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 $GOMAXPROCS limit,
28 // and then unlocks g from m.
30 // The above description skipped over the possibility of the gcc-compiled
31 // function f calling back into Go. If that happens, we continue down
32 // the rabbit hole during the execution of f.
34 // To make it possible for gcc-compiled C code to call a Go function p.GoF,
35 // cgo writes a gcc-compiled function named GoF (not p.GoF, since gcc doesn't
36 // know about packages). The gcc-compiled C function f calls GoF.
38 // GoF calls crosscall2(_cgoexp_GoF, frame, framesize). Crosscall2
39 // (in cgo/gcc_$GOARCH.S, a gcc-compiled assembly file) is a two-argument
40 // adapter from the gcc function call ABI to the 6c function call ABI.
41 // It is called from gcc to call 6c functions. In this case it calls
42 // _cgoexp_GoF(frame, framesize), still running on m->g0's stack
43 // and outside the $GOMAXPROCS limit. Thus, this code cannot yet
44 // call arbitrary Go code directly and must be careful not to allocate
45 // memory or use up m->g0's stack.
47 // _cgoexp_GoF calls runtime.cgocallback(p.GoF, frame, framesize, ctxt).
48 // (The reason for having _cgoexp_GoF instead of writing a crosscall3
49 // to make this call directly is that _cgoexp_GoF, because it is compiled
50 // with 6c instead of gcc, can refer to dotted names like
51 // runtime.cgocallback and p.GoF.)
53 // runtime.cgocallback (in asm_$GOARCH.s) switches from m->g0's
54 // stack to the original g (m->curg)'s stack, on which it calls
55 // runtime.cgocallbackg(p.GoF, frame, framesize).
56 // As part of the stack switch, runtime.cgocallback saves the current
57 // SP as m->g0->sched.sp, so that any use of m->g0's stack during the
58 // execution of the callback will be done below the existing stack frames.
59 // Before overwriting m->g0->sched.sp, it pushes the old value on the
60 // m->g0 stack, so that it can be restored later.
62 // runtime.cgocallbackg (below) is now running on a real goroutine
63 // stack (not an m->g0 stack). First it calls runtime.exitsyscall, which will
64 // block until the $GOMAXPROCS limit allows running this goroutine.
65 // Once exitsyscall has returned, it is safe to do things like call the memory
66 // allocator or invoke the Go callback function p.GoF. runtime.cgocallbackg
67 // first defers a function to unwind m->g0.sched.sp, so that if p.GoF
68 // panics, m->g0.sched.sp will be restored to its old value: the m->g0 stack
69 // and the m->curg stack will be unwound in lock step.
70 // Then it calls p.GoF. Finally it pops but does not execute the deferred
71 // function, calls runtime.entersyscall, and returns to runtime.cgocallback.
73 // After it regains control, runtime.cgocallback switches back to
74 // m->g0's stack (the pointer is still in m->g0.sched.sp), restores the old
75 // m->g0.sched.sp value from the stack, and returns to _cgoexp_GoF.
77 // _cgoexp_GoF immediately returns to crosscall2, which restores the
78 // callee-save registers for gcc and returns to GoF, which returns to f.
83 "runtime/internal/atomic"
84 "runtime/internal/sys"
88 // Addresses collected in a cgo backtrace when crashing.
89 // Length must match arg.Max in x_cgo_callers in runtime/cgo/gcc_traceback.c.
90 type cgoCallers [32]uintptr
94 func cgocall(fn, arg unsafe.Pointer) int32 {
95 if !iscgo && GOOS != "solaris" && GOOS != "windows" {
96 throw("cgocall unavailable")
104 racereleasemerge(unsafe.Pointer(&racecgosync))
114 // Announce we are entering a system call
115 // so that the scheduler knows to create another
116 // M to run goroutines while we are in the
119 // The call to asmcgocall is guaranteed not to
120 // grow the stack and does not allocate memory,
121 // so it is safe to call while "in a system call", outside
122 // the $GOMAXPROCS accounting.
124 // fn may call back into Go code, in which case we'll exit the
125 // "system call", run the Go code (which may grow the stack),
126 // and then re-enter the "system call" reusing the PC and SP
127 // saved by entersyscall here.
131 errno := asmcgocall(fn, arg)
133 // Update accounting before exitsyscall because exitsyscall may
134 // reschedule us on to a different M.
140 // Note that raceacquire must be called only after exitsyscall has
141 // wired this M to a P.
143 raceacquire(unsafe.Pointer(&racecgosync))
146 // From the garbage collector's perspective, time can move
147 // backwards in the sequence above. If there's a callback into
148 // Go code, GC will see this function at the call to
149 // asmcgocall. When the Go call later returns to C, the
150 // syscall PC/SP is rolled back and the GC sees this function
151 // back at the call to entersyscall. Normally, fn and arg
152 // would be live at entersyscall and dead at asmcgocall, so if
153 // time moved backwards, GC would see these arguments as dead
154 // and then live. Prevent these undead arguments from crashing
155 // GC by forcing them to stay live across this time warp.
163 // Call from C back to Go.
165 func cgocallbackg(ctxt uintptr) {
168 println("runtime: bad g in cgocallback")
172 // The call from C is on gp.m's g0 stack, so we must ensure
173 // that we stay on that M. We have to do this before calling
174 // exitsyscall, since it would otherwise be free to move us to
175 // a different M. The call to unlockOSThread is in unwindm.
178 // Save current syscall parameters, so m.syscall can be
179 // used again if callback decide to make syscall.
180 syscall := gp.m.syscall
182 // entersyscall saves the caller's SP to allow the GC to trace the Go
183 // stack. However, since we're returning to an earlier stack frame and
184 // need to pair with the entersyscall() call made by cgocall, we must
185 // save syscall* and let reentersyscall restore them.
186 savedsp := unsafe.Pointer(gp.syscallsp)
187 savedpc := gp.syscallpc
188 exitsyscall() // coming out of cgo call
193 // At this point unlockOSThread has been called.
194 // The following code must not change to a different m.
195 // This is enforced by checking incgo in the schedule function.
198 // going back to cgo call
199 reentersyscall(savedpc, uintptr(savedsp))
201 gp.m.syscall = syscall
204 func cgocallbackg1(ctxt uintptr) {
206 if gp.m.needextram || atomic.Load(&extraMWaiters) > 0 {
207 gp.m.needextram = false
208 systemstack(newextram)
212 s := append(gp.cgoCtxt, ctxt)
214 // Now we need to set gp.cgoCtxt = s, but we could get
215 // a SIGPROF signal while manipulating the slice, and
216 // the SIGPROF handler could pick up gp.cgoCtxt while
217 // tracing up the stack. We need to ensure that the
218 // handler always sees a valid slice, so set the
219 // values in an order such that it always does.
220 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
221 atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0]))
226 // Decrease the length of the slice by one, safely.
227 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
233 // The C call to Go came from a thread not currently running
234 // any Go. In the case of -buildmode=c-archive or c-shared,
235 // this call may be coming in before package initialization
236 // is complete. Wait until it is.
240 // Add entry to defer stack in case of panic.
242 defer unwindm(&restore)
245 raceacquire(unsafe.Pointer(&racecgosync))
255 // Location of callback arguments depends on stack frame layout
256 // and size of stack frame of cgocallback_gofunc.
257 sp := gp.m.g0.sched.sp
260 throw("cgocallbackg is unimplemented on arch")
262 // On arm, stack frame is two words and there's a saved LR between
263 // SP and the stack frame and between the stack frame and the arguments.
264 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
266 // On arm64, stack frame is four words and there's a saved LR between
267 // SP and the stack frame and between the stack frame and the arguments.
268 // Additional two words (16-byte alignment) are for saving FP.
269 cb = (*args)(unsafe.Pointer(sp + 7*sys.PtrSize))
271 // On amd64, stack frame is two words, plus caller PC.
272 if framepointer_enabled {
273 // In this case, there's also saved BP.
274 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
277 cb = (*args)(unsafe.Pointer(sp + 3*sys.PtrSize))
279 // On 386, stack frame is three words, plus caller PC.
280 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
281 case "ppc64", "ppc64le", "s390x":
282 // On ppc64 and s390x, the callback arguments are in the arguments area of
283 // cgocallback's stack frame. The stack looks like this:
284 // +--------------------+------------------------------+
286 // | cgoexp_$fn +------------------------------+
287 // | | fixed frame area |
288 // +--------------------+------------------------------+
289 // | | arguments area |
290 // | cgocallback +------------------------------+ <- sp + 2*minFrameSize + 2*ptrSize
291 // | | fixed frame area |
292 // +--------------------+------------------------------+ <- sp + minFrameSize + 2*ptrSize
293 // | | local variables (2 pointers) |
294 // | cgocallback_gofunc +------------------------------+ <- sp + minFrameSize
295 // | | fixed frame area |
296 // +--------------------+------------------------------+ <- sp
297 cb = (*args)(unsafe.Pointer(sp + 2*sys.MinFrameSize + 2*sys.PtrSize))
298 case "mips64", "mips64le":
299 // On mips64x, stack frame is two words and there's a saved LR between
300 // SP and the stack frame and between the stack frame and the arguments.
301 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
302 case "mips", "mipsle":
303 // On mipsx, stack frame is two words and there's a saved LR between
304 // SP and the stack frame and between the stack frame and the arguments.
305 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
309 // NOTE(rsc): passing nil for argtype means that the copying of the
310 // results back into cb.arg happens without any corresponding write barriers.
311 // For cgo, cb.arg points into a C stack frame and therefore doesn't
312 // hold any pointers that the GC can find anyway - the write barrier
314 reflectcall(nil, unsafe.Pointer(cb.fn), cb.arg, uint32(cb.argsize), 0)
317 racereleasemerge(unsafe.Pointer(&racecgosync))
320 // Tell msan that we wrote to the entire argument block.
321 // This tells msan that we set the results.
322 // Since we have already called the function it doesn't
323 // matter that we are writing to the non-result parameters.
324 msanwrite(cb.arg, cb.argsize)
327 // Do not unwind m->g0->sched.sp.
328 // Our caller, cgocallback, will do that.
332 func unwindm(restore *bool) {
334 // Restore sp saved by cgocallback during
335 // unwind of g's stack (see comment at top of file).
337 sched := &mp.g0.sched
340 throw("unwindm not implemented")
341 case "386", "amd64", "arm", "ppc64", "ppc64le", "mips64", "mips64le", "s390x", "mips", "mipsle":
342 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + sys.MinFrameSize))
344 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + 16))
347 // Do the accounting that cgocall will not have a chance to do
350 // In the case where a Go call originates from C, ncgo is 0
351 // and there is no matching cgocall to end.
360 // Undo the call to lockOSThread in cgocallbackg.
361 // We must still stay on the same m.
365 // called from assembly
366 func badcgocallback() {
367 throw("misaligned stack in cgocallback")
370 // called from (incomplete) assembly
372 throw("cgo not implemented")
375 var racecgosync uint64 // represents possible synchronization in C code
377 // Pointer checking for cgo code.
379 // We want to detect all cases where a program that does not use
380 // unsafe makes a cgo call passing a Go pointer to memory that
381 // contains a Go pointer. Here a Go pointer is defined as a pointer
382 // to memory allocated by the Go runtime. Programs that use unsafe
383 // can evade this restriction easily, so we don't try to catch them.
384 // The cgo program will rewrite all possibly bad pointer arguments to
385 // call cgoCheckPointer, where we can catch cases of a Go pointer
386 // pointing to a Go pointer.
388 // Complicating matters, taking the address of a slice or array
389 // element permits the C program to access all elements of the slice
390 // or array. In that case we will see a pointer to a single element,
391 // but we need to check the entire data structure.
393 // The cgoCheckPointer call takes additional arguments indicating that
394 // it was called on an address expression. An additional argument of
395 // true means that it only needs to check a single element. An
396 // additional argument of a slice or array means that it needs to
397 // check the entire slice/array, but nothing else. Otherwise, the
398 // pointer could be anything, and we check the entire heap object,
399 // which is conservative but safe.
401 // When and if we implement a moving garbage collector,
402 // cgoCheckPointer will pin the pointer for the duration of the cgo
403 // call. (This is necessary but not sufficient; the cgo program will
404 // also have to change to pin Go pointers that cannot point to Go
407 // cgoCheckPointer checks if the argument contains a Go pointer that
408 // points to a Go pointer, and panics if it does.
409 func cgoCheckPointer(ptr interface{}, args ...interface{}) {
410 if debug.cgocheck == 0 {
414 ep := (*eface)(unsafe.Pointer(&ptr))
418 if len(args) > 0 && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) {
420 if t.kind&kindDirectIface == 0 {
421 p = *(*unsafe.Pointer)(p)
423 if !cgoIsGoPointer(p) {
426 aep := (*eface)(unsafe.Pointer(&args[0]))
427 switch aep._type.kind & kindMask {
429 if t.kind&kindMask == kindUnsafePointer {
430 // We don't know the type of the element.
433 pt := (*ptrtype)(unsafe.Pointer(t))
434 cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail)
437 // Check the slice rather than the pointer.
441 // Check the array rather than the pointer.
442 // Pass top as false since we have a pointer
448 throw("can't happen")
452 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail)
455 const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer"
456 const cgoResultFail = "cgo result has Go pointer"
458 // cgoCheckArg is the real work of cgoCheckPointer. The argument p
459 // is either a pointer to the value (of type t), or the value itself,
460 // depending on indir. The top parameter is whether we are at the top
461 // level, where Go pointers are allowed.
462 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) {
464 // If the type has no pointers there is nothing to do.
468 switch t.kind & kindMask {
470 throw("can't happen")
472 at := (*arraytype)(unsafe.Pointer(t))
475 throw("can't happen")
477 cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg)
480 for i := uintptr(0); i < at.len; i++ {
481 cgoCheckArg(at.elem, p, true, top, msg)
482 p = add(p, at.elem.size)
484 case kindChan, kindMap:
485 // These types contain internal pointers that will
486 // always be allocated in the Go heap. It's never OK
487 // to pass them to C.
488 panic(errorString(msg))
491 p = *(*unsafe.Pointer)(p)
493 if !cgoIsGoPointer(p) {
496 panic(errorString(msg))
502 // A type known at compile time is OK since it's
503 // constant. A type not known at compile time will be
504 // in the heap and will not be OK.
505 if inheap(uintptr(unsafe.Pointer(it))) {
506 panic(errorString(msg))
508 p = *(*unsafe.Pointer)(add(p, sys.PtrSize))
509 if !cgoIsGoPointer(p) {
513 panic(errorString(msg))
515 cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg)
517 st := (*slicetype)(unsafe.Pointer(t))
520 if !cgoIsGoPointer(p) {
524 panic(errorString(msg))
526 if st.elem.ptrdata == 0 {
529 for i := 0; i < s.cap; i++ {
530 cgoCheckArg(st.elem, p, true, false, msg)
531 p = add(p, st.elem.size)
534 ss := (*stringStruct)(p)
535 if !cgoIsGoPointer(ss.str) {
539 panic(errorString(msg))
542 st := (*structtype)(unsafe.Pointer(t))
544 if len(st.fields) != 1 {
545 throw("can't happen")
547 cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg)
550 for _, f := range st.fields {
551 cgoCheckArg(f.typ, add(p, f.offset()), true, top, msg)
553 case kindPtr, kindUnsafePointer:
555 p = *(*unsafe.Pointer)(p)
558 if !cgoIsGoPointer(p) {
562 panic(errorString(msg))
565 cgoCheckUnknownPointer(p, msg)
569 // cgoCheckUnknownPointer is called for an arbitrary pointer into Go
570 // memory. It checks whether that Go memory contains any other
571 // pointer into Go memory. If it does, we panic.
572 // The return values are unused but useful to see in panic tracebacks.
573 func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) {
574 if inheap(uintptr(p)) {
575 b, span, _ := findObject(uintptr(p), 0, 0)
580 hbits := heapBitsForAddr(base)
582 for i = uintptr(0); i < n; i += sys.PtrSize {
583 if i != 1*sys.PtrSize && !hbits.morePointers() {
584 // No more possible pointers.
587 if hbits.isPointer() && cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(base + i))) {
588 panic(errorString(msg))
596 for _, datap := range activeModules() {
597 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
598 // We have no way to know the size of the object.
599 // We have to assume that it might contain a pointer.
600 panic(errorString(msg))
602 // In the text or noptr sections, we know that the
603 // pointer does not point to a Go pointer.
609 // cgoIsGoPointer reports whether the pointer is a Go pointer--a
610 // pointer to Go memory. We only care about Go memory that might
613 //go:nowritebarrierrec
614 func cgoIsGoPointer(p unsafe.Pointer) bool {
619 if inHeapOrStack(uintptr(p)) {
623 for _, datap := range activeModules() {
624 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
632 // cgoInRange reports whether p is between start and end.
634 //go:nowritebarrierrec
635 func cgoInRange(p unsafe.Pointer, start, end uintptr) bool {
636 return start <= uintptr(p) && uintptr(p) < end
639 // cgoCheckResult is called to check the result parameter of an
640 // exported Go function. It panics if the result is or contains a Go
642 func cgoCheckResult(val interface{}) {
643 if debug.cgocheck == 0 {
647 ep := (*eface)(unsafe.Pointer(&val))
649 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail)