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 $GOMAXPROCS limit,
28 // and then unlocks g from m.
30 // The above description skipped over the possibility of the gcc-compiled
31 // function f calling back into Go. If that happens, we continue down
32 // the rabbit hole during the execution of f.
34 // To make it possible for gcc-compiled C code to call a Go function p.GoF,
35 // cgo writes a gcc-compiled function named GoF (not p.GoF, since gcc doesn't
36 // know about packages). The gcc-compiled C function f calls GoF.
38 // GoF calls crosscall2(_cgoexp_GoF, frame, framesize, ctxt).
39 // Crosscall2 (in cgo/asm_$GOARCH.s) is a four-argument adapter from
40 // the gcc function call ABI to the gc function call ABI.
41 // It is called from gcc to call gc functions. In this case it calls
42 // _cgoexp_GoF(frame, framesize), still running on m.g0's stack
43 // and outside the $GOMAXPROCS limit. Thus, this code cannot yet
44 // call arbitrary Go code directly and must be careful not to allocate
45 // memory or use up m.g0's stack.
47 // _cgoexp_GoF (generated by cmd/cgo) calls
48 // runtime.cgocallback(funcPC(p.GoF), frame, framesize, ctxt).
49 // (The reason for having _cgoexp_GoF instead of writing a crosscall3
50 // to make this call directly is that _cgoexp_GoF, because it is compiled
51 // with gc instead of gcc, can refer to dotted names like
52 // runtime.cgocallback and p.GoF.)
54 // runtime.cgocallback (in asm_$GOARCH.s) turns the raw PC of p.GoF
55 // into a Go function value and calls runtime.cgocallback_gofunc.
57 // runtime.cgocallback_gofunc (in asm_$GOARCH.s) switches from m.g0's
58 // stack to the original g (m.curg)'s stack, on which it calls
59 // runtime.cgocallbackg(p.GoF, frame, framesize).
60 // As part of the stack switch, runtime.cgocallback saves the current
61 // SP as m.g0.sched.sp, so that any use of m.g0's stack during the
62 // execution of the callback will be done below the existing stack frames.
63 // Before overwriting m.g0.sched.sp, it pushes the old value on the
64 // m.g0 stack, so that it can be restored later.
66 // runtime.cgocallbackg (below) is now running on a real goroutine
67 // stack (not an m.g0 stack). First it calls runtime.exitsyscall, which will
68 // block until the $GOMAXPROCS limit allows running this goroutine.
69 // Once exitsyscall has returned, it is safe to do things like call the memory
70 // allocator or invoke the Go callback function p.GoF. runtime.cgocallbackg
71 // first defers a function to unwind m.g0.sched.sp, so that if p.GoF
72 // panics, m.g0.sched.sp will be restored to its old value: the m.g0 stack
73 // and the m.curg stack will be unwound in lock step.
74 // Then it calls p.GoF. Finally it pops but does not execute the deferred
75 // function, calls runtime.entersyscall, and returns to runtime.cgocallback.
77 // After it regains control, runtime.cgocallback switches back to
78 // m.g0's stack (the pointer is still in m.g0.sched.sp), restores the old
79 // m.g0.sched.sp value from the stack, and returns to _cgoexp_GoF.
81 // _cgoexp_GoF immediately returns to crosscall2, which restores the
82 // callee-save registers for gcc and returns to GoF, which returns to f.
87 "runtime/internal/atomic"
88 "runtime/internal/sys"
92 // Addresses collected in a cgo backtrace when crashing.
93 // Length must match arg.Max in x_cgo_callers in runtime/cgo/gcc_traceback.c.
94 type cgoCallers [32]uintptr
98 // This must be nosplit because it's used for syscalls on some
99 // platforms. Syscalls may have untyped arguments on the stack, so
100 // it's not safe to grow or scan the stack.
103 func cgocall(fn, arg unsafe.Pointer) int32 {
104 if !iscgo && GOOS != "solaris" && GOOS != "illumos" && GOOS != "windows" {
105 throw("cgocall unavailable")
113 racereleasemerge(unsafe.Pointer(&racecgosync))
123 // Announce we are entering a system call
124 // so that the scheduler knows to create another
125 // M to run goroutines while we are in the
128 // The call to asmcgocall is guaranteed not to
129 // grow the stack and does not allocate memory,
130 // so it is safe to call while "in a system call", outside
131 // the $GOMAXPROCS accounting.
133 // fn may call back into Go code, in which case we'll exit the
134 // "system call", run the Go code (which may grow the stack),
135 // and then re-enter the "system call" reusing the PC and SP
136 // saved by entersyscall here.
139 // Tell asynchronous preemption that we're entering external
140 // code. We do this after entersyscall because this may block
141 // and cause an async preemption to fail, but at this point a
142 // sync preemption will succeed (though this is not a matter
144 osPreemptExtEnter(mp)
147 errno := asmcgocall(fn, arg)
149 // Update accounting before exitsyscall because exitsyscall may
150 // reschedule us on to a different M.
158 // Note that raceacquire must be called only after exitsyscall has
159 // wired this M to a P.
161 raceacquire(unsafe.Pointer(&racecgosync))
164 // From the garbage collector's perspective, time can move
165 // backwards in the sequence above. If there's a callback into
166 // Go code, GC will see this function at the call to
167 // asmcgocall. When the Go call later returns to C, the
168 // syscall PC/SP is rolled back and the GC sees this function
169 // back at the call to entersyscall. Normally, fn and arg
170 // would be live at entersyscall and dead at asmcgocall, so if
171 // time moved backwards, GC would see these arguments as dead
172 // and then live. Prevent these undead arguments from crashing
173 // GC by forcing them to stay live across this time warp.
181 // Call from C back to Go.
183 func cgocallbackg(ctxt uintptr) {
186 println("runtime: bad g in cgocallback")
190 // The call from C is on gp.m's g0 stack, so we must ensure
191 // that we stay on that M. We have to do this before calling
192 // exitsyscall, since it would otherwise be free to move us to
193 // a different M. The call to unlockOSThread is in unwindm.
196 // Save current syscall parameters, so m.syscall can be
197 // used again if callback decide to make syscall.
198 syscall := gp.m.syscall
200 // entersyscall saves the caller's SP to allow the GC to trace the Go
201 // stack. However, since we're returning to an earlier stack frame and
202 // need to pair with the entersyscall() call made by cgocall, we must
203 // save syscall* and let reentersyscall restore them.
204 savedsp := unsafe.Pointer(gp.syscallsp)
205 savedpc := gp.syscallpc
206 exitsyscall() // coming out of cgo call
209 osPreemptExtExit(gp.m)
213 // At this point unlockOSThread has been called.
214 // The following code must not change to a different m.
215 // This is enforced by checking incgo in the schedule function.
217 osPreemptExtEnter(gp.m)
220 // going back to cgo call
221 reentersyscall(savedpc, uintptr(savedsp))
223 gp.m.syscall = syscall
226 func cgocallbackg1(ctxt uintptr) {
228 if gp.m.needextram || atomic.Load(&extraMWaiters) > 0 {
229 gp.m.needextram = false
230 systemstack(newextram)
234 s := append(gp.cgoCtxt, ctxt)
236 // Now we need to set gp.cgoCtxt = s, but we could get
237 // a SIGPROF signal while manipulating the slice, and
238 // the SIGPROF handler could pick up gp.cgoCtxt while
239 // tracing up the stack. We need to ensure that the
240 // handler always sees a valid slice, so set the
241 // values in an order such that it always does.
242 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
243 atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0]))
248 // Decrease the length of the slice by one, safely.
249 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
255 // The C call to Go came from a thread not currently running
256 // any Go. In the case of -buildmode=c-archive or c-shared,
257 // this call may be coming in before package initialization
258 // is complete. Wait until it is.
262 // Add entry to defer stack in case of panic.
264 defer unwindm(&restore)
267 raceacquire(unsafe.Pointer(&racecgosync))
277 // Location of callback arguments depends on stack frame layout
278 // and size of stack frame of cgocallback_gofunc.
279 sp := gp.m.g0.sched.sp
282 throw("cgocallbackg is unimplemented on arch")
284 // On arm, stack frame is two words and there's a saved LR between
285 // SP and the stack frame and between the stack frame and the arguments.
286 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
288 // On arm64, stack frame is four words and there's a saved LR between
289 // SP and the stack frame and between the stack frame and the arguments.
290 // Additional two words (16-byte alignment) are for saving FP.
291 cb = (*args)(unsafe.Pointer(sp + 7*sys.PtrSize))
293 // On amd64, stack frame is two words, plus caller PC and BP.
294 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
296 // On 386, stack frame is three words, plus caller PC.
297 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
298 case "ppc64", "ppc64le", "s390x":
299 // On ppc64 and s390x, the callback arguments are in the arguments area of
300 // cgocallback's stack frame. The stack looks like this:
301 // +--------------------+------------------------------+
303 // | cgoexp_$fn +------------------------------+
304 // | | fixed frame area |
305 // +--------------------+------------------------------+
306 // | | arguments area |
307 // | cgocallback +------------------------------+ <- sp + 2*minFrameSize + 2*ptrSize
308 // | | fixed frame area |
309 // +--------------------+------------------------------+ <- sp + minFrameSize + 2*ptrSize
310 // | | local variables (2 pointers) |
311 // | cgocallback_gofunc +------------------------------+ <- sp + minFrameSize
312 // | | fixed frame area |
313 // +--------------------+------------------------------+ <- sp
314 cb = (*args)(unsafe.Pointer(sp + 2*sys.MinFrameSize + 2*sys.PtrSize))
315 case "mips64", "mips64le":
316 // On mips64x, stack frame is two words and there's a saved LR between
317 // SP and the stack frame and between the stack frame and the arguments.
318 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
319 case "mips", "mipsle":
320 // On mipsx, stack frame is two words and there's a saved LR between
321 // SP and the stack frame and between the stack frame and the arguments.
322 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
326 // NOTE(rsc): passing nil for argtype means that the copying of the
327 // results back into cb.arg happens without any corresponding write barriers.
328 // For cgo, cb.arg points into a C stack frame and therefore doesn't
329 // hold any pointers that the GC can find anyway - the write barrier
331 reflectcall(nil, unsafe.Pointer(cb.fn), cb.arg, uint32(cb.argsize), 0)
334 racereleasemerge(unsafe.Pointer(&racecgosync))
337 // Tell msan that we wrote to the entire argument block.
338 // This tells msan that we set the results.
339 // Since we have already called the function it doesn't
340 // matter that we are writing to the non-result parameters.
341 msanwrite(cb.arg, cb.argsize)
344 // Do not unwind m->g0->sched.sp.
345 // Our caller, cgocallback, will do that.
349 func unwindm(restore *bool) {
351 // Restore sp saved by cgocallback during
352 // unwind of g's stack (see comment at top of file).
354 sched := &mp.g0.sched
357 throw("unwindm not implemented")
358 case "386", "amd64", "arm", "ppc64", "ppc64le", "mips64", "mips64le", "s390x", "mips", "mipsle":
359 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + sys.MinFrameSize))
361 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + 16))
364 // Do the accounting that cgocall will not have a chance to do
367 // In the case where a Go call originates from C, ncgo is 0
368 // and there is no matching cgocall to end.
378 // Undo the call to lockOSThread in cgocallbackg.
379 // We must still stay on the same m.
383 // called from assembly
384 func badcgocallback() {
385 throw("misaligned stack in cgocallback")
388 // called from (incomplete) assembly
390 throw("cgo not implemented")
393 var racecgosync uint64 // represents possible synchronization in C code
395 // Pointer checking for cgo code.
397 // We want to detect all cases where a program that does not use
398 // unsafe makes a cgo call passing a Go pointer to memory that
399 // contains a Go pointer. Here a Go pointer is defined as a pointer
400 // to memory allocated by the Go runtime. Programs that use unsafe
401 // can evade this restriction easily, so we don't try to catch them.
402 // The cgo program will rewrite all possibly bad pointer arguments to
403 // call cgoCheckPointer, where we can catch cases of a Go pointer
404 // pointing to a Go pointer.
406 // Complicating matters, taking the address of a slice or array
407 // element permits the C program to access all elements of the slice
408 // or array. In that case we will see a pointer to a single element,
409 // but we need to check the entire data structure.
411 // The cgoCheckPointer call takes additional arguments indicating that
412 // it was called on an address expression. An additional argument of
413 // true means that it only needs to check a single element. An
414 // additional argument of a slice or array means that it needs to
415 // check the entire slice/array, but nothing else. Otherwise, the
416 // pointer could be anything, and we check the entire heap object,
417 // which is conservative but safe.
419 // When and if we implement a moving garbage collector,
420 // cgoCheckPointer will pin the pointer for the duration of the cgo
421 // call. (This is necessary but not sufficient; the cgo program will
422 // also have to change to pin Go pointers that cannot point to Go
425 // cgoCheckPointer checks if the argument contains a Go pointer that
426 // points to a Go pointer, and panics if it does.
427 func cgoCheckPointer(ptr interface{}, arg interface{}) {
428 if debug.cgocheck == 0 {
436 if arg != nil && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) {
438 if t.kind&kindDirectIface == 0 {
439 p = *(*unsafe.Pointer)(p)
441 if p == nil || !cgoIsGoPointer(p) {
445 switch aep._type.kind & kindMask {
447 if t.kind&kindMask == kindUnsafePointer {
448 // We don't know the type of the element.
451 pt := (*ptrtype)(unsafe.Pointer(t))
452 cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail)
455 // Check the slice rather than the pointer.
459 // Check the array rather than the pointer.
460 // Pass top as false since we have a pointer
466 throw("can't happen")
470 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail)
473 const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer"
474 const cgoResultFail = "cgo result has Go pointer"
476 // cgoCheckArg is the real work of cgoCheckPointer. The argument p
477 // is either a pointer to the value (of type t), or the value itself,
478 // depending on indir. The top parameter is whether we are at the top
479 // level, where Go pointers are allowed.
480 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) {
481 if t.ptrdata == 0 || p == nil {
482 // If the type has no pointers there is nothing to do.
486 switch t.kind & kindMask {
488 throw("can't happen")
490 at := (*arraytype)(unsafe.Pointer(t))
493 throw("can't happen")
495 cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg)
498 for i := uintptr(0); i < at.len; i++ {
499 cgoCheckArg(at.elem, p, true, top, msg)
500 p = add(p, at.elem.size)
502 case kindChan, kindMap:
503 // These types contain internal pointers that will
504 // always be allocated in the Go heap. It's never OK
505 // to pass them to C.
506 panic(errorString(msg))
509 p = *(*unsafe.Pointer)(p)
511 if !cgoIsGoPointer(p) {
514 panic(errorString(msg))
520 // A type known at compile time is OK since it's
521 // constant. A type not known at compile time will be
522 // in the heap and will not be OK.
523 if inheap(uintptr(unsafe.Pointer(it))) {
524 panic(errorString(msg))
526 p = *(*unsafe.Pointer)(add(p, sys.PtrSize))
527 if !cgoIsGoPointer(p) {
531 panic(errorString(msg))
533 cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg)
535 st := (*slicetype)(unsafe.Pointer(t))
538 if p == nil || !cgoIsGoPointer(p) {
542 panic(errorString(msg))
544 if st.elem.ptrdata == 0 {
547 for i := 0; i < s.cap; i++ {
548 cgoCheckArg(st.elem, p, true, false, msg)
549 p = add(p, st.elem.size)
552 ss := (*stringStruct)(p)
553 if !cgoIsGoPointer(ss.str) {
557 panic(errorString(msg))
560 st := (*structtype)(unsafe.Pointer(t))
562 if len(st.fields) != 1 {
563 throw("can't happen")
565 cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg)
568 for _, f := range st.fields {
569 if f.typ.ptrdata == 0 {
572 cgoCheckArg(f.typ, add(p, f.offset()), true, top, msg)
574 case kindPtr, kindUnsafePointer:
576 p = *(*unsafe.Pointer)(p)
582 if !cgoIsGoPointer(p) {
586 panic(errorString(msg))
589 cgoCheckUnknownPointer(p, msg)
593 // cgoCheckUnknownPointer is called for an arbitrary pointer into Go
594 // memory. It checks whether that Go memory contains any other
595 // pointer into Go memory. If it does, we panic.
596 // The return values are unused but useful to see in panic tracebacks.
597 func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) {
598 if inheap(uintptr(p)) {
599 b, span, _ := findObject(uintptr(p), 0, 0)
604 hbits := heapBitsForAddr(base)
606 for i = uintptr(0); i < n; i += sys.PtrSize {
607 if !hbits.morePointers() {
608 // No more possible pointers.
611 if hbits.isPointer() && cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(base + i))) {
612 panic(errorString(msg))
620 for _, datap := range activeModules() {
621 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
622 // We have no way to know the size of the object.
623 // We have to assume that it might contain a pointer.
624 panic(errorString(msg))
626 // In the text or noptr sections, we know that the
627 // pointer does not point to a Go pointer.
633 // cgoIsGoPointer reports whether the pointer is a Go pointer--a
634 // pointer to Go memory. We only care about Go memory that might
637 //go:nowritebarrierrec
638 func cgoIsGoPointer(p unsafe.Pointer) bool {
643 if inHeapOrStack(uintptr(p)) {
647 for _, datap := range activeModules() {
648 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
656 // cgoInRange reports whether p is between start and end.
658 //go:nowritebarrierrec
659 func cgoInRange(p unsafe.Pointer, start, end uintptr) bool {
660 return start <= uintptr(p) && uintptr(p) < end
663 // cgoCheckResult is called to check the result parameter of an
664 // exported Go function. It panics if the result is or contains a Go
666 func cgoCheckResult(val interface{}) {
667 if debug.cgocheck == 0 {
673 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail)