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 // Call endcgo before exitsyscall because exitsyscall may
134 // reschedule us on to a different M.
139 // From the garbage collector's perspective, time can move
140 // backwards in the sequence above. If there's a callback into
141 // Go code, GC will see this function at the call to
142 // asmcgocall. When the Go call later returns to C, the
143 // syscall PC/SP is rolled back and the GC sees this function
144 // back at the call to entersyscall. Normally, fn and arg
145 // would be live at entersyscall and dead at asmcgocall, so if
146 // time moved backwards, GC would see these arguments as dead
147 // and then live. Prevent these undead arguments from crashing
148 // GC by forcing them to stay live across this time warp.
162 raceacquire(unsafe.Pointer(&racecgosync))
166 // Call from C back to Go.
168 func cgocallbackg(ctxt uintptr) {
171 println("runtime: bad g in cgocallback")
175 // The call from C is on gp.m's g0 stack, so we must ensure
176 // that we stay on that M. We have to do this before calling
177 // exitsyscall, since it would otherwise be free to move us to
178 // a different M. The call to unlockOSThread is in unwindm.
181 // Save current syscall parameters, so m.syscall can be
182 // used again if callback decide to make syscall.
183 syscall := gp.m.syscall
185 // entersyscall saves the caller's SP to allow the GC to trace the Go
186 // stack. However, since we're returning to an earlier stack frame and
187 // need to pair with the entersyscall() call made by cgocall, we must
188 // save syscall* and let reentersyscall restore them.
189 savedsp := unsafe.Pointer(gp.syscallsp)
190 savedpc := gp.syscallpc
191 exitsyscall() // coming out of cgo call
196 // At this point unlockOSThread has been called.
197 // The following code must not change to a different m.
198 // This is enforced by checking incgo in the schedule function.
201 // going back to cgo call
202 reentersyscall(savedpc, uintptr(savedsp))
204 gp.m.syscall = syscall
207 func cgocallbackg1(ctxt uintptr) {
209 if gp.m.needextram || atomic.Load(&extraMWaiters) > 0 {
210 gp.m.needextram = false
211 systemstack(newextram)
215 s := append(gp.cgoCtxt, ctxt)
217 // Now we need to set gp.cgoCtxt = s, but we could get
218 // a SIGPROF signal while manipulating the slice, and
219 // the SIGPROF handler could pick up gp.cgoCtxt while
220 // tracing up the stack. We need to ensure that the
221 // handler always sees a valid slice, so set the
222 // values in an order such that it always does.
223 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
224 atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0]))
229 // Decrease the length of the slice by one, safely.
230 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
236 // The C call to Go came from a thread not currently running
237 // any Go. In the case of -buildmode=c-archive or c-shared,
238 // this call may be coming in before package initialization
239 // is complete. Wait until it is.
243 // Add entry to defer stack in case of panic.
245 defer unwindm(&restore)
248 raceacquire(unsafe.Pointer(&racecgosync))
258 // Location of callback arguments depends on stack frame layout
259 // and size of stack frame of cgocallback_gofunc.
260 sp := gp.m.g0.sched.sp
263 throw("cgocallbackg is unimplemented on arch")
265 // On arm, stack frame is two words and there's a saved LR between
266 // SP and the stack frame and between the stack frame and the arguments.
267 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
269 // On arm64, stack frame is four words and there's a saved LR between
270 // SP and the stack frame and between the stack frame and the arguments.
271 // Additional two words (16-byte alignment) are for saving FP.
272 cb = (*args)(unsafe.Pointer(sp + 7*sys.PtrSize))
274 // On amd64, stack frame is two words, plus caller PC.
275 if framepointer_enabled {
276 // In this case, there's also saved BP.
277 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
280 cb = (*args)(unsafe.Pointer(sp + 3*sys.PtrSize))
282 // On 386, stack frame is three words, plus caller PC.
283 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
284 case "ppc64", "ppc64le", "s390x":
285 // On ppc64 and s390x, the callback arguments are in the arguments area of
286 // cgocallback's stack frame. The stack looks like this:
287 // +--------------------+------------------------------+
289 // | cgoexp_$fn +------------------------------+
290 // | | fixed frame area |
291 // +--------------------+------------------------------+
292 // | | arguments area |
293 // | cgocallback +------------------------------+ <- sp + 2*minFrameSize + 2*ptrSize
294 // | | fixed frame area |
295 // +--------------------+------------------------------+ <- sp + minFrameSize + 2*ptrSize
296 // | | local variables (2 pointers) |
297 // | cgocallback_gofunc +------------------------------+ <- sp + minFrameSize
298 // | | fixed frame area |
299 // +--------------------+------------------------------+ <- sp
300 cb = (*args)(unsafe.Pointer(sp + 2*sys.MinFrameSize + 2*sys.PtrSize))
301 case "mips64", "mips64le":
302 // On mips64x, stack frame is two words and there's a saved LR between
303 // SP and the stack frame and between the stack frame and the arguments.
304 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
305 case "mips", "mipsle":
306 // On mipsx, stack frame is two words and there's a saved LR between
307 // SP and the stack frame and between the stack frame and the arguments.
308 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
312 // NOTE(rsc): passing nil for argtype means that the copying of the
313 // results back into cb.arg happens without any corresponding write barriers.
314 // For cgo, cb.arg points into a C stack frame and therefore doesn't
315 // hold any pointers that the GC can find anyway - the write barrier
317 reflectcall(nil, unsafe.Pointer(cb.fn), cb.arg, uint32(cb.argsize), 0)
320 racereleasemerge(unsafe.Pointer(&racecgosync))
323 // Tell msan that we wrote to the entire argument block.
324 // This tells msan that we set the results.
325 // Since we have already called the function it doesn't
326 // matter that we are writing to the non-result parameters.
327 msanwrite(cb.arg, cb.argsize)
330 // Do not unwind m->g0->sched.sp.
331 // Our caller, cgocallback, will do that.
335 func unwindm(restore *bool) {
337 // Restore sp saved by cgocallback during
338 // unwind of g's stack (see comment at top of file).
340 sched := &mp.g0.sched
343 throw("unwindm not implemented")
344 case "386", "amd64", "arm", "ppc64", "ppc64le", "mips64", "mips64le", "s390x", "mips", "mipsle":
345 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + sys.MinFrameSize))
347 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + 16))
350 // Call endcgo to do the accounting that cgocall will not have a
351 // chance to do during an unwind.
353 // In the case where a Go call originates from C, ncgo is 0
354 // and there is no matching cgocall to end.
362 // Undo the call to lockOSThread in cgocallbackg.
363 // We must still stay on the same m.
367 // called from assembly
368 func badcgocallback() {
369 throw("misaligned stack in cgocallback")
372 // called from (incomplete) assembly
374 throw("cgo not implemented")
377 var racecgosync uint64 // represents possible synchronization in C code
379 // Pointer checking for cgo code.
381 // We want to detect all cases where a program that does not use
382 // unsafe makes a cgo call passing a Go pointer to memory that
383 // contains a Go pointer. Here a Go pointer is defined as a pointer
384 // to memory allocated by the Go runtime. Programs that use unsafe
385 // can evade this restriction easily, so we don't try to catch them.
386 // The cgo program will rewrite all possibly bad pointer arguments to
387 // call cgoCheckPointer, where we can catch cases of a Go pointer
388 // pointing to a Go pointer.
390 // Complicating matters, taking the address of a slice or array
391 // element permits the C program to access all elements of the slice
392 // or array. In that case we will see a pointer to a single element,
393 // but we need to check the entire data structure.
395 // The cgoCheckPointer call takes additional arguments indicating that
396 // it was called on an address expression. An additional argument of
397 // true means that it only needs to check a single element. An
398 // additional argument of a slice or array means that it needs to
399 // check the entire slice/array, but nothing else. Otherwise, the
400 // pointer could be anything, and we check the entire heap object,
401 // which is conservative but safe.
403 // When and if we implement a moving garbage collector,
404 // cgoCheckPointer will pin the pointer for the duration of the cgo
405 // call. (This is necessary but not sufficient; the cgo program will
406 // also have to change to pin Go pointers that cannot point to Go
409 // cgoCheckPointer checks if the argument contains a Go pointer that
410 // points to a Go pointer, and panics if it does.
411 func cgoCheckPointer(ptr interface{}, args ...interface{}) {
412 if debug.cgocheck == 0 {
416 ep := (*eface)(unsafe.Pointer(&ptr))
420 if len(args) > 0 && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) {
422 if t.kind&kindDirectIface == 0 {
423 p = *(*unsafe.Pointer)(p)
425 if !cgoIsGoPointer(p) {
428 aep := (*eface)(unsafe.Pointer(&args[0]))
429 switch aep._type.kind & kindMask {
431 if t.kind&kindMask == kindUnsafePointer {
432 // We don't know the type of the element.
435 pt := (*ptrtype)(unsafe.Pointer(t))
436 cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail)
439 // Check the slice rather than the pointer.
443 // Check the array rather than the pointer.
444 // Pass top as false since we have a pointer
450 throw("can't happen")
454 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail)
457 const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer"
458 const cgoResultFail = "cgo result has Go pointer"
460 // cgoCheckArg is the real work of cgoCheckPointer. The argument p
461 // is either a pointer to the value (of type t), or the value itself,
462 // depending on indir. The top parameter is whether we are at the top
463 // level, where Go pointers are allowed.
464 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) {
465 if t.kind&kindNoPointers != 0 {
466 // If the type has no pointers there is nothing to do.
470 switch t.kind & kindMask {
472 throw("can't happen")
474 at := (*arraytype)(unsafe.Pointer(t))
477 throw("can't happen")
479 cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg)
482 for i := uintptr(0); i < at.len; i++ {
483 cgoCheckArg(at.elem, p, true, top, msg)
484 p = add(p, at.elem.size)
486 case kindChan, kindMap:
487 // These types contain internal pointers that will
488 // always be allocated in the Go heap. It's never OK
489 // to pass them to C.
490 panic(errorString(msg))
493 p = *(*unsafe.Pointer)(p)
495 if !cgoIsGoPointer(p) {
498 panic(errorString(msg))
504 // A type known at compile time is OK since it's
505 // constant. A type not known at compile time will be
506 // in the heap and will not be OK.
507 if inheap(uintptr(unsafe.Pointer(it))) {
508 panic(errorString(msg))
510 p = *(*unsafe.Pointer)(add(p, sys.PtrSize))
511 if !cgoIsGoPointer(p) {
515 panic(errorString(msg))
517 cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg)
519 st := (*slicetype)(unsafe.Pointer(t))
522 if !cgoIsGoPointer(p) {
526 panic(errorString(msg))
528 if st.elem.kind&kindNoPointers != 0 {
531 for i := 0; i < s.cap; i++ {
532 cgoCheckArg(st.elem, p, true, false, msg)
533 p = add(p, st.elem.size)
536 ss := (*stringStruct)(p)
537 if !cgoIsGoPointer(ss.str) {
541 panic(errorString(msg))
544 st := (*structtype)(unsafe.Pointer(t))
546 if len(st.fields) != 1 {
547 throw("can't happen")
549 cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg)
552 for _, f := range st.fields {
553 cgoCheckArg(f.typ, add(p, f.offset()), true, top, msg)
555 case kindPtr, kindUnsafePointer:
557 p = *(*unsafe.Pointer)(p)
560 if !cgoIsGoPointer(p) {
564 panic(errorString(msg))
567 cgoCheckUnknownPointer(p, msg)
571 // cgoCheckUnknownPointer is called for an arbitrary pointer into Go
572 // memory. It checks whether that Go memory contains any other
573 // pointer into Go memory. If it does, we panic.
574 // The return values are unused but useful to see in panic tracebacks.
575 func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) {
576 if inheap(uintptr(p)) {
577 b, span, _ := findObject(uintptr(p), 0, 0)
582 hbits := heapBitsForAddr(base)
584 for i = uintptr(0); i < n; i += sys.PtrSize {
585 if i != 1*sys.PtrSize && !hbits.morePointers() {
586 // No more possible pointers.
589 if hbits.isPointer() && cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(base + i))) {
590 panic(errorString(msg))
598 for _, datap := range activeModules() {
599 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
600 // We have no way to know the size of the object.
601 // We have to assume that it might contain a pointer.
602 panic(errorString(msg))
604 // In the text or noptr sections, we know that the
605 // pointer does not point to a Go pointer.
611 // cgoIsGoPointer returns whether the pointer is a Go pointer--a
612 // pointer to Go memory. We only care about Go memory that might
615 //go:nowritebarrierrec
616 func cgoIsGoPointer(p unsafe.Pointer) bool {
621 if inHeapOrStack(uintptr(p)) {
625 for _, datap := range activeModules() {
626 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
634 // cgoInRange returns whether p is between start and end.
636 //go:nowritebarrierrec
637 func cgoInRange(p unsafe.Pointer, start, end uintptr) bool {
638 return start <= uintptr(p) && uintptr(p) < end
641 // cgoCheckResult is called to check the result parameter of an
642 // exported Go function. It panics if the result is or contains a Go
644 func cgoCheckResult(val interface{}) {
645 if debug.cgocheck == 0 {
649 ep := (*eface)(unsafe.Pointer(&val))
651 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail)