github.com/4ad/go@v0.0.0-20161219182952-69a12818b605/src/runtime/cgocall.go (about) 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) locks g to m, calls entersyscall 12 // so as not to block other goroutines or the garbage collector, 13 // and then calls 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 != "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 /* 108 * Lock g to m to ensure we stay on the same stack if we do a 109 * cgo callback. Add entry to defer stack in case of panic. 110 */ 111 lockOSThread() 112 mp := getg().m 113 mp.ncgocall++ 114 mp.ncgo++ 115 defer endcgo(mp) 116 117 // Reset traceback. 118 mp.cgoCallers[0] = 0 119 120 /* 121 * Announce we are entering a system call 122 * so that the scheduler knows to create another 123 * M to run goroutines while we are in the 124 * foreign code. 125 * 126 * The call to asmcgocall is guaranteed not to 127 * split the stack and does not allocate memory, 128 * so it is safe to call while "in a system call", outside 129 * the $GOMAXPROCS accounting. 130 */ 131 entersyscall(0) 132 errno := asmcgocall(fn, arg) 133 exitsyscall(0) 134 135 return errno 136 } 137 138 //go:nosplit 139 func endcgo(mp *m) { 140 mp.ncgo-- 141 142 if raceenabled { 143 raceacquire(unsafe.Pointer(&racecgosync)) 144 } 145 146 unlockOSThread() // invalidates mp 147 } 148 149 // Call from C back to Go. 150 //go:nosplit 151 func cgocallbackg(ctxt uintptr) { 152 gp := getg() 153 if gp != gp.m.curg { 154 println("runtime: bad g in cgocallback") 155 exit(2) 156 } 157 158 // Save current syscall parameters, so m.syscall can be 159 // used again if callback decide to make syscall. 160 syscall := gp.m.syscall 161 162 // entersyscall saves the caller's SP to allow the GC to trace the Go 163 // stack. However, since we're returning to an earlier stack frame and 164 // need to pair with the entersyscall() call made by cgocall, we must 165 // save syscall* and let reentersyscall restore them. 166 savedsp := unsafe.Pointer(gp.syscallsp) 167 savedpc := gp.syscallpc 168 exitsyscall(0) // coming out of cgo call 169 170 cgocallbackg1(ctxt) 171 172 // going back to cgo call 173 reentersyscall(savedpc, uintptr(savedsp)) 174 175 gp.m.syscall = syscall 176 } 177 178 func cgocallbackg1(ctxt uintptr) { 179 gp := getg() 180 if gp.m.needextram || atomic.Load(&extraMWaiters) > 0 { 181 gp.m.needextram = false 182 systemstack(newextram) 183 } 184 185 if ctxt != 0 { 186 s := append(gp.cgoCtxt, ctxt) 187 188 // Now we need to set gp.cgoCtxt = s, but we could get 189 // a SIGPROF signal while manipulating the slice, and 190 // the SIGPROF handler could pick up gp.cgoCtxt while 191 // tracing up the stack. We need to ensure that the 192 // handler always sees a valid slice, so set the 193 // values in an order such that it always does. 194 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt)) 195 atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0])) 196 p.cap = cap(s) 197 p.len = len(s) 198 199 defer func(gp *g) { 200 // Decrease the length of the slice by one, safely. 201 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt)) 202 p.len-- 203 }(gp) 204 } 205 206 if gp.m.ncgo == 0 { 207 // The C call to Go came from a thread not currently running 208 // any Go. In the case of -buildmode=c-archive or c-shared, 209 // this call may be coming in before package initialization 210 // is complete. Wait until it is. 211 <-main_init_done 212 } 213 214 // Add entry to defer stack in case of panic. 215 restore := true 216 defer unwindm(&restore) 217 218 if raceenabled { 219 raceacquire(unsafe.Pointer(&racecgosync)) 220 } 221 222 type args struct { 223 fn *funcval 224 arg unsafe.Pointer 225 argsize uintptr 226 } 227 var cb *args 228 229 // Location of callback arguments depends on stack frame layout 230 // and size of stack frame of cgocallback_gofunc. 231 sp := gp.m.g0.sched.sp 232 switch GOARCH { 233 default: 234 throw("cgocallbackg is unimplemented on arch") 235 case "arm": 236 // On arm, stack frame is two words and there's a saved LR between 237 // SP and the stack frame and between the stack frame and the arguments. 238 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize)) 239 case "arm64": 240 // On arm64, stack frame is four words and there's a saved LR between 241 // SP and the stack frame and between the stack frame and the arguments. 242 cb = (*args)(unsafe.Pointer(sp + 5*sys.PtrSize)) 243 case "amd64": 244 // On amd64, stack frame is two words, plus caller PC. 245 if framepointer_enabled { 246 // In this case, there's also saved BP. 247 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize)) 248 break 249 } 250 cb = (*args)(unsafe.Pointer(sp + 3*sys.PtrSize)) 251 case "386": 252 // On 386, stack frame is three words, plus caller PC. 253 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize)) 254 case "ppc64", "ppc64le", "s390x", "sparc64": 255 // On ppc64, s390x, and sparc64, the callback arguments are in 256 // the arguments area of cgocallback's stack frame. The stack 257 // looks like this: 258 // +--------------------+------------------------------+ 259 // | | ... | 260 // | cgoexp_$fn +------------------------------+ 261 // | | fixed frame area | 262 // +--------------------+------------------------------+ 263 // | | arguments area | 264 // | cgocallback +------------------------------+ <- sp + 2*minFrameSize + 2*ptrSize 265 // | | fixed frame area | 266 // +--------------------+------------------------------+ <- sp + minFrameSize + 2*ptrSize 267 // | | local variables (2 pointers) | 268 // | cgocallback_gofunc +------------------------------+ <- sp + minFrameSize 269 // | | fixed frame area | 270 // +--------------------+------------------------------+ <- sp 271 cb = (*args)(unsafe.Pointer(sp + 2*sys.MinFrameSize + 2*sys.PtrSize)) 272 case "mips64", "mips64le": 273 // On mips64x, stack frame is two words and there's a saved LR between 274 // SP and the stack frame and between the stack frame and the arguments. 275 cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize)) 276 } 277 278 // Invoke callback. 279 // NOTE(rsc): passing nil for argtype means that the copying of the 280 // results back into cb.arg happens without any corresponding write barriers. 281 // For cgo, cb.arg points into a C stack frame and therefore doesn't 282 // hold any pointers that the GC can find anyway - the write barrier 283 // would be a no-op. 284 reflectcall(nil, unsafe.Pointer(cb.fn), cb.arg, uint32(cb.argsize), 0) 285 286 if raceenabled { 287 racereleasemerge(unsafe.Pointer(&racecgosync)) 288 } 289 if msanenabled { 290 // Tell msan that we wrote to the entire argument block. 291 // This tells msan that we set the results. 292 // Since we have already called the function it doesn't 293 // matter that we are writing to the non-result parameters. 294 msanwrite(cb.arg, cb.argsize) 295 } 296 297 // Do not unwind m->g0->sched.sp. 298 // Our caller, cgocallback, will do that. 299 restore = false 300 } 301 302 func unwindm(restore *bool) { 303 if !*restore { 304 return 305 } 306 // Restore sp saved by cgocallback during 307 // unwind of g's stack (see comment at top of file). 308 mp := acquirem() 309 sched := &mp.g0.sched 310 switch GOARCH { 311 default: 312 throw("unwindm not implemented") 313 case "386", "amd64", "arm", "ppc64", "ppc64le", "mips64", "mips64le", "s390x", "sparc64": 314 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + sys.MinFrameSize)) 315 case "arm64": 316 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + 16)) 317 } 318 releasem(mp) 319 } 320 321 // called from assembly 322 func badcgocallback() { 323 throw("misaligned stack in cgocallback") 324 } 325 326 // called from (incomplete) assembly 327 func cgounimpl() { 328 throw("cgo not implemented") 329 } 330 331 var racecgosync uint64 // represents possible synchronization in C code 332 333 // Pointer checking for cgo code. 334 335 // We want to detect all cases where a program that does not use 336 // unsafe makes a cgo call passing a Go pointer to memory that 337 // contains a Go pointer. Here a Go pointer is defined as a pointer 338 // to memory allocated by the Go runtime. Programs that use unsafe 339 // can evade this restriction easily, so we don't try to catch them. 340 // The cgo program will rewrite all possibly bad pointer arguments to 341 // call cgoCheckPointer, where we can catch cases of a Go pointer 342 // pointing to a Go pointer. 343 344 // Complicating matters, taking the address of a slice or array 345 // element permits the C program to access all elements of the slice 346 // or array. In that case we will see a pointer to a single element, 347 // but we need to check the entire data structure. 348 349 // The cgoCheckPointer call takes additional arguments indicating that 350 // it was called on an address expression. An additional argument of 351 // true means that it only needs to check a single element. An 352 // additional argument of a slice or array means that it needs to 353 // check the entire slice/array, but nothing else. Otherwise, the 354 // pointer could be anything, and we check the entire heap object, 355 // which is conservative but safe. 356 357 // When and if we implement a moving garbage collector, 358 // cgoCheckPointer will pin the pointer for the duration of the cgo 359 // call. (This is necessary but not sufficient; the cgo program will 360 // also have to change to pin Go pointers that cannot point to Go 361 // pointers.) 362 363 // cgoCheckPointer checks if the argument contains a Go pointer that 364 // points to a Go pointer, and panics if it does. It returns the pointer. 365 func cgoCheckPointer(ptr interface{}, args ...interface{}) interface{} { 366 if debug.cgocheck == 0 { 367 return ptr 368 } 369 370 ep := (*eface)(unsafe.Pointer(&ptr)) 371 t := ep._type 372 373 top := true 374 if len(args) > 0 && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) { 375 p := ep.data 376 if t.kind&kindDirectIface == 0 { 377 p = *(*unsafe.Pointer)(p) 378 } 379 if !cgoIsGoPointer(p) { 380 return ptr 381 } 382 aep := (*eface)(unsafe.Pointer(&args[0])) 383 switch aep._type.kind & kindMask { 384 case kindBool: 385 if t.kind&kindMask == kindUnsafePointer { 386 // We don't know the type of the element. 387 break 388 } 389 pt := (*ptrtype)(unsafe.Pointer(t)) 390 cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail) 391 return ptr 392 case kindSlice: 393 // Check the slice rather than the pointer. 394 ep = aep 395 t = ep._type 396 case kindArray: 397 // Check the array rather than the pointer. 398 // Pass top as false since we have a pointer 399 // to the array. 400 ep = aep 401 t = ep._type 402 top = false 403 default: 404 throw("can't happen") 405 } 406 } 407 408 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail) 409 return ptr 410 } 411 412 const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer" 413 const cgoResultFail = "cgo result has Go pointer" 414 415 // cgoCheckArg is the real work of cgoCheckPointer. The argument p 416 // is either a pointer to the value (of type t), or the value itself, 417 // depending on indir. The top parameter is whether we are at the top 418 // level, where Go pointers are allowed. 419 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) { 420 if t.kind&kindNoPointers != 0 { 421 // If the type has no pointers there is nothing to do. 422 return 423 } 424 425 switch t.kind & kindMask { 426 default: 427 throw("can't happen") 428 case kindArray: 429 at := (*arraytype)(unsafe.Pointer(t)) 430 if !indir { 431 if at.len != 1 { 432 throw("can't happen") 433 } 434 cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg) 435 return 436 } 437 for i := uintptr(0); i < at.len; i++ { 438 cgoCheckArg(at.elem, p, true, top, msg) 439 p = add(p, at.elem.size) 440 } 441 case kindChan, kindMap: 442 // These types contain internal pointers that will 443 // always be allocated in the Go heap. It's never OK 444 // to pass them to C. 445 panic(errorString(msg)) 446 case kindFunc: 447 if indir { 448 p = *(*unsafe.Pointer)(p) 449 } 450 if !cgoIsGoPointer(p) { 451 return 452 } 453 panic(errorString(msg)) 454 case kindInterface: 455 it := *(**_type)(p) 456 if it == nil { 457 return 458 } 459 // A type known at compile time is OK since it's 460 // constant. A type not known at compile time will be 461 // in the heap and will not be OK. 462 if inheap(uintptr(unsafe.Pointer(it))) { 463 panic(errorString(msg)) 464 } 465 p = *(*unsafe.Pointer)(add(p, sys.PtrSize)) 466 if !cgoIsGoPointer(p) { 467 return 468 } 469 if !top { 470 panic(errorString(msg)) 471 } 472 cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg) 473 case kindSlice: 474 st := (*slicetype)(unsafe.Pointer(t)) 475 s := (*slice)(p) 476 p = s.array 477 if !cgoIsGoPointer(p) { 478 return 479 } 480 if !top { 481 panic(errorString(msg)) 482 } 483 if st.elem.kind&kindNoPointers != 0 { 484 return 485 } 486 for i := 0; i < s.cap; i++ { 487 cgoCheckArg(st.elem, p, true, false, msg) 488 p = add(p, st.elem.size) 489 } 490 case kindString: 491 ss := (*stringStruct)(p) 492 if !cgoIsGoPointer(ss.str) { 493 return 494 } 495 if !top { 496 panic(errorString(msg)) 497 } 498 case kindStruct: 499 st := (*structtype)(unsafe.Pointer(t)) 500 if !indir { 501 if len(st.fields) != 1 { 502 throw("can't happen") 503 } 504 cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg) 505 return 506 } 507 for _, f := range st.fields { 508 cgoCheckArg(f.typ, add(p, f.offset), true, top, msg) 509 } 510 case kindPtr, kindUnsafePointer: 511 if indir { 512 p = *(*unsafe.Pointer)(p) 513 } 514 515 if !cgoIsGoPointer(p) { 516 return 517 } 518 if !top { 519 panic(errorString(msg)) 520 } 521 522 cgoCheckUnknownPointer(p, msg) 523 } 524 } 525 526 // cgoCheckUnknownPointer is called for an arbitrary pointer into Go 527 // memory. It checks whether that Go memory contains any other 528 // pointer into Go memory. If it does, we panic. 529 // The return values are unused but useful to see in panic tracebacks. 530 func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) { 531 if cgoInRange(p, mheap_.arena_start, mheap_.arena_used) { 532 if !inheap(uintptr(p)) { 533 // On 32-bit systems it is possible for C's allocated memory 534 // to have addresses between arena_start and arena_used. 535 // Either this pointer is a stack or an unused span or it's 536 // a C allocation. Escape analysis should prevent the first, 537 // garbage collection should prevent the second, 538 // and the third is completely OK. 539 return 540 } 541 542 b, hbits, span, _ := heapBitsForObject(uintptr(p), 0, 0) 543 base = b 544 if base == 0 { 545 return 546 } 547 n := span.elemsize 548 for i = uintptr(0); i < n; i += sys.PtrSize { 549 if i != 1*sys.PtrSize && !hbits.morePointers() { 550 // No more possible pointers. 551 break 552 } 553 if hbits.isPointer() { 554 if cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(base + i))) { 555 panic(errorString(msg)) 556 } 557 } 558 hbits = hbits.next() 559 } 560 561 return 562 } 563 564 for datap := &firstmoduledata; datap != nil; datap = datap.next { 565 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) { 566 // We have no way to know the size of the object. 567 // We have to assume that it might contain a pointer. 568 panic(errorString(msg)) 569 } 570 // In the text or noptr sections, we know that the 571 // pointer does not point to a Go pointer. 572 } 573 574 return 575 } 576 577 // cgoIsGoPointer returns whether the pointer is a Go pointer--a 578 // pointer to Go memory. We only care about Go memory that might 579 // contain pointers. 580 //go:nosplit 581 //go:nowritebarrierrec 582 func cgoIsGoPointer(p unsafe.Pointer) bool { 583 if p == nil { 584 return false 585 } 586 587 if inHeapOrStack(uintptr(p)) { 588 return true 589 } 590 591 for datap := &firstmoduledata; datap != nil; datap = datap.next { 592 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) { 593 return true 594 } 595 } 596 597 return false 598 } 599 600 // cgoInRange returns whether p is between start and end. 601 //go:nosplit 602 //go:nowritebarrierrec 603 func cgoInRange(p unsafe.Pointer, start, end uintptr) bool { 604 return start <= uintptr(p) && uintptr(p) < end 605 } 606 607 // cgoCheckResult is called to check the result parameter of an 608 // exported Go function. It panics if the result is or contains a Go 609 // pointer. 610 func cgoCheckResult(val interface{}) { 611 if debug.cgocheck == 0 { 612 return 613 } 614 615 ep := (*eface)(unsafe.Pointer(&val)) 616 t := ep._type 617 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail) 618 }