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 // This must be nosplit because it's used for syscalls on some
95 // platforms. Syscalls may have untyped arguments on the stack, so
96 // it's not safe to grow or scan the stack.
99 func cgocall(fn, arg unsafe.Pointer) int32 {
100 if !iscgo && GOOS != "solaris" && GOOS != "illumos" && GOOS != "windows" {
101 throw("cgocall unavailable")
109 racereleasemerge(unsafe.Pointer(&racecgosync))
119 // Announce we are entering a system call
120 // so that the scheduler knows to create another
121 // M to run goroutines while we are in the
124 // The call to asmcgocall is guaranteed not to
125 // grow the stack and does not allocate memory,
126 // so it is safe to call while "in a system call", outside
127 // the $GOMAXPROCS accounting.
129 // fn may call back into Go code, in which case we'll exit the
130 // "system call", run the Go code (which may grow the stack),
131 // and then re-enter the "system call" reusing the PC and SP
132 // saved by entersyscall here.
135 // Tell asynchronous preemption that we're entering external
136 // code. We do this after entersyscall because this may block
137 // and cause an async preemption to fail, but at this point a
138 // sync preemption will succeed (though this is not a matter
140 osPreemptExtEnter(mp)
143 errno := asmcgocall(fn, arg)
145 // Update accounting before exitsyscall because exitsyscall may
146 // reschedule us on to a different M.
154 // Note that raceacquire must be called only after exitsyscall has
155 // wired this M to a P.
157 raceacquire(unsafe.Pointer(&racecgosync))
160 // From the garbage collector's perspective, time can move
161 // backwards in the sequence above. If there's a callback into
162 // Go code, GC will see this function at the call to
163 // asmcgocall. When the Go call later returns to C, the
164 // syscall PC/SP is rolled back and the GC sees this function
165 // back at the call to entersyscall. Normally, fn and arg
166 // would be live at entersyscall and dead at asmcgocall, so if
167 // time moved backwards, GC would see these arguments as dead
168 // and then live. Prevent these undead arguments from crashing
169 // GC by forcing them to stay live across this time warp.
177 // Call from C back to Go.
179 func cgocallbackg(ctxt uintptr) {
182 println("runtime: bad g in cgocallback")
186 // The call from C is on gp.m's g0 stack, so we must ensure
187 // that we stay on that M. We have to do this before calling
188 // exitsyscall, since it would otherwise be free to move us to
189 // a different M. The call to unlockOSThread is in unwindm.
192 // Save current syscall parameters, so m.syscall can be
193 // used again if callback decide to make syscall.
194 syscall := gp.m.syscall
196 // entersyscall saves the caller's SP to allow the GC to trace the Go
197 // stack. However, since we're returning to an earlier stack frame and
198 // need to pair with the entersyscall() call made by cgocall, we must
199 // save syscall* and let reentersyscall restore them.
200 savedsp := unsafe.Pointer(gp.syscallsp)
201 savedpc := gp.syscallpc
202 exitsyscall() // coming out of cgo call
205 osPreemptExtExit(gp.m)
209 // At this point unlockOSThread has been called.
210 // The following code must not change to a different m.
211 // This is enforced by checking incgo in the schedule function.
213 osPreemptExtEnter(gp.m)
216 // going back to cgo call
217 reentersyscall(savedpc, uintptr(savedsp))
219 gp.m.syscall = syscall
222 func cgocallbackg1(ctxt uintptr) {
224 if gp.m.needextram || atomic.Load(&extraMWaiters) > 0 {
225 gp.m.needextram = false
226 systemstack(newextram)
230 s := append(gp.cgoCtxt, ctxt)
232 // Now we need to set gp.cgoCtxt = s, but we could get
233 // a SIGPROF signal while manipulating the slice, and
234 // the SIGPROF handler could pick up gp.cgoCtxt while
235 // tracing up the stack. We need to ensure that the
236 // handler always sees a valid slice, so set the
237 // values in an order such that it always does.
238 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
239 atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0]))
244 // Decrease the length of the slice by one, safely.
245 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
251 // The C call to Go came from a thread not currently running
252 // any Go. In the case of -buildmode=c-archive or c-shared,
253 // this call may be coming in before package initialization
254 // is complete. Wait until it is.
258 // Add entry to defer stack in case of panic.
260 defer unwindm(&restore)
263 raceacquire(unsafe.Pointer(&racecgosync))
273 // Location of callback arguments depends on stack frame layout
274 // and size of stack frame of cgocallback_gofunc.
275 sp := gp.m.g0.sched.sp
278 throw("cgocallbackg is unimplemented on arch")
280 // On arm, stack frame is two words and there's a saved LR between
281 // SP and the stack frame and between the stack frame and the arguments.
282 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
284 // On arm64, stack frame is four words and there's a saved LR between
285 // SP and the stack frame and between the stack frame and the arguments.
286 // Additional two words (16-byte alignment) are for saving FP.
287 cb = (*args)(unsafe.Pointer(sp + 7*sys.PtrSize))
289 // On amd64, stack frame is two words, plus caller PC.
290 if framepointer_enabled {
291 // In this case, there's also saved BP.
292 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
295 cb = (*args)(unsafe.Pointer(sp + 3*sys.PtrSize))
297 // On 386, stack frame is three words, plus caller PC.
298 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
299 case "ppc64", "ppc64le", "s390x":
300 // On ppc64 and s390x, the callback arguments are in the arguments area of
301 // cgocallback's stack frame. The stack looks like this:
302 // +--------------------+------------------------------+
304 // | cgoexp_$fn +------------------------------+
305 // | | fixed frame area |
306 // +--------------------+------------------------------+
307 // | | arguments area |
308 // | cgocallback +------------------------------+ <- sp + 2*minFrameSize + 2*ptrSize
309 // | | fixed frame area |
310 // +--------------------+------------------------------+ <- sp + minFrameSize + 2*ptrSize
311 // | | local variables (2 pointers) |
312 // | cgocallback_gofunc +------------------------------+ <- sp + minFrameSize
313 // | | fixed frame area |
314 // +--------------------+------------------------------+ <- sp
315 cb = (*args)(unsafe.Pointer(sp + 2*sys.MinFrameSize + 2*sys.PtrSize))
316 case "mips64", "mips64le":
317 // On mips64x, stack frame is two words and there's a saved LR between
318 // SP and the stack frame and between the stack frame and the arguments.
319 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
320 case "mips", "mipsle":
321 // On mipsx, stack frame is two words and there's a saved LR between
322 // SP and the stack frame and between the stack frame and the arguments.
323 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
327 // NOTE(rsc): passing nil for argtype means that the copying of the
328 // results back into cb.arg happens without any corresponding write barriers.
329 // For cgo, cb.arg points into a C stack frame and therefore doesn't
330 // hold any pointers that the GC can find anyway - the write barrier
332 reflectcall(nil, unsafe.Pointer(cb.fn), cb.arg, uint32(cb.argsize), 0)
335 racereleasemerge(unsafe.Pointer(&racecgosync))
338 // Tell msan that we wrote to the entire argument block.
339 // This tells msan that we set the results.
340 // Since we have already called the function it doesn't
341 // matter that we are writing to the non-result parameters.
342 msanwrite(cb.arg, cb.argsize)
345 // Do not unwind m->g0->sched.sp.
346 // Our caller, cgocallback, will do that.
350 func unwindm(restore *bool) {
352 // Restore sp saved by cgocallback during
353 // unwind of g's stack (see comment at top of file).
355 sched := &mp.g0.sched
358 throw("unwindm not implemented")
359 case "386", "amd64", "arm", "ppc64", "ppc64le", "mips64", "mips64le", "s390x", "mips", "mipsle":
360 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + sys.MinFrameSize))
362 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + 16))
365 // Do the accounting that cgocall will not have a chance to do
368 // In the case where a Go call originates from C, ncgo is 0
369 // and there is no matching cgocall to end.
379 // Undo the call to lockOSThread in cgocallbackg.
380 // We must still stay on the same m.
384 // called from assembly
385 func badcgocallback() {
386 throw("misaligned stack in cgocallback")
389 // called from (incomplete) assembly
391 throw("cgo not implemented")
394 var racecgosync uint64 // represents possible synchronization in C code
396 // Pointer checking for cgo code.
398 // We want to detect all cases where a program that does not use
399 // unsafe makes a cgo call passing a Go pointer to memory that
400 // contains a Go pointer. Here a Go pointer is defined as a pointer
401 // to memory allocated by the Go runtime. Programs that use unsafe
402 // can evade this restriction easily, so we don't try to catch them.
403 // The cgo program will rewrite all possibly bad pointer arguments to
404 // call cgoCheckPointer, where we can catch cases of a Go pointer
405 // pointing to a Go pointer.
407 // Complicating matters, taking the address of a slice or array
408 // element permits the C program to access all elements of the slice
409 // or array. In that case we will see a pointer to a single element,
410 // but we need to check the entire data structure.
412 // The cgoCheckPointer call takes additional arguments indicating that
413 // it was called on an address expression. An additional argument of
414 // true means that it only needs to check a single element. An
415 // additional argument of a slice or array means that it needs to
416 // check the entire slice/array, but nothing else. Otherwise, the
417 // pointer could be anything, and we check the entire heap object,
418 // which is conservative but safe.
420 // When and if we implement a moving garbage collector,
421 // cgoCheckPointer will pin the pointer for the duration of the cgo
422 // call. (This is necessary but not sufficient; the cgo program will
423 // also have to change to pin Go pointers that cannot point to Go
426 // cgoCheckPointer checks if the argument contains a Go pointer that
427 // points to a Go pointer, and panics if it does.
428 func cgoCheckPointer(ptr interface{}, arg interface{}) {
429 if debug.cgocheck == 0 {
437 if arg != nil && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) {
439 if t.kind&kindDirectIface == 0 {
440 p = *(*unsafe.Pointer)(p)
442 if p == nil || !cgoIsGoPointer(p) {
446 switch aep._type.kind & kindMask {
448 if t.kind&kindMask == kindUnsafePointer {
449 // We don't know the type of the element.
452 pt := (*ptrtype)(unsafe.Pointer(t))
453 cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail)
456 // Check the slice rather than the pointer.
460 // Check the array rather than the pointer.
461 // Pass top as false since we have a pointer
467 throw("can't happen")
471 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail)
474 const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer"
475 const cgoResultFail = "cgo result has Go pointer"
477 // cgoCheckArg is the real work of cgoCheckPointer. The argument p
478 // is either a pointer to the value (of type t), or the value itself,
479 // depending on indir. The top parameter is whether we are at the top
480 // level, where Go pointers are allowed.
481 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) {
482 if t.ptrdata == 0 || p == nil {
483 // If the type has no pointers there is nothing to do.
487 switch t.kind & kindMask {
489 throw("can't happen")
491 at := (*arraytype)(unsafe.Pointer(t))
494 throw("can't happen")
496 cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg)
499 for i := uintptr(0); i < at.len; i++ {
500 cgoCheckArg(at.elem, p, true, top, msg)
501 p = add(p, at.elem.size)
503 case kindChan, kindMap:
504 // These types contain internal pointers that will
505 // always be allocated in the Go heap. It's never OK
506 // to pass them to C.
507 panic(errorString(msg))
510 p = *(*unsafe.Pointer)(p)
512 if !cgoIsGoPointer(p) {
515 panic(errorString(msg))
521 // A type known at compile time is OK since it's
522 // constant. A type not known at compile time will be
523 // in the heap and will not be OK.
524 if inheap(uintptr(unsafe.Pointer(it))) {
525 panic(errorString(msg))
527 p = *(*unsafe.Pointer)(add(p, sys.PtrSize))
528 if !cgoIsGoPointer(p) {
532 panic(errorString(msg))
534 cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg)
536 st := (*slicetype)(unsafe.Pointer(t))
539 if p == nil || !cgoIsGoPointer(p) {
543 panic(errorString(msg))
545 if st.elem.ptrdata == 0 {
548 for i := 0; i < s.cap; i++ {
549 cgoCheckArg(st.elem, p, true, false, msg)
550 p = add(p, st.elem.size)
553 ss := (*stringStruct)(p)
554 if !cgoIsGoPointer(ss.str) {
558 panic(errorString(msg))
561 st := (*structtype)(unsafe.Pointer(t))
563 if len(st.fields) != 1 {
564 throw("can't happen")
566 cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg)
569 for _, f := range st.fields {
570 if f.typ.ptrdata == 0 {
573 cgoCheckArg(f.typ, add(p, f.offset()), true, top, msg)
575 case kindPtr, kindUnsafePointer:
577 p = *(*unsafe.Pointer)(p)
583 if !cgoIsGoPointer(p) {
587 panic(errorString(msg))
590 cgoCheckUnknownPointer(p, msg)
594 // cgoCheckUnknownPointer is called for an arbitrary pointer into Go
595 // memory. It checks whether that Go memory contains any other
596 // pointer into Go memory. If it does, we panic.
597 // The return values are unused but useful to see in panic tracebacks.
598 func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) {
599 if inheap(uintptr(p)) {
600 b, span, _ := findObject(uintptr(p), 0, 0)
605 hbits := heapBitsForAddr(base)
607 for i = uintptr(0); i < n; i += sys.PtrSize {
608 if i != 1*sys.PtrSize && !hbits.morePointers() {
609 // No more possible pointers.
612 if hbits.isPointer() && cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(base + i))) {
613 panic(errorString(msg))
621 for _, datap := range activeModules() {
622 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
623 // We have no way to know the size of the object.
624 // We have to assume that it might contain a pointer.
625 panic(errorString(msg))
627 // In the text or noptr sections, we know that the
628 // pointer does not point to a Go pointer.
634 // cgoIsGoPointer reports whether the pointer is a Go pointer--a
635 // pointer to Go memory. We only care about Go memory that might
638 //go:nowritebarrierrec
639 func cgoIsGoPointer(p unsafe.Pointer) bool {
644 if inHeapOrStack(uintptr(p)) {
648 for _, datap := range activeModules() {
649 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
657 // cgoInRange reports whether p is between start and end.
659 //go:nowritebarrierrec
660 func cgoInRange(p unsafe.Pointer, start, end uintptr) bool {
661 return start <= uintptr(p) && uintptr(p) < end
664 // cgoCheckResult is called to check the result parameter of an
665 // exported Go function. It panics if the result is or contains a Go
667 func cgoCheckResult(val interface{}) {
668 if debug.cgocheck == 0 {
674 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail)