we panic.
544 // The return values are unused but useful to see in panic tracebacks.
545 func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) {
546 if inheap(uintptr(p)) {
547 b, span, _ := findObject(uintptr(p), 0, 0)
552 hbits := heapBitsForAddr(base)
554 for i = uintptr(0); i < n; i += goarch.PtrSize {
555 if !hbits.morePointers() {
556 // No more possible pointers.
559 if hbits.isPointer() && cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(base + i))) {
560 panic(errorString(msg))
568 for _, datap := range activeModules() {
569 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
570 // We have no way to know the size of the object.
571 // We have to assume that it might contain a pointer.
572 panic(errorString(msg))
574 // In the text or noptr sections, we know that the
575 // pointer does not point to a Go pointer.
581 // cgoIsGoPointer reports whether the pointer is a Go pointer--a
582 // pointer to Go memory. We only care about Go memory that might
585 //go:nowritebarrierrec
586 func cgoIsGoPointer(p unsafe.Pointer) bool {
591 if inHeapOrStack(uintptr(p)) {
595 for _, datap := range activeModules() {
596 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
604 // cgoInRange reports whether p is between start and end.
606 //go:nowritebarrierrec
607 func cgoInRange(p unsafe.Pointer, start, end uintptr) bool {
608 return start <= uintptr(p) && uintptr(p) < end
611 // cgoCheckResult is called to check the result parameter of an
612 // exported Go function. It panics if the result is or contains a Go
614 func cgoCheckResult(val interface{}) {
615 if debug.cgocheck == 0 {
621 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail)