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.
89 "internal/goexperiment"
90 "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.
106 //go:linkname syscall_cgocaller syscall.cgocaller
109 func syscall_cgocaller(fn unsafe.Pointer, args ...uintptr) uintptr {
110 as := argset{args: unsafe.Pointer(&args[0])}
111 cgocall(fn, unsafe.Pointer(&as))
115 var ncgocall uint64 // number of cgo calls in total for dead m
117 // Call from Go to C.
119 // This must be nosplit because it's used for syscalls on some
120 // platforms. Syscalls may have untyped arguments on the stack, so
121 // it's not safe to grow or scan the stack.
124 func cgocall(fn, arg unsafe.Pointer) int32 {
125 if !iscgo && GOOS != "solaris" && GOOS != "illumos" && GOOS != "windows" {
126 throw("cgocall unavailable")
134 racereleasemerge(unsafe.Pointer(&racecgosync))
144 // Announce we are entering a system call
145 // so that the scheduler knows to create another
146 // M to run goroutines while we are in the
149 // The call to asmcgocall is guaranteed not to
150 // grow the stack and does not allocate memory,
151 // so it is safe to call while "in a system call", outside
152 // the $GOMAXPROCS accounting.
154 // fn may call back into Go code, in which case we'll exit the
155 // "system call", run the Go code (which may grow the stack),
156 // and then re-enter the "system call" reusing the PC and SP
157 // saved by entersyscall here.
160 // Tell asynchronous preemption that we're entering external
161 // code. We do this after entersyscall because this may block
162 // and cause an async preemption to fail, but at this point a
163 // sync preemption will succeed (though this is not a matter
165 osPreemptExtEnter(mp)
168 errno := asmcgocall(fn, arg)
170 // Update accounting before exitsyscall because exitsyscall may
171 // reschedule us on to a different M.
179 // Note that raceacquire must be called only after exitsyscall has
180 // wired this M to a P.
182 raceacquire(unsafe.Pointer(&racecgosync))
185 // From the garbage collector's perspective, time can move
186 // backwards in the sequence above. If there's a callback into
187 // Go code, GC will see this function at the call to
188 // asmcgocall. When the Go call later returns to C, the
189 // syscall PC/SP is rolled back and the GC sees this function
190 // back at the call to entersyscall. Normally, fn and arg
191 // would be live at entersyscall and dead at asmcgocall, so if
192 // time moved backwards, GC would see these arguments as dead
193 // and then live. Prevent these undead arguments from crashing
194 // GC by forcing them to stay live across this time warp.
202 // Call from C back to Go. fn must point to an ABIInternal Go entry-point.
205 func cgocallbackg(fn, frame unsafe.Pointer, ctxt uintptr) {
208 println("runtime: bad g in cgocallback")
212 // The call from C is on gp.m's g0 stack, so we must ensure
213 // that we stay on that M. We have to do this before calling
214 // exitsyscall, since it would otherwise be free to move us to
215 // a different M. The call to unlockOSThread is in unwindm.
220 // Save current syscall parameters, so m.syscall can be
221 // used again if callback decide to make syscall.
222 syscall := gp.m.syscall
224 // entersyscall saves the caller's SP to allow the GC to trace the Go
225 // stack. However, since we're returning to an earlier stack frame and
226 // need to pair with the entersyscall() call made by cgocall, we must
227 // save syscall* and let reentersyscall restore them.
228 savedsp := unsafe.Pointer(gp.syscallsp)
229 savedpc := gp.syscallpc
230 exitsyscall() // coming out of cgo call
233 gp.m.isExtraInC = false
236 osPreemptExtExit(gp.m)
238 cgocallbackg1(fn, frame, ctxt) // will call unlockOSThread
240 // At this point unlockOSThread has been called.
241 // The following code must not change to a different m.
242 // This is enforced by checking incgo in the schedule function.
246 gp.m.isExtraInC = true
250 throw("m changed unexpectedly in cgocallbackg")
253 osPreemptExtEnter(gp.m)
255 // going back to cgo call
256 reentersyscall(savedpc, uintptr(savedsp))
258 gp.m.syscall = syscall
261 func cgocallbackg1(fn, frame unsafe.Pointer, ctxt uintptr) {
264 // When we return, undo the call to lockOSThread in cgocallbackg.
265 // We must still stay on the same m.
266 defer unlockOSThread()
268 if gp.m.needextram || extraMWaiters.Load() > 0 {
269 gp.m.needextram = false
270 systemstack(newextram)
274 s := append(gp.cgoCtxt, ctxt)
276 // Now we need to set gp.cgoCtxt = s, but we could get
277 // a SIGPROF signal while manipulating the slice, and
278 // the SIGPROF handler could pick up gp.cgoCtxt while
279 // tracing up the stack. We need to ensure that the
280 // handler always sees a valid slice, so set the
281 // values in an order such that it always does.
282 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
283 atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0]))
288 // Decrease the length of the slice by one, safely.
289 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt))
295 // The C call to Go came from a thread not currently running
296 // any Go. In the case of -buildmode=c-archive or c-shared,
297 // this call may be coming in before package initialization
298 // is complete. Wait until it is.
302 // Check whether the profiler needs to be turned on or off; this route to
303 // run Go code does not use runtime.execute, so bypasses the check there.
304 hz := sched.profilehz
305 if gp.m.profilehz != hz {
306 setThreadCPUProfiler(hz)
309 // Add entry to defer stack in case of panic.
311 defer unwindm(&restore)
314 raceacquire(unsafe.Pointer(&racecgosync))
317 // Invoke callback. This function is generated by cmd/cgo and
318 // will unpack the argument frame and call the Go function.
319 var cb func(frame unsafe.Pointer)
320 cbFV := funcval{uintptr(fn)}
321 *(*unsafe.Pointer)(unsafe.Pointer(&cb)) = noescape(unsafe.Pointer(&cbFV))
325 racereleasemerge(unsafe.Pointer(&racecgosync))
328 // Do not unwind m->g0->sched.sp.
329 // Our caller, cgocallback, will do that.
333 func unwindm(restore *bool) {
335 // Restore sp saved by cgocallback during
336 // unwind of g's stack (see comment at top of file).
338 sched := &mp.g0.sched
339 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + alignUp(sys.MinFrameSize, sys.StackAlign)))
341 // Do the accounting that cgocall will not have a chance to do
344 // In the case where a Go call originates from C, ncgo is 0
345 // and there is no matching cgocall to end.
356 // called from assembly.
357 func badcgocallback() {
358 throw("misaligned stack in cgocallback")
361 // called from (incomplete) assembly.
363 throw("cgo not implemented")
366 var racecgosync uint64 // represents possible synchronization in C code
368 // Pointer checking for cgo code.
370 // We want to detect all cases where a program that does not use
371 // unsafe makes a cgo call passing a Go pointer to memory that
372 // contains a Go pointer. Here a Go pointer is defined as a pointer
373 // to memory allocated by the Go runtime. Programs that use unsafe
374 // can evade this restriction easily, so we don't try to catch them.
375 // The cgo program will rewrite all possibly bad pointer arguments to
376 // call cgoCheckPointer, where we can catch cases of a Go pointer
377 // pointing to a Go pointer.
379 // Complicating matters, taking the address of a slice or array
380 // element permits the C program to access all elements of the slice
381 // or array. In that case we will see a pointer to a single element,
382 // but we need to check the entire data structure.
384 // The cgoCheckPointer call takes additional arguments indicating that
385 // it was called on an address expression. An additional argument of
386 // true means that it only needs to check a single element. An
387 // additional argument of a slice or array means that it needs to
388 // check the entire slice/array, but nothing else. Otherwise, the
389 // pointer could be anything, and we check the entire heap object,
390 // which is conservative but safe.
392 // When and if we implement a moving garbage collector,
393 // cgoCheckPointer will pin the pointer for the duration of the cgo
394 // call. (This is necessary but not sufficient; the cgo program will
395 // also have to change to pin Go pointers that cannot point to Go
398 // cgoCheckPointer checks if the argument contains a Go pointer that
399 // points to a Go pointer, and panics if it does.
400 func cgoCheckPointer(ptr any, arg any) {
401 if !goexperiment.CgoCheck2 && debug.cgocheck == 0 {
409 if arg != nil && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) {
411 if t.kind&kindDirectIface == 0 {
412 p = *(*unsafe.Pointer)(p)
414 if p == nil || !cgoIsGoPointer(p) {
418 switch aep._type.kind & kindMask {
420 if t.kind&kindMask == kindUnsafePointer {
421 // We don't know the type of the element.
424 pt := (*ptrtype)(unsafe.Pointer(t))
425 cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail)
428 // Check the slice rather than the pointer.
432 // Check the array rather than the pointer.
433 // Pass top as false since we have a pointer
439 throw("can't happen")
443 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail)
446 const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer"
447 const cgoResultFail = "cgo result has Go pointer"
449 // cgoCheckArg is the real work of cgoCheckPointer. The argument p
450 // is either a pointer to the value (of type t), or the value itself,
451 // depending on indir. The top parameter is whether we are at the top
452 // level, where Go pointers are allowed.
453 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) {
454 if t.ptrdata == 0 || p == nil {
455 // If the type has no pointers there is nothing to do.
459 switch t.kind & kindMask {
461 throw("can't happen")
463 at := (*arraytype)(unsafe.Pointer(t))
466 throw("can't happen")
468 cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg)
471 for i := uintptr(0); i < at.len; i++ {
472 cgoCheckArg(at.elem, p, true, top, msg)
473 p = add(p, at.elem.size)
475 case kindChan, kindMap:
476 // These types contain internal pointers that will
477 // always be allocated in the Go heap. It's never OK
478 // to pass them to C.
479 panic(errorString(msg))
482 p = *(*unsafe.Pointer)(p)
484 if !cgoIsGoPointer(p) {
487 panic(errorString(msg))
493 // A type known at compile time is OK since it's
494 // constant. A type not known at compile time will be
495 // in the heap and will not be OK.
496 if inheap(uintptr(unsafe.Pointer(it))) {
497 panic(errorString(msg))
499 p = *(*unsafe.Pointer)(add(p, goarch.PtrSize))
500 if !cgoIsGoPointer(p) {
504 panic(errorString(msg))
506 cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg)
508 st := (*slicetype)(unsafe.Pointer(t))
511 if p == nil || !cgoIsGoPointer(p) {
515 panic(errorString(msg))
517 if st.elem.ptrdata == 0 {
520 for i := 0; i < s.cap; i++ {
521 cgoCheckArg(st.elem, p, true, false, msg)
522 p = add(p, st.elem.size)
525 ss := (*stringStruct)(p)
526 if !cgoIsGoPointer(ss.str) {
530 panic(errorString(msg))
533 st := (*structtype)(unsafe.Pointer(t))
535 if len(st.fields) != 1 {
536 throw("can't happen")
538 cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg)
541 for _, f := range st.fields {
542 if f.typ.ptrdata == 0 {
545 cgoCheckArg(f.typ, add(p, f.offset), true, top, msg)
547 case kindPtr, kindUnsafePointer:
549 p = *(*unsafe.Pointer)(p)
555 if !cgoIsGoPointer(p) {
559 panic(errorString(msg))
562 cgoCheckUnknownPointer(p, msg)
566 // cgoCheckUnknownPointer is called for an arbitrary pointer into Go
567 // memory. It checks whether that Go memory contains any other
568 // pointer into Go memory. If it does, we panic.
569 // The return values are unused but useful to see in panic tracebacks.
570 func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) {
571 if inheap(uintptr(p)) {
572 b, span, _ := findObject(uintptr(p), 0, 0)
578 hbits := heapBitsForAddr(base, n)
581 if hbits, addr = hbits.next(); addr == 0 {
584 if cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(addr))) {
585 panic(errorString(msg))
592 for _, datap := range activeModules() {
593 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
594 // We have no way to know the size of the object.
595 // We have to assume that it might contain a pointer.
596 panic(errorString(msg))
598 // In the text or noptr sections, we know that the
599 // pointer does not point to a Go pointer.
605 // cgoIsGoPointer reports whether the pointer is a Go pointer--a
606 // pointer to Go memory. We only care about Go memory that might
610 //go:nowritebarrierrec
611 func cgoIsGoPointer(p unsafe.Pointer) bool {
616 if inHeapOrStack(uintptr(p)) {
620 for _, datap := range activeModules() {
621 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
629 // cgoInRange reports whether p is between start and end.
632 //go:nowritebarrierrec
633 func cgoInRange(p unsafe.Pointer, start, end uintptr) bool {
634 return start <= uintptr(p) && uintptr(p) < end
637 // cgoCheckResult is called to check the result parameter of an
638 // exported Go function. It panics if the result is or contains a Go
640 func cgoCheckResult(val any) {
641 if !goexperiment.CgoCheck2 && debug.cgocheck == 0 {
647 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail)