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 $GOMAXPROCS limit, 28 // and then unlocks g from m. 29 // 30 // The above description skipped over the possibility of the gcc-compiled 31 // function f calling back into Go. If that happens, we continue down 32 // the rabbit hole during the execution of f. 33 // 34 // To make it possible for gcc-compiled C code to call a Go function p.GoF, 35 // cgo writes a gcc-compiled function named GoF (not p.GoF, since gcc doesn't 36 // know about packages). The gcc-compiled C function f calls GoF. 37 // 38 // GoF calls crosscall2(_cgoexp_GoF, frame, framesize). Crosscall2 39 // (in cgo/gcc_$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 $GOMAXPROCS limit. Thus, this code cannot yet 44 // call arbitrary Go code directly and must be careful not to allocate 45 // memory or use up m->g0's stack. 46 // 47 // _cgoexp_GoF calls runtime.cgocallback(p.GoF, frame, framesize, ctxt). 48 // (The reason for having _cgoexp_GoF instead of writing a crosscall3 49 // to make this call directly is that _cgoexp_GoF, because it is compiled 50 // with 6c instead of gcc, can refer to dotted names like 51 // runtime.cgocallback and p.GoF.) 52 // 53 // runtime.cgocallback (in asm_$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. 61 // 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. 72 // 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. 76 // 77 // _cgoexp_GoF immediately returns to crosscall2, which restores the 78 // callee-save registers for gcc and returns to GoF, which returns to f. 79 80 package runtime 81 82 import ( 83 "runtime/internal/atomic" 84 "runtime/internal/sys" 85 "unsafe" 86 ) 87 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 91 92 // Call from Go to C. 93 //go:nosplit 94 func cgocall(fn, arg unsafe.Pointer) int32 { 95 if !iscgo && GOOS != "solaris" && GOOS != "illumos" && GOOS != "windows" { 96 throw("cgocall unavailable") 97 } 98 99 if fn == nil { 100 throw("cgocall nil") 101 } 102 103 if raceenabled { 104 racereleasemerge(unsafe.Pointer(&racecgosync)) 105 } 106 107 mp := getg().m 108 mp.ncgocall++ 109 mp.ncgo++ 110 111 // Reset traceback. 112 mp.cgoCallers[0] = 0 113 114 // 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 117 // foreign code. 118 // 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. 123 // 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. 128 entersyscall() 129 130 mp.incgo = true 131 errno := asmcgocall(fn, arg) 132 133 // Update accounting before exitsyscall because exitsyscall may 134 // reschedule us on to a different M. 135 mp.incgo = false 136 mp.ncgo-- 137 138 exitsyscall() 139 140 // Note that raceacquire must be called only after exitsyscall has 141 // wired this M to a P. 142 if raceenabled { 143 raceacquire(unsafe.Pointer(&racecgosync)) 144 } 145 146 // From the garbage collector's perspective, time can move 147 // backwards in the sequence above. If there's a callback into 148 // Go code, GC will see this function at the call to 149 // asmcgocall. When the Go call later returns to C, the 150 // syscall PC/SP is rolled back and the GC sees this function 151 // back at the call to entersyscall. Normally, fn and arg 152 // would be live at entersyscall and dead at asmcgocall, so if 153 // time moved backwards, GC would see these arguments as dead 154 // and then live. Prevent these undead arguments from crashing 155 // GC by forcing them to stay live across this time warp. 156 KeepAlive(fn) 157 KeepAlive(arg) 158 KeepAlive(mp) 159 160 return errno 161 } 162 163 // Call from C back to Go. 164 //go:nosplit 165 func cgocallbackg(ctxt uintptr) { 166 gp := getg() 167 if gp != gp.m.curg { 168 println("runtime: bad g in cgocallback") 169 exit(2) 170 } 171 172 // The call from C is on gp.m's g0 stack, so we must ensure 173 // that we stay on that M. We have to do this before calling 174 // exitsyscall, since it would otherwise be free to move us to 175 // a different M. The call to unlockOSThread is in unwindm. 176 lockOSThread() 177 178 // Save current syscall parameters, so m.syscall can be 179 // used again if callback decide to make syscall. 180 syscall := gp.m.syscall 181 182 // entersyscall saves the caller's SP to allow the GC to trace the Go 183 // stack. However, since we're returning to an earlier stack frame and 184 // need to pair with the entersyscall() call made by cgocall, we must 185 // save syscall* and let reentersyscall restore them. 186 savedsp := unsafe.Pointer(gp.syscallsp) 187 savedpc := gp.syscallpc 188 exitsyscall() // coming out of cgo call 189 gp.m.incgo = false 190 191 cgocallbackg1(ctxt) 192 193 // At this point unlockOSThread has been called. 194 // The following code must not change to a different m. 195 // This is enforced by checking incgo in the schedule function. 196 197 gp.m.incgo = true 198 // going back to cgo call 199 reentersyscall(savedpc, uintptr(savedsp)) 200 201 gp.m.syscall = syscall 202 } 203 204 func cgocallbackg1(ctxt uintptr) { 205 gp := getg() 206 if gp.m.needextram || atomic.Load(&extraMWaiters) > 0 { 207 gp.m.needextram = false 208 systemstack(newextram) 209 } 210 211 if ctxt != 0 { 212 s := append(gp.cgoCtxt, ctxt) 213 214 // Now we need to set gp.cgoCtxt = s, but we could get 215 // a SIGPROF signal while manipulating the slice, and 216 // the SIGPROF handler could pick up gp.cgoCtxt while 217 // tracing up the stack. We need to ensure that the 218 // handler always sees a valid slice, so set the 219 // values in an order such that it always does. 220 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt)) 221 atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0])) 222 p.cap = cap(s) 223 p.len = len(s) 224 225 defer func(gp *g) { 226 // Decrease the length of the slice by one, safely. 227 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt)) 228 p.len-- 229 }(gp) 230 } 231 232 if gp.m.ncgo == 0 { 233 // The C call to Go came from a thread not currently running 234 // any Go. In the case of -buildmode=c-archive or c-shared, 235 // this call may be coming in before package initialization 236 // is complete. Wait until it is. 237 <-main_init_done 238 } 239 240 // Add entry to defer stack in case of panic. 241 restore := true 242 defer unwindm(&restore) 243 244 if raceenabled { 245 raceacquire(unsafe.Pointer(&racecgosync)) 246 } 247 248 type args struct { 249 fn *funcval 250 arg unsafe.Pointer 251 argsize uintptr 252 } 253 var cb *args 254 255 // Location of callback arguments depends on stack frame layout 256 // and size of stack frame of cgocallback_gofunc. 257 sp := gp.m.g0.sched.sp 258 switch GOARCH { 259 default: 260 throw("cgocallbackg is unimplemented on arch") 261 case "arm": 262 // On arm, stack frame is two words and there's a saved LR between 263 // SP and the stack frame and between the stack frame and the arguments. 264 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize)) 265 case "arm64": 266 // On arm64, stack frame is four words and there's a saved LR between 267 // SP and the stack frame and between the stack frame and the arguments. 268 // Additional two words (16-byte alignment) are for saving FP. 269 cb = (*args)(unsafe.Pointer(sp + 7*sys.PtrSize)) 270 case "amd64": 271 // On amd64, stack frame is two words, plus caller PC. 272 if framepointer_enabled { 273 // In this case, there's also saved BP. 274 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize)) 275 break 276 } 277 cb = (*args)(unsafe.Pointer(sp + 3*sys.PtrSize)) 278 case "386": 279 // On 386, stack frame is three words, plus caller PC. 280 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize)) 281 case "ppc64", "ppc64le", "s390x": 282 // On ppc64 and s390x, the callback arguments are in the arguments area of 283 // cgocallback's stack frame. The stack looks like this: 284 // +--------------------+------------------------------+ 285 // | | ... | 286 // | cgoexp_$fn +------------------------------+ 287 // | | fixed frame area | 288 // +--------------------+------------------------------+ 289 // | | arguments area | 290 // | cgocallback +------------------------------+ <- sp + 2*minFrameSize + 2*ptrSize 291 // | | fixed frame area | 292 // +--------------------+------------------------------+ <- sp + minFrameSize + 2*ptrSize 293 // | | local variables (2 pointers) | 294 // | cgocallback_gofunc +------------------------------+ <- sp + minFrameSize 295 // | | fixed frame area | 296 // +--------------------+------------------------------+ <- sp 297 cb = (*args)(unsafe.Pointer(sp + 2*sys.MinFrameSize + 2*sys.PtrSize)) 298 case "mips64", "mips64le": 299 // On mips64x, stack frame is two words and there's a saved LR between 300 // SP and the stack frame and between the stack frame and the arguments. 301 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize)) 302 case "mips", "mipsle": 303 // On mipsx, stack frame is two words and there's a saved LR between 304 // SP and the stack frame and between the stack frame and the arguments. 305 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize)) 306 } 307 308 // Invoke callback. 309 // NOTE(rsc): passing nil for argtype means that the copying of the 310 // results back into cb.arg happens without any corresponding write barriers. 311 // For cgo, cb.arg points into a C stack frame and therefore doesn't 312 // hold any pointers that the GC can find anyway - the write barrier 313 // would be a no-op. 314 reflectcall(nil, unsafe.Pointer(cb.fn), cb.arg, uint32(cb.argsize), 0) 315 316 if raceenabled { 317 racereleasemerge(unsafe.Pointer(&racecgosync)) 318 } 319 if msanenabled { 320 // Tell msan that we wrote to the entire argument block. 321 // This tells msan that we set the results. 322 // Since we have already called the function it doesn't 323 // matter that we are writing to the non-result parameters. 324 msanwrite(cb.arg, cb.argsize) 325 } 326 327 // Do not unwind m->g0->sched.sp. 328 // Our caller, cgocallback, will do that. 329 restore = false 330 } 331 332 func unwindm(restore *bool) { 333 if *restore { 334 // Restore sp saved by cgocallback during 335 // unwind of g's stack (see comment at top of file). 336 mp := acquirem() 337 sched := &mp.g0.sched 338 switch GOARCH { 339 default: 340 throw("unwindm not implemented") 341 case "386", "amd64", "arm", "ppc64", "ppc64le", "mips64", "mips64le", "s390x", "mips", "mipsle": 342 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + sys.MinFrameSize)) 343 case "arm64": 344 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + 16)) 345 } 346 347 // Do the accounting that cgocall will not have a chance to do 348 // during an unwind. 349 // 350 // In the case where a Go call originates from C, ncgo is 0 351 // and there is no matching cgocall to end. 352 if mp.ncgo > 0 { 353 mp.incgo = false 354 mp.ncgo-- 355 } 356 357 releasem(mp) 358 } 359 360 // Undo the call to lockOSThread in cgocallbackg. 361 // We must still stay on the same m. 362 unlockOSThread() 363 } 364 365 // called from assembly 366 func badcgocallback() { 367 throw("misaligned stack in cgocallback") 368 } 369 370 // called from (incomplete) assembly 371 func cgounimpl() { 372 throw("cgo not implemented") 373 } 374 375 var racecgosync uint64 // represents possible synchronization in C code 376 377 // Pointer checking for cgo code. 378 379 // We want to detect all cases where a program that does not use 380 // unsafe makes a cgo call passing a Go pointer to memory that 381 // contains a Go pointer. Here a Go pointer is defined as a pointer 382 // to memory allocated by the Go runtime. Programs that use unsafe 383 // can evade this restriction easily, so we don't try to catch them. 384 // The cgo program will rewrite all possibly bad pointer arguments to 385 // call cgoCheckPointer, where we can catch cases of a Go pointer 386 // pointing to a Go pointer. 387 388 // Complicating matters, taking the address of a slice or array 389 // element permits the C program to access all elements of the slice 390 // or array. In that case we will see a pointer to a single element, 391 // but we need to check the entire data structure. 392 393 // The cgoCheckPointer call takes additional arguments indicating that 394 // it was called on an address expression. An additional argument of 395 // true means that it only needs to check a single element. An 396 // additional argument of a slice or array means that it needs to 397 // check the entire slice/array, but nothing else. Otherwise, the 398 // pointer could be anything, and we check the entire heap object, 399 // which is conservative but safe. 400 401 // When and if we implement a moving garbage collector, 402 // cgoCheckPointer will pin the pointer for the duration of the cgo 403 // call. (This is necessary but not sufficient; the cgo program will 404 // also have to change to pin Go pointers that cannot point to Go 405 // pointers.) 406 407 // cgoCheckPointer checks if the argument contains a Go pointer that 408 // points to a Go pointer, and panics if it does. 409 func cgoCheckPointer(ptr interface{}, args ...interface{}) { 410 if debug.cgocheck == 0 { 411 return 412 } 413 414 ep := (*eface)(unsafe.Pointer(&ptr)) 415 t := ep._type 416 417 top := true 418 if len(args) > 0 && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) { 419 p := ep.data 420 if t.kind&kindDirectIface == 0 { 421 p = *(*unsafe.Pointer)(p) 422 } 423 if !cgoIsGoPointer(p) { 424 return 425 } 426 aep := (*eface)(unsafe.Pointer(&args[0])) 427 switch aep._type.kind & kindMask { 428 case kindBool: 429 if t.kind&kindMask == kindUnsafePointer { 430 // We don't know the type of the element. 431 break 432 } 433 pt := (*ptrtype)(unsafe.Pointer(t)) 434 cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail) 435 return 436 case kindSlice: 437 // Check the slice rather than the pointer. 438 ep = aep 439 t = ep._type 440 case kindArray: 441 // Check the array rather than the pointer. 442 // Pass top as false since we have a pointer 443 // to the array. 444 ep = aep 445 t = ep._type 446 top = false 447 default: 448 throw("can't happen") 449 } 450 } 451 452 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail) 453 } 454 455 const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer" 456 const cgoResultFail = "cgo result has Go pointer" 457 458 // cgoCheckArg is the real work of cgoCheckPointer. The argument p 459 // is either a pointer to the value (of type t), or the value itself, 460 // depending on indir. The top parameter is whether we are at the top 461 // level, where Go pointers are allowed. 462 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) { 463 if t.ptrdata == 0 { 464 // If the type has no pointers there is nothing to do. 465 return 466 } 467 468 switch t.kind & kindMask { 469 default: 470 throw("can't happen") 471 case kindArray: 472 at := (*arraytype)(unsafe.Pointer(t)) 473 if !indir { 474 if at.len != 1 { 475 throw("can't happen") 476 } 477 cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg) 478 return 479 } 480 for i := uintptr(0); i < at.len; i++ { 481 cgoCheckArg(at.elem, p, true, top, msg) 482 p = add(p, at.elem.size) 483 } 484 case kindChan, kindMap: 485 // These types contain internal pointers that will 486 // always be allocated in the Go heap. It's never OK 487 // to pass them to C. 488 panic(errorString(msg)) 489 case kindFunc: 490 if indir { 491 p = *(*unsafe.Pointer)(p) 492 } 493 if !cgoIsGoPointer(p) { 494 return 495 } 496 panic(errorString(msg)) 497 case kindInterface: 498 it := *(**_type)(p) 499 if it == nil { 500 return 501 } 502 // A type known at compile time is OK since it's 503 // constant. A type not known at compile time will be 504 // in the heap and will not be OK. 505 if inheap(uintptr(unsafe.Pointer(it))) { 506 panic(errorString(msg)) 507 } 508 p = *(*unsafe.Pointer)(add(p, sys.PtrSize)) 509 if !cgoIsGoPointer(p) { 510 return 511 } 512 if !top { 513 panic(errorString(msg)) 514 } 515 cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg) 516 case kindSlice: 517 st := (*slicetype)(unsafe.Pointer(t)) 518 s := (*slice)(p) 519 p = s.array 520 if !cgoIsGoPointer(p) { 521 return 522 } 523 if !top { 524 panic(errorString(msg)) 525 } 526 if st.elem.ptrdata == 0 { 527 return 528 } 529 for i := 0; i < s.cap; i++ { 530 cgoCheckArg(st.elem, p, true, false, msg) 531 p = add(p, st.elem.size) 532 } 533 case kindString: 534 ss := (*stringStruct)(p) 535 if !cgoIsGoPointer(ss.str) { 536 return 537 } 538 if !top { 539 panic(errorString(msg)) 540 } 541 case kindStruct: 542 st := (*structtype)(unsafe.Pointer(t)) 543 if !indir { 544 if len(st.fields) != 1 { 545 throw("can't happen") 546 } 547 cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg) 548 return 549 } 550 for _, f := range st.fields { 551 cgoCheckArg(f.typ, add(p, f.offset()), true, top, msg) 552 } 553 case kindPtr, kindUnsafePointer: 554 if indir { 555 p = *(*unsafe.Pointer)(p) 556 } 557 558 if !cgoIsGoPointer(p) { 559 return 560 } 561 if !top { 562 panic(errorString(msg)) 563 } 564 565 cgoCheckUnknownPointer(p, msg) 566 } 567 } 568 569 // cgoCheckUnknownPointer is called for an arbitrary pointer into Go 570 // memory. It checks whether that Go memory contains any other 571 // pointer into Go memory. If it does, we panic. 572 // The return values are unused but useful to see in panic tracebacks. 573 func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) { 574 if inheap(uintptr(p)) { 575 b, span, _ := findObject(uintptr(p), 0, 0) 576 base = b 577 if base == 0 { 578 return 579 } 580 hbits := heapBitsForAddr(base) 581 n := span.elemsize 582 for i = uintptr(0); i < n; i += sys.PtrSize { 583 if i != 1*sys.PtrSize && !hbits.morePointers() { 584 // No more possible pointers. 585 break 586 } 587 if hbits.isPointer() && cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(base + i))) { 588 panic(errorString(msg)) 589 } 590 hbits = hbits.next() 591 } 592 593 return 594 } 595 596 for _, datap := range activeModules() { 597 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) { 598 // We have no way to know the size of the object. 599 // We have to assume that it might contain a pointer. 600 panic(errorString(msg)) 601 } 602 // In the text or noptr sections, we know that the 603 // pointer does not point to a Go pointer. 604 } 605 606 return 607 } 608 609 // cgoIsGoPointer reports whether the pointer is a Go pointer--a 610 // pointer to Go memory. We only care about Go memory that might 611 // contain pointers. 612 //go:nosplit 613 //go:nowritebarrierrec 614 func cgoIsGoPointer(p unsafe.Pointer) bool { 615 if p == nil { 616 return false 617 } 618 619 if inHeapOrStack(uintptr(p)) { 620 return true 621 } 622 623 for _, datap := range activeModules() { 624 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) { 625 return true 626 } 627 } 628 629 return false 630 } 631 632 // cgoInRange reports whether p is between start and end. 633 //go:nosplit 634 //go:nowritebarrierrec 635 func cgoInRange(p unsafe.Pointer, start, end uintptr) bool { 636 return start <= uintptr(p) && uintptr(p) < end 637 } 638 639 // cgoCheckResult is called to check the result parameter of an 640 // exported Go function. It panics if the result is or contains a Go 641 // pointer. 642 func cgoCheckResult(val interface{}) { 643 if debug.cgocheck == 0 { 644 return 645 } 646 647 ep := (*eface)(unsafe.Pointer(&val)) 648 t := ep._type 649 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail) 650 } 651