Source file src/runtime/cgocall.go
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. 4 5 // Cgo call and callback support. 6 // 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. 10 // 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). 14 // 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). 18 // 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. 22 // 23 // After it regains control, runtime.asmcgocall switches back to the 24 // original g (m->curg)'s stack and returns to runtime.cgocall. 25 // 26 // After it regains control, runtime.cgocall calls exitsyscall, which blocks 27 // until this m can run Go code without violating the $GOMAXPROCS limit, 28 // and then unlocks g from m. 29 // 30 // The above description skipped over the possibility of the gcc-compiled 31 // function f calling back into Go. If that happens, we continue down 32 // the rabbit hole during the execution of f. 33 // 34 // To make it possible for gcc-compiled C code to call a Go function p.GoF, 35 // cgo writes a gcc-compiled function named GoF (not p.GoF, since gcc doesn't 36 // know about packages). The gcc-compiled C function f calls GoF. 37 // 38 // GoF initializes "frame", a structure containing all of its 39 // arguments and slots for p.GoF's results. It calls 40 // crosscall2(_cgoexp_GoF, frame, framesize, ctxt) using the gcc ABI. 41 // 42 // crosscall2 (in cgo/asm_$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 $GOMAXPROCS limit. crosscall2 calls 46 // runtime.cgocallback(_cgoexp_GoF, frame, ctxt) using the gc ABI. 47 // (crosscall2's framesize argument is no longer used, but there's one 48 // case where SWIG calls crosscall2 directly and expects to pass this 49 // argument. See _cgo_panic.) 50 // 51 // runtime.cgocallback (in asm_$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. 59 // 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). 69 // 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. 73 // 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. 77 // 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. 81 // 82 // crosscall2 restores the callee-save registers for gcc and returns 83 // to GoF, which unpacks any result values and returns to f. 84 85 package runtime 86 87 import ( 88 "internal/goarch" 89 "internal/goexperiment" 90 "runtime/internal/sys" 91 "unsafe" 92 ) 93 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 97 98 // argset matches runtime/cgo/linux_syscall.c:argset_t 99 type argset struct { 100 args unsafe.Pointer 101 retval uintptr 102 } 103 104 // wrapper for syscall package to call cgocall for libc (cgo) calls. 105 // 106 //go:linkname syscall_cgocaller syscall.cgocaller 107 //go:nosplit 108 //go:uintptrescapes 109 func syscall_cgocaller(fn unsafe.Pointer, args ...uintptr) uintptr { 110 as := argset{args: unsafe.Pointer(&args[0])} 111 cgocall(fn, unsafe.Pointer(&as)) 112 return as.retval 113 } 114 115 var ncgocall uint64 // number of cgo calls in total for dead m 116 117 // Call from Go to C. 118 // 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. 122 // 123 //go:nosplit 124 func cgocall(fn, arg unsafe.Pointer) int32 { 125 if !iscgo && GOOS != "solaris" && GOOS != "illumos" && GOOS != "windows" { 126 throw("cgocall unavailable") 127 } 128 129 if fn == nil { 130 throw("cgocall nil") 131 } 132 133 if raceenabled { 134 racereleasemerge(unsafe.Pointer(&racecgosync)) 135 } 136 137 mp := getg().m 138 mp.ncgocall++ 139 140 // Reset traceback. 141 mp.cgoCallers[0] = 0 142 143 // Announce we are entering a system call 144 // so that the scheduler knows to create another 145 // M to run goroutines while we are in the 146 // foreign code. 147 // 148 // The call to asmcgocall is guaranteed not to 149 // grow the stack and does not allocate memory, 150 // so it is safe to call while "in a system call", outside 151 // the $GOMAXPROCS accounting. 152 // 153 // fn may call back into Go code, in which case we'll exit the 154 // "system call", run the Go code (which may grow the stack), 155 // and then re-enter the "system call" reusing the PC and SP 156 // saved by entersyscall here. 157 entersyscall() 158 159 // Tell asynchronous preemption that we're entering external 160 // code. We do this after entersyscall because this may block 161 // and cause an async preemption to fail, but at this point a 162 // sync preemption will succeed (though this is not a matter 163 // of correctness). 164 osPreemptExtEnter(mp) 165 166 mp.incgo = true 167 // We use ncgo as a check during execution tracing for whether there is 168 // any C on the call stack, which there will be after this point. If 169 // there isn't, we can use frame pointer unwinding to collect call 170 // stacks efficiently. This will be the case for the first Go-to-C call 171 // on a stack, so it's prefereable to update it here, after we emit a 172 // trace event in entersyscall above. 173 mp.ncgo++ 174 175 errno := asmcgocall(fn, arg) 176 177 // Update accounting before exitsyscall because exitsyscall may 178 // reschedule us on to a different M. 179 mp.incgo = false 180 mp.ncgo-- 181 182 osPreemptExtExit(mp) 183 184 exitsyscall() 185 186 // Note that raceacquire must be called only after exitsyscall has 187 // wired this M to a P. 188 if raceenabled { 189 raceacquire(unsafe.Pointer(&racecgosync)) 190 } 191 192 // From the garbage collector's perspective, time can move 193 // backwards in the sequence above. If there's a callback into 194 // Go code, GC will see this function at the call to 195 // asmcgocall. When the Go call later returns to C, the 196 // syscall PC/SP is rolled back and the GC sees this function 197 // back at the call to entersyscall. Normally, fn and arg 198 // would be live at entersyscall and dead at asmcgocall, so if 199 // time moved backwards, GC would see these arguments as dead 200 // and then live. Prevent these undead arguments from crashing 201 // GC by forcing them to stay live across this time warp. 202 KeepAlive(fn) 203 KeepAlive(arg) 204 KeepAlive(mp) 205 206 return errno 207 } 208 209 // Call from C back to Go. fn must point to an ABIInternal Go entry-point. 210 // 211 //go:nosplit 212 func cgocallbackg(fn, frame unsafe.Pointer, ctxt uintptr) { 213 gp := getg() 214 if gp != gp.m.curg { 215 println("runtime: bad g in cgocallback") 216 exit(2) 217 } 218 219 // The call from C is on gp.m's g0 stack, so we must ensure 220 // that we stay on that M. We have to do this before calling 221 // exitsyscall, since it would otherwise be free to move us to 222 // a different M. The call to unlockOSThread is in unwindm. 223 lockOSThread() 224 225 checkm := gp.m 226 227 // Save current syscall parameters, so m.syscall can be 228 // used again if callback decide to make syscall. 229 syscall := gp.m.syscall 230 231 // entersyscall saves the caller's SP to allow the GC to trace the Go 232 // stack. However, since we're returning to an earlier stack frame and 233 // need to pair with the entersyscall() call made by cgocall, we must 234 // save syscall* and let reentersyscall restore them. 235 savedsp := unsafe.Pointer(gp.syscallsp) 236 savedpc := gp.syscallpc 237 exitsyscall() // coming out of cgo call 238 gp.m.incgo = false 239 if gp.m.isextra { 240 gp.m.isExtraInC = false 241 } 242 243 osPreemptExtExit(gp.m) 244 245 cgocallbackg1(fn, frame, ctxt) // will call unlockOSThread 246 247 // At this point unlockOSThread has been called. 248 // The following code must not change to a different m. 249 // This is enforced by checking incgo in the schedule function. 250 251 gp.m.incgo = true 252 if gp.m.isextra { 253 gp.m.isExtraInC = true 254 } 255 256 if gp.m != checkm { 257 throw("m changed unexpectedly in cgocallbackg") 258 } 259 260 osPreemptExtEnter(gp.m) 261 262 // going back to cgo call 263 reentersyscall(savedpc, uintptr(savedsp)) 264 265 gp.m.syscall = syscall 266 } 267 268 func cgocallbackg1(fn, frame unsafe.Pointer, ctxt uintptr) { 269 gp := getg() 270 271 // When we return, undo the call to lockOSThread in cgocallbackg. 272 // We must still stay on the same m. 273 defer unlockOSThread() 274 275 if gp.m.needextram || extraMWaiters.Load() > 0 { 276 gp.m.needextram = false 277 systemstack(newextram) 278 } 279 280 if ctxt != 0 { 281 s := append(gp.cgoCtxt, ctxt) 282 283 // Now we need to set gp.cgoCtxt = s, but we could get 284 // a SIGPROF signal while manipulating the slice, and 285 // the SIGPROF handler could pick up gp.cgoCtxt while 286 // tracing up the stack. We need to ensure that the 287 // handler always sees a valid slice, so set the 288 // values in an order such that it always does. 289 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt)) 290 atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0])) 291 p.cap = cap(s) 292 p.len = len(s) 293 294 defer func(gp *g) { 295 // Decrease the length of the slice by one, safely. 296 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt)) 297 p.len-- 298 }(gp) 299 } 300 301 if gp.m.ncgo == 0 { 302 // The C call to Go came from a thread not currently running 303 // any Go. In the case of -buildmode=c-archive or c-shared, 304 // this call may be coming in before package initialization 305 // is complete. Wait until it is. 306 <-main_init_done 307 } 308 309 // Check whether the profiler needs to be turned on or off; this route to 310 // run Go code does not use runtime.execute, so bypasses the check there. 311 hz := sched.profilehz 312 if gp.m.profilehz != hz { 313 setThreadCPUProfiler(hz) 314 } 315 316 // Add entry to defer stack in case of panic. 317 restore := true 318 defer unwindm(&restore) 319 320 if raceenabled { 321 raceacquire(unsafe.Pointer(&racecgosync)) 322 } 323 324 // Invoke callback. This function is generated by cmd/cgo and 325 // will unpack the argument frame and call the Go function. 326 var cb func(frame unsafe.Pointer) 327 cbFV := funcval{uintptr(fn)} 328 *(*unsafe.Pointer)(unsafe.Pointer(&cb)) = noescape(unsafe.Pointer(&cbFV)) 329 cb(frame) 330 331 if raceenabled { 332 racereleasemerge(unsafe.Pointer(&racecgosync)) 333 } 334 335 // Do not unwind m->g0->sched.sp. 336 // Our caller, cgocallback, will do that. 337 restore = false 338 } 339 340 func unwindm(restore *bool) { 341 if *restore { 342 // Restore sp saved by cgocallback during 343 // unwind of g's stack (see comment at top of file). 344 mp := acquirem() 345 sched := &mp.g0.sched 346 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + alignUp(sys.MinFrameSize, sys.StackAlign))) 347 348 // Do the accounting that cgocall will not have a chance to do 349 // during an unwind. 350 // 351 // In the case where a Go call originates from C, ncgo is 0 352 // and there is no matching cgocall to end. 353 if mp.ncgo > 0 { 354 mp.incgo = false 355 mp.ncgo-- 356 osPreemptExtExit(mp) 357 } 358 359 releasem(mp) 360 } 361 } 362 363 // called from assembly. 364 func badcgocallback() { 365 throw("misaligned stack in cgocallback") 366 } 367 368 // called from (incomplete) assembly. 369 func cgounimpl() { 370 throw("cgo not implemented") 371 } 372 373 var racecgosync uint64 // represents possible synchronization in C code 374 375 // Pointer checking for cgo code. 376 377 // We want to detect all cases where a program that does not use 378 // unsafe makes a cgo call passing a Go pointer to memory that 379 // contains an unpinned Go pointer. Here a Go pointer is defined as a 380 // pointer to memory allocated by the Go runtime. Programs that use 381 // unsafe can evade this restriction easily, so we don't try to catch 382 // them. The cgo program will rewrite all possibly bad pointer 383 // arguments to call cgoCheckPointer, where we can catch cases of a Go 384 // pointer pointing to an unpinned Go pointer. 385 386 // Complicating matters, taking the address of a slice or array 387 // element permits the C program to access all elements of the slice 388 // or array. In that case we will see a pointer to a single element, 389 // but we need to check the entire data structure. 390 391 // The cgoCheckPointer call takes additional arguments indicating that 392 // it was called on an address expression. An additional argument of 393 // true means that it only needs to check a single element. An 394 // additional argument of a slice or array means that it needs to 395 // check the entire slice/array, but nothing else. Otherwise, the 396 // pointer could be anything, and we check the entire heap object, 397 // which is conservative but safe. 398 399 // When and if we implement a moving garbage collector, 400 // cgoCheckPointer will pin the pointer for the duration of the cgo 401 // call. (This is necessary but not sufficient; the cgo program will 402 // also have to change to pin Go pointers that cannot point to Go 403 // pointers.) 404 405 // cgoCheckPointer checks if the argument contains a Go pointer that 406 // points to an unpinned Go pointer, and panics if it does. 407 func cgoCheckPointer(ptr any, arg any) { 408 if !goexperiment.CgoCheck2 && debug.cgocheck == 0 { 409 return 410 } 411 412 ep := efaceOf(&ptr) 413 t := ep._type 414 415 top := true 416 if arg != nil && (t.Kind_&kindMask == kindPtr || t.Kind_&kindMask == kindUnsafePointer) { 417 p := ep.data 418 if t.Kind_&kindDirectIface == 0 { 419 p = *(*unsafe.Pointer)(p) 420 } 421 if p == nil || !cgoIsGoPointer(p) { 422 return 423 } 424 aep := efaceOf(&arg) 425 switch aep._type.Kind_ & kindMask { 426 case kindBool: 427 if t.Kind_&kindMask == kindUnsafePointer { 428 // We don't know the type of the element. 429 break 430 } 431 pt := (*ptrtype)(unsafe.Pointer(t)) 432 cgoCheckArg(pt.Elem, p, true, false, cgoCheckPointerFail) 433 return 434 case kindSlice: 435 // Check the slice rather than the pointer. 436 ep = aep 437 t = ep._type 438 case kindArray: 439 // Check the array rather than the pointer. 440 // Pass top as false since we have a pointer 441 // to the array. 442 ep = aep 443 t = ep._type 444 top = false 445 default: 446 throw("can't happen") 447 } 448 } 449 450 cgoCheckArg(t, ep.data, t.Kind_&kindDirectIface == 0, top, cgoCheckPointerFail) 451 } 452 453 const cgoCheckPointerFail = "cgo argument has Go pointer to unpinned Go pointer" 454 const cgoResultFail = "cgo result has Go pointer" 455 456 // cgoCheckArg is the real work of cgoCheckPointer. The argument p 457 // is either a pointer to the value (of type t), or the value itself, 458 // depending on indir. The top parameter is whether we are at the top 459 // level, where Go pointers are allowed. Go pointers to pinned objects are 460 // always allowed. 461 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) { 462 if t.PtrBytes == 0 || p == nil { 463 // If the type has no pointers there is nothing to do. 464 return 465 } 466 467 switch t.Kind_ & kindMask { 468 default: 469 throw("can't happen") 470 case kindArray: 471 at := (*arraytype)(unsafe.Pointer(t)) 472 if !indir { 473 if at.Len != 1 { 474 throw("can't happen") 475 } 476 cgoCheckArg(at.Elem, p, at.Elem.Kind_&kindDirectIface == 0, top, msg) 477 return 478 } 479 for i := uintptr(0); i < at.Len; i++ { 480 cgoCheckArg(at.Elem, p, true, top, msg) 481 p = add(p, at.Elem.Size_) 482 } 483 case kindChan, kindMap: 484 // These types contain internal pointers that will 485 // always be allocated in the Go heap. It's never OK 486 // to pass them to C. 487 panic(errorString(msg)) 488 case kindFunc: 489 if indir { 490 p = *(*unsafe.Pointer)(p) 491 } 492 if !cgoIsGoPointer(p) { 493 return 494 } 495 panic(errorString(msg)) 496 case kindInterface: 497 it := *(**_type)(p) 498 if it == nil { 499 return 500 } 501 // A type known at compile time is OK since it's 502 // constant. A type not known at compile time will be 503 // in the heap and will not be OK. 504 if inheap(uintptr(unsafe.Pointer(it))) { 505 panic(errorString(msg)) 506 } 507 p = *(*unsafe.Pointer)(add(p, goarch.PtrSize)) 508 if !cgoIsGoPointer(p) { 509 return 510 } 511 if !top && !isPinned(p) { 512 panic(errorString(msg)) 513 } 514 cgoCheckArg(it, p, it.Kind_&kindDirectIface == 0, false, msg) 515 case kindSlice: 516 st := (*slicetype)(unsafe.Pointer(t)) 517 s := (*slice)(p) 518 p = s.array 519 if p == nil || !cgoIsGoPointer(p) { 520 return 521 } 522 if !top && !isPinned(p) { 523 panic(errorString(msg)) 524 } 525 if st.Elem.PtrBytes == 0 { 526 return 527 } 528 for i := 0; i < s.cap; i++ { 529 cgoCheckArg(st.Elem, p, true, false, msg) 530 p = add(p, st.Elem.Size_) 531 } 532 case kindString: 533 ss := (*stringStruct)(p) 534 if !cgoIsGoPointer(ss.str) { 535 return 536 } 537 if !top && !isPinned(ss.str) { 538 panic(errorString(msg)) 539 } 540 case kindStruct: 541 st := (*structtype)(unsafe.Pointer(t)) 542 if !indir { 543 if len(st.Fields) != 1 { 544 throw("can't happen") 545 } 546 cgoCheckArg(st.Fields[0].Typ, p, st.Fields[0].Typ.Kind_&kindDirectIface == 0, top, msg) 547 return 548 } 549 for _, f := range st.Fields { 550 if f.Typ.PtrBytes == 0 { 551 continue 552 } 553 cgoCheckArg(f.Typ, add(p, f.Offset), true, top, msg) 554 } 555 case kindPtr, kindUnsafePointer: 556 if indir { 557 p = *(*unsafe.Pointer)(p) 558 if p == nil { 559 return 560 } 561 } 562 563 if !cgoIsGoPointer(p) { 564 return 565 } 566 if !top && !isPinned(p) { 567 panic(errorString(msg)) 568 } 569 570 cgoCheckUnknownPointer(p, msg) 571 } 572 } 573 574 // cgoCheckUnknownPointer is called for an arbitrary pointer into Go 575 // memory. It checks whether that Go memory contains any other 576 // pointer into unpinned Go memory. If it does, we panic. 577 // The return values are unused but useful to see in panic tracebacks. 578 func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) { 579 if inheap(uintptr(p)) { 580 b, span, _ := findObject(uintptr(p), 0, 0) 581 base = b 582 if base == 0 { 583 return 584 } 585 n := span.elemsize 586 hbits := heapBitsForAddr(base, n) 587 for { 588 var addr uintptr 589 if hbits, addr = hbits.next(); addr == 0 { 590 break 591 } 592 pp := *(*unsafe.Pointer)(unsafe.Pointer(addr)) 593 if cgoIsGoPointer(pp) && !isPinned(pp) { 594 panic(errorString(msg)) 595 } 596 } 597 598 return 599 } 600 601 for _, datap := range activeModules() { 602 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) { 603 // We have no way to know the size of the object. 604 // We have to assume that it might contain a pointer. 605 panic(errorString(msg)) 606 } 607 // In the text or noptr sections, we know that the 608 // pointer does not point to a Go pointer. 609 } 610 611 return 612 } 613 614 // cgoIsGoPointer reports whether the pointer is a Go pointer--a 615 // pointer to Go memory. We only care about Go memory that might 616 // contain pointers. 617 // 618 //go:nosplit 619 //go:nowritebarrierrec 620 func cgoIsGoPointer(p unsafe.Pointer) bool { 621 if p == nil { 622 return false 623 } 624 625 if inHeapOrStack(uintptr(p)) { 626 return true 627 } 628 629 for _, datap := range activeModules() { 630 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) { 631 return true 632 } 633 } 634 635 return false 636 } 637 638 // cgoInRange reports whether p is between start and end. 639 // 640 //go:nosplit 641 //go:nowritebarrierrec 642 func cgoInRange(p unsafe.Pointer, start, end uintptr) bool { 643 return start <= uintptr(p) && uintptr(p) < end 644 } 645 646 // cgoCheckResult is called to check the result parameter of an 647 // exported Go function. It panics if the result is or contains a Go 648 // pointer. 649 func cgoCheckResult(val any) { 650 if !goexperiment.CgoCheck2 && debug.cgocheck == 0 { 651 return 652 } 653 654 ep := efaceOf(&val) 655 t := ep._type 656 cgoCheckArg(t, ep.data, t.Kind_&kindDirectIface == 0, false, cgoResultFail) 657 } 658