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 "runtime/internal/sys" 90 "unsafe" 91 ) 92 93 // Addresses collected in a cgo backtrace when crashing. 94 // Length must match arg.Max in x_cgo_callers in runtime/cgo/gcc_traceback.c. 95 type cgoCallers [32]uintptr 96 97 // argset matches runtime/cgo/linux_syscall.c:argset_t 98 type argset struct { 99 args unsafe.Pointer 100 retval uintptr 101 } 102 103 // wrapper for syscall package to call cgocall for libc (cgo) calls. 104 // 105 //go:linkname syscall_cgocaller syscall.cgocaller 106 //go:nosplit 107 //go:uintptrescapes 108 func syscall_cgocaller(fn unsafe.Pointer, args ...uintptr) uintptr { 109 as := argset{args: unsafe.Pointer(&args[0])} 110 cgocall(fn, unsafe.Pointer(&as)) 111 return as.retval 112 } 113 114 var ncgocall uint64 // number of cgo calls in total for dead m 115 116 // Call from Go to C. 117 // 118 // This must be nosplit because it's used for syscalls on some 119 // platforms. Syscalls may have untyped arguments on the stack, so 120 // it's not safe to grow or scan the stack. 121 // 122 //go:nosplit 123 func cgocall(fn, arg unsafe.Pointer) int32 { 124 if !iscgo && GOOS != "solaris" && GOOS != "illumos" && GOOS != "windows" { 125 throw("cgocall unavailable") 126 } 127 128 if fn == nil { 129 throw("cgocall nil") 130 } 131 132 if raceenabled { 133 racereleasemerge(unsafe.Pointer(&racecgosync)) 134 } 135 136 mp := getg().m 137 mp.ncgocall++ 138 mp.ncgo++ 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 errno := asmcgocall(fn, arg) 168 169 // Update accounting before exitsyscall because exitsyscall may 170 // reschedule us on to a different M. 171 mp.incgo = false 172 mp.ncgo-- 173 174 osPreemptExtExit(mp) 175 176 exitsyscall() 177 178 // Note that raceacquire must be called only after exitsyscall has 179 // wired this M to a P. 180 if raceenabled { 181 raceacquire(unsafe.Pointer(&racecgosync)) 182 } 183 184 // From the garbage collector's perspective, time can move 185 // backwards in the sequence above. If there's a callback into 186 // Go code, GC will see this function at the call to 187 // asmcgocall. When the Go call later returns to C, the 188 // syscall PC/SP is rolled back and the GC sees this function 189 // back at the call to entersyscall. Normally, fn and arg 190 // would be live at entersyscall and dead at asmcgocall, so if 191 // time moved backwards, GC would see these arguments as dead 192 // and then live. Prevent these undead arguments from crashing 193 // GC by forcing them to stay live across this time warp. 194 KeepAlive(fn) 195 KeepAlive(arg) 196 KeepAlive(mp) 197 198 return errno 199 } 200 201 // Call from C back to Go. fn must point to an ABIInternal Go entry-point. 202 // 203 //go:nosplit 204 func cgocallbackg(fn, frame unsafe.Pointer, ctxt uintptr) { 205 gp := getg() 206 if gp != gp.m.curg { 207 println("runtime: bad g in cgocallback") 208 exit(2) 209 } 210 211 // The call from C is on gp.m's g0 stack, so we must ensure 212 // that we stay on that M. We have to do this before calling 213 // exitsyscall, since it would otherwise be free to move us to 214 // a different M. The call to unlockOSThread is in unwindm. 215 lockOSThread() 216 217 checkm := gp.m 218 219 // Save current syscall parameters, so m.syscall can be 220 // used again if callback decide to make syscall. 221 syscall := gp.m.syscall 222 223 // entersyscall saves the caller's SP to allow the GC to trace the Go 224 // stack. However, since we're returning to an earlier stack frame and 225 // need to pair with the entersyscall() call made by cgocall, we must 226 // save syscall* and let reentersyscall restore them. 227 savedsp := unsafe.Pointer(gp.syscallsp) 228 savedpc := gp.syscallpc 229 exitsyscall() // coming out of cgo call 230 gp.m.incgo = false 231 232 osPreemptExtExit(gp.m) 233 234 cgocallbackg1(fn, frame, ctxt) // will call unlockOSThread 235 236 // At this point unlockOSThread has been called. 237 // The following code must not change to a different m. 238 // This is enforced by checking incgo in the schedule function. 239 240 gp.m.incgo = true 241 242 if gp.m != checkm { 243 throw("m changed unexpectedly in cgocallbackg") 244 } 245 246 osPreemptExtEnter(gp.m) 247 248 // going back to cgo call 249 reentersyscall(savedpc, uintptr(savedsp)) 250 251 gp.m.syscall = syscall 252 } 253 254 func cgocallbackg1(fn, frame unsafe.Pointer, ctxt uintptr) { 255 gp := getg() 256 257 // When we return, undo the call to lockOSThread in cgocallbackg. 258 // We must still stay on the same m. 259 defer unlockOSThread() 260 261 if gp.m.needextram || extraMWaiters.Load() > 0 { 262 gp.m.needextram = false 263 systemstack(newextram) 264 } 265 266 if ctxt != 0 { 267 s := append(gp.cgoCtxt, ctxt) 268 269 // Now we need to set gp.cgoCtxt = s, but we could get 270 // a SIGPROF signal while manipulating the slice, and 271 // the SIGPROF handler could pick up gp.cgoCtxt while 272 // tracing up the stack. We need to ensure that the 273 // handler always sees a valid slice, so set the 274 // values in an order such that it always does. 275 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt)) 276 atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0])) 277 p.cap = cap(s) 278 p.len = len(s) 279 280 defer func(gp *g) { 281 // Decrease the length of the slice by one, safely. 282 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt)) 283 p.len-- 284 }(gp) 285 } 286 287 if gp.m.ncgo == 0 { 288 // The C call to Go came from a thread not currently running 289 // any Go. In the case of -buildmode=c-archive or c-shared, 290 // this call may be coming in before package initialization 291 // is complete. Wait until it is. 292 <-main_init_done 293 } 294 295 // Check whether the profiler needs to be turned on or off; this route to 296 // run Go code does not use runtime.execute, so bypasses the check there. 297 hz := sched.profilehz 298 if gp.m.profilehz != hz { 299 setThreadCPUProfiler(hz) 300 } 301 302 // Add entry to defer stack in case of panic. 303 restore := true 304 defer unwindm(&restore) 305 306 if raceenabled { 307 raceacquire(unsafe.Pointer(&racecgosync)) 308 } 309 310 // Invoke callback. This function is generated by cmd/cgo and 311 // will unpack the argument frame and call the Go function. 312 var cb func(frame unsafe.Pointer) 313 cbFV := funcval{uintptr(fn)} 314 *(*unsafe.Pointer)(unsafe.Pointer(&cb)) = noescape(unsafe.Pointer(&cbFV)) 315 cb(frame) 316 317 if raceenabled { 318 racereleasemerge(unsafe.Pointer(&racecgosync)) 319 } 320 321 // Do not unwind m->g0->sched.sp. 322 // Our caller, cgocallback, will do that. 323 restore = false 324 } 325 326 func unwindm(restore *bool) { 327 if *restore { 328 // Restore sp saved by cgocallback during 329 // unwind of g's stack (see comment at top of file). 330 mp := acquirem() 331 sched := &mp.g0.sched 332 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + alignUp(sys.MinFrameSize, sys.StackAlign))) 333 334 // Do the accounting that cgocall will not have a chance to do 335 // during an unwind. 336 // 337 // In the case where a Go call originates from C, ncgo is 0 338 // and there is no matching cgocall to end. 339 if mp.ncgo > 0 { 340 mp.incgo = false 341 mp.ncgo-- 342 osPreemptExtExit(mp) 343 } 344 345 releasem(mp) 346 } 347 } 348 349 // called from assembly. 350 func badcgocallback() { 351 throw("misaligned stack in cgocallback") 352 } 353 354 // called from (incomplete) assembly. 355 func cgounimpl() { 356 throw("cgo not implemented") 357 } 358 359 var racecgosync uint64 // represents possible synchronization in C code 360 361 // Pointer checking for cgo code. 362 363 // We want to detect all cases where a program that does not use 364 // unsafe makes a cgo call passing a Go pointer to memory that 365 // contains a Go pointer. Here a Go pointer is defined as a pointer 366 // to memory allocated by the Go runtime. Programs that use unsafe 367 // can evade this restriction easily, so we don't try to catch them. 368 // The cgo program will rewrite all possibly bad pointer arguments to 369 // call cgoCheckPointer, where we can catch cases of a Go pointer 370 // pointing to a Go pointer. 371 372 // Complicating matters, taking the address of a slice or array 373 // element permits the C program to access all elements of the slice 374 // or array. In that case we will see a pointer to a single element, 375 // but we need to check the entire data structure. 376 377 // The cgoCheckPointer call takes additional arguments indicating that 378 // it was called on an address expression. An additional argument of 379 // true means that it only needs to check a single element. An 380 // additional argument of a slice or array means that it needs to 381 // check the entire slice/array, but nothing else. Otherwise, the 382 // pointer could be anything, and we check the entire heap object, 383 // which is conservative but safe. 384 385 // When and if we implement a moving garbage collector, 386 // cgoCheckPointer will pin the pointer for the duration of the cgo 387 // call. (This is necessary but not sufficient; the cgo program will 388 // also have to change to pin Go pointers that cannot point to Go 389 // pointers.) 390 391 // cgoCheckPointer checks if the argument contains a Go pointer that 392 // points to a Go pointer, and panics if it does. 393 func cgoCheckPointer(ptr any, arg any) { 394 if debug.cgocheck == 0 { 395 return 396 } 397 398 ep := efaceOf(&ptr) 399 t := ep._type 400 401 top := true 402 if arg != nil && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) { 403 p := ep.data 404 if t.kind&kindDirectIface == 0 { 405 p = *(*unsafe.Pointer)(p) 406 } 407 if p == nil || !cgoIsGoPointer(p) { 408 return 409 } 410 aep := efaceOf(&arg) 411 switch aep._type.kind & kindMask { 412 case kindBool: 413 if t.kind&kindMask == kindUnsafePointer { 414 // We don't know the type of the element. 415 break 416 } 417 pt := (*ptrtype)(unsafe.Pointer(t)) 418 cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail) 419 return 420 case kindSlice: 421 // Check the slice rather than the pointer. 422 ep = aep 423 t = ep._type 424 case kindArray: 425 // Check the array rather than the pointer. 426 // Pass top as false since we have a pointer 427 // to the array. 428 ep = aep 429 t = ep._type 430 top = false 431 default: 432 throw("can't happen") 433 } 434 } 435 436 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail) 437 } 438 439 const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer" 440 const cgoResultFail = "cgo result has Go pointer" 441 442 // cgoCheckArg is the real work of cgoCheckPointer. The argument p 443 // is either a pointer to the value (of type t), or the value itself, 444 // depending on indir. The top parameter is whether we are at the top 445 // level, where Go pointers are allowed. 446 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) { 447 if t.ptrdata == 0 || p == nil { 448 // If the type has no pointers there is nothing to do. 449 return 450 } 451 452 switch t.kind & kindMask { 453 default: 454 throw("can't happen") 455 case kindArray: 456 at := (*arraytype)(unsafe.Pointer(t)) 457 if !indir { 458 if at.len != 1 { 459 throw("can't happen") 460 } 461 cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg) 462 return 463 } 464 for i := uintptr(0); i < at.len; i++ { 465 cgoCheckArg(at.elem, p, true, top, msg) 466 p = add(p, at.elem.size) 467 } 468 case kindChan, kindMap: 469 // These types contain internal pointers that will 470 // always be allocated in the Go heap. It's never OK 471 // to pass them to C. 472 panic(errorString(msg)) 473 case kindFunc: 474 if indir { 475 p = *(*unsafe.Pointer)(p) 476 } 477 if !cgoIsGoPointer(p) { 478 return 479 } 480 panic(errorString(msg)) 481 case kindInterface: 482 it := *(**_type)(p) 483 if it == nil { 484 return 485 } 486 // A type known at compile time is OK since it's 487 // constant. A type not known at compile time will be 488 // in the heap and will not be OK. 489 if inheap(uintptr(unsafe.Pointer(it))) { 490 panic(errorString(msg)) 491 } 492 p = *(*unsafe.Pointer)(add(p, goarch.PtrSize)) 493 if !cgoIsGoPointer(p) { 494 return 495 } 496 if !top { 497 panic(errorString(msg)) 498 } 499 cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg) 500 case kindSlice: 501 st := (*slicetype)(unsafe.Pointer(t)) 502 s := (*slice)(p) 503 p = s.array 504 if p == nil || !cgoIsGoPointer(p) { 505 return 506 } 507 if !top { 508 panic(errorString(msg)) 509 } 510 if st.elem.ptrdata == 0 { 511 return 512 } 513 for i := 0; i < s.cap; i++ { 514 cgoCheckArg(st.elem, p, true, false, msg) 515 p = add(p, st.elem.size) 516 } 517 case kindString: 518 ss := (*stringStruct)(p) 519 if !cgoIsGoPointer(ss.str) { 520 return 521 } 522 if !top { 523 panic(errorString(msg)) 524 } 525 case kindStruct: 526 st := (*structtype)(unsafe.Pointer(t)) 527 if !indir { 528 if len(st.fields) != 1 { 529 throw("can't happen") 530 } 531 cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg) 532 return 533 } 534 for _, f := range st.fields { 535 if f.typ.ptrdata == 0 { 536 continue 537 } 538 cgoCheckArg(f.typ, add(p, f.offset), true, top, msg) 539 } 540 case kindPtr, kindUnsafePointer: 541 if indir { 542 p = *(*unsafe.Pointer)(p) 543 if p == nil { 544 return 545 } 546 } 547 548 if !cgoIsGoPointer(p) { 549 return 550 } 551 if !top { 552 panic(errorString(msg)) 553 } 554 555 cgoCheckUnknownPointer(p, msg) 556 } 557 } 558 559 // cgoCheckUnknownPointer is called for an arbitrary pointer into Go 560 // memory. It checks whether that Go memory contains any other 561 // pointer into Go memory. If it does, we panic. 562 // The return values are unused but useful to see in panic tracebacks. 563 func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) { 564 if inheap(uintptr(p)) { 565 b, span, _ := findObject(uintptr(p), 0, 0) 566 base = b 567 if base == 0 { 568 return 569 } 570 n := span.elemsize 571 hbits := heapBitsForAddr(base, n) 572 for { 573 var addr uintptr 574 if hbits, addr = hbits.next(); addr == 0 { 575 break 576 } 577 if cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(addr))) { 578 panic(errorString(msg)) 579 } 580 } 581 582 return 583 } 584 585 for _, datap := range activeModules() { 586 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) { 587 // We have no way to know the size of the object. 588 // We have to assume that it might contain a pointer. 589 panic(errorString(msg)) 590 } 591 // In the text or noptr sections, we know that the 592 // pointer does not point to a Go pointer. 593 } 594 595 return 596 } 597 598 // cgoIsGoPointer reports whether the pointer is a Go pointer--a 599 // pointer to Go memory. We only care about Go memory that might 600 // contain pointers. 601 // 602 //go:nosplit 603 //go:nowritebarrierrec 604 func cgoIsGoPointer(p unsafe.Pointer) bool { 605 if p == nil { 606 return false 607 } 608 609 if inHeapOrStack(uintptr(p)) { 610 return true 611 } 612 613 for _, datap := range activeModules() { 614 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) { 615 return true 616 } 617 } 618 619 return false 620 } 621 622 // cgoInRange reports whether p is between start and end. 623 // 624 //go:nosplit 625 //go:nowritebarrierrec 626 func cgoInRange(p unsafe.Pointer, start, end uintptr) bool { 627 return start <= uintptr(p) && uintptr(p) < end 628 } 629 630 // cgoCheckResult is called to check the result parameter of an 631 // exported Go function. It panics if the result is or contains a Go 632 // pointer. 633 func cgoCheckResult(val any) { 634 if debug.cgocheck == 0 { 635 return 636 } 637 638 ep := efaceOf(&val) 639 t := ep._type 640 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail) 641 } 642