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/atomic" 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 mp.ncgo++ 140 141 // Reset traceback. 142 mp.cgoCallers[0] = 0 143 144 // 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 147 // foreign code. 148 // 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. 153 // 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. 158 entersyscall() 159 160 // 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 164 // of correctness). 165 osPreemptExtEnter(mp) 166 167 mp.incgo = true 168 errno := asmcgocall(fn, arg) 169 170 // Update accounting before exitsyscall because exitsyscall may 171 // reschedule us on to a different M. 172 mp.incgo = false 173 mp.ncgo-- 174 175 osPreemptExtExit(mp) 176 177 exitsyscall() 178 179 // Note that raceacquire must be called only after exitsyscall has 180 // wired this M to a P. 181 if raceenabled { 182 raceacquire(unsafe.Pointer(&racecgosync)) 183 } 184 185 // 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. 195 KeepAlive(fn) 196 KeepAlive(arg) 197 KeepAlive(mp) 198 199 return errno 200 } 201 202 // Call from C back to Go. fn must point to an ABIInternal Go entry-point. 203 // 204 //go:nosplit 205 func cgocallbackg(fn, frame unsafe.Pointer, ctxt uintptr) { 206 gp := getg() 207 if gp != gp.m.curg { 208 println("runtime: bad g in cgocallback") 209 exit(2) 210 } 211 212 // 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. 216 lockOSThread() 217 218 checkm := gp.m 219 220 // Save current syscall parameters, so m.syscall can be 221 // used again if callback decide to make syscall. 222 syscall := gp.m.syscall 223 224 // 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 231 gp.m.incgo = false 232 233 osPreemptExtExit(gp.m) 234 235 cgocallbackg1(fn, frame, ctxt) // will call unlockOSThread 236 237 // At this point unlockOSThread has been called. 238 // The following code must not change to a different m. 239 // This is enforced by checking incgo in the schedule function. 240 241 gp.m.incgo = true 242 243 if gp.m != checkm { 244 throw("m changed unexpectedly in cgocallbackg") 245 } 246 247 osPreemptExtEnter(gp.m) 248 249 // going back to cgo call 250 reentersyscall(savedpc, uintptr(savedsp)) 251 252 gp.m.syscall = syscall 253 } 254 255 func cgocallbackg1(fn, frame unsafe.Pointer, ctxt uintptr) { 256 gp := getg() 257 258 // When we return, undo the call to lockOSThread in cgocallbackg. 259 // We must still stay on the same m. 260 defer unlockOSThread() 261 262 if gp.m.needextram || atomic.Load(&extraMWaiters) > 0 { 263 gp.m.needextram = false 264 systemstack(newextram) 265 } 266 267 if ctxt != 0 { 268 s := append(gp.cgoCtxt, ctxt) 269 270 // Now we need to set gp.cgoCtxt = s, but we could get 271 // a SIGPROF signal while manipulating the slice, and 272 // the SIGPROF handler could pick up gp.cgoCtxt while 273 // tracing up the stack. We need to ensure that the 274 // handler always sees a valid slice, so set the 275 // values in an order such that it always does. 276 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt)) 277 atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0])) 278 p.cap = cap(s) 279 p.len = len(s) 280 281 defer func(gp *g) { 282 // Decrease the length of the slice by one, safely. 283 p := (*slice)(unsafe.Pointer(&gp.cgoCtxt)) 284 p.len-- 285 }(gp) 286 } 287 288 if gp.m.ncgo == 0 { 289 // The C call to Go came from a thread not currently running 290 // any Go. In the case of -buildmode=c-archive or c-shared, 291 // this call may be coming in before package initialization 292 // is complete. Wait until it is. 293 <-main_init_done 294 } 295 296 // Check whether the profiler needs to be turned on or off; this route to 297 // run Go code does not use runtime.execute, so bypasses the check there. 298 hz := sched.profilehz 299 if gp.m.profilehz != hz { 300 setThreadCPUProfiler(hz) 301 } 302 303 // Add entry to defer stack in case of panic. 304 restore := true 305 defer unwindm(&restore) 306 307 if raceenabled { 308 raceacquire(unsafe.Pointer(&racecgosync)) 309 } 310 311 // Invoke callback. This function is generated by cmd/cgo and 312 // will unpack the argument frame and call the Go function. 313 var cb func(frame unsafe.Pointer) 314 cbFV := funcval{uintptr(fn)} 315 *(*unsafe.Pointer)(unsafe.Pointer(&cb)) = noescape(unsafe.Pointer(&cbFV)) 316 cb(frame) 317 318 if raceenabled { 319 racereleasemerge(unsafe.Pointer(&racecgosync)) 320 } 321 322 // Do not unwind m->g0->sched.sp. 323 // Our caller, cgocallback, will do that. 324 restore = false 325 } 326 327 func unwindm(restore *bool) { 328 if *restore { 329 // Restore sp saved by cgocallback during 330 // unwind of g's stack (see comment at top of file). 331 mp := acquirem() 332 sched := &mp.g0.sched 333 sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + alignUp(sys.MinFrameSize, sys.StackAlign))) 334 335 // Do the accounting that cgocall will not have a chance to do 336 // during an unwind. 337 // 338 // In the case where a Go call originates from C, ncgo is 0 339 // and there is no matching cgocall to end. 340 if mp.ncgo > 0 { 341 mp.incgo = false 342 mp.ncgo-- 343 osPreemptExtExit(mp) 344 } 345 346 releasem(mp) 347 } 348 } 349 350 // called from assembly 351 func badcgocallback() { 352 throw("misaligned stack in cgocallback") 353 } 354 355 // called from (incomplete) assembly 356 func cgounimpl() { 357 throw("cgo not implemented") 358 } 359 360 var racecgosync uint64 // represents possible synchronization in C code 361 362 // Pointer checking for cgo code. 363 364 // We want to detect all cases where a program that does not use 365 // unsafe makes a cgo call passing a Go pointer to memory that 366 // contains a Go pointer. Here a Go pointer is defined as a pointer 367 // to memory allocated by the Go runtime. Programs that use unsafe 368 // can evade this restriction easily, so we don't try to catch them. 369 // The cgo program will rewrite all possibly bad pointer arguments to 370 // call cgoCheckPointer, where we can catch cases of a Go pointer 371 // pointing to a Go pointer. 372 373 // Complicating matters, taking the address of a slice or array 374 // element permits the C program to access all elements of the slice 375 // or array. In that case we will see a pointer to a single element, 376 // but we need to check the entire data structure. 377 378 // The cgoCheckPointer call takes additional arguments indicating that 379 // it was called on an address expression. An additional argument of 380 // true means that it only needs to check a single element. An 381 // additional argument of a slice or array means that it needs to 382 // check the entire slice/array, but nothing else. Otherwise, the 383 // pointer could be anything, and we check the entire heap object, 384 // which is conservative but safe. 385 386 // When and if we implement a moving garbage collector, 387 // cgoCheckPointer will pin the pointer for the duration of the cgo 388 // call. (This is necessary but not sufficient; the cgo program will 389 // also have to change to pin Go pointers that cannot point to Go 390 // pointers.) 391 392 // cgoCheckPointer checks if the argument contains a Go pointer that 393 // points to a Go pointer, and panics if it does. 394 func cgoCheckPointer(ptr any, arg any) { 395 if debug.cgocheck == 0 { 396 return 397 } 398 399 ep := efaceOf(&ptr) 400 t := ep._type 401 402 top := true 403 if arg != nil && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) { 404 p := ep.data 405 if t.kind&kindDirectIface == 0 { 406 p = *(*unsafe.Pointer)(p) 407 } 408 if p == nil || !cgoIsGoPointer(p) { 409 return 410 } 411 aep := efaceOf(&arg) 412 switch aep._type.kind & kindMask { 413 case kindBool: 414 if t.kind&kindMask == kindUnsafePointer { 415 // We don't know the type of the element. 416 break 417 } 418 pt := (*ptrtype)(unsafe.Pointer(t)) 419 cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail) 420 return 421 case kindSlice: 422 // Check the slice rather than the pointer. 423 ep = aep 424 t = ep._type 425 case kindArray: 426 // Check the array rather than the pointer. 427 // Pass top as false since we have a pointer 428 // to the array. 429 ep = aep 430 t = ep._type 431 top = false 432 default: 433 throw("can't happen") 434 } 435 } 436 437 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail) 438 } 439 440 const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer" 441 const cgoResultFail = "cgo result has Go pointer" 442 443 // cgoCheckArg is the real work of cgoCheckPointer. The argument p 444 // is either a pointer to the value (of type t), or the value itself, 445 // depending on indir. The top parameter is whether we are at the top 446 // level, where Go pointers are allowed. 447 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) { 448 if t.ptrdata == 0 || p == nil { 449 // If the type has no pointers there is nothing to do. 450 return 451 } 452 453 switch t.kind & kindMask { 454 default: 455 throw("can't happen") 456 case kindArray: 457 at := (*arraytype)(unsafe.Pointer(t)) 458 if !indir { 459 if at.len != 1 { 460 throw("can't happen") 461 } 462 cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg) 463 return 464 } 465 for i := uintptr(0); i < at.len; i++ { 466 cgoCheckArg(at.elem, p, true, top, msg) 467 p = add(p, at.elem.size) 468 } 469 case kindChan, kindMap: 470 // These types contain internal pointers that will 471 // always be allocated in the Go heap. It's never OK 472 // to pass them to C. 473 panic(errorString(msg)) 474 case kindFunc: 475 if indir { 476 p = *(*unsafe.Pointer)(p) 477 } 478 if !cgoIsGoPointer(p) { 479 return 480 } 481 panic(errorString(msg)) 482 case kindInterface: 483 it := *(**_type)(p) 484 if it == nil { 485 return 486 } 487 // A type known at compile time is OK since it's 488 // constant. A type not known at compile time will be 489 // in the heap and will not be OK. 490 if inheap(uintptr(unsafe.Pointer(it))) { 491 panic(errorString(msg)) 492 } 493 p = *(*unsafe.Pointer)(add(p, goarch.PtrSize)) 494 if !cgoIsGoPointer(p) { 495 return 496 } 497 if !top { 498 panic(errorString(msg)) 499 } 500 cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg) 501 case kindSlice: 502 st := (*slicetype)(unsafe.Pointer(t)) 503 s := (*slice)(p) 504 p = s.array 505 if p == nil || !cgoIsGoPointer(p) { 506 return 507 } 508 if !top { 509 panic(errorString(msg)) 510 } 511 if st.elem.ptrdata == 0 { 512 return 513 } 514 for i := 0; i < s.cap; i++ { 515 cgoCheckArg(st.elem, p, true, false, msg) 516 p = add(p, st.elem.size) 517 } 518 case kindString: 519 ss := (*stringStruct)(p) 520 if !cgoIsGoPointer(ss.str) { 521 return 522 } 523 if !top { 524 panic(errorString(msg)) 525 } 526 case kindStruct: 527 st := (*structtype)(unsafe.Pointer(t)) 528 if !indir { 529 if len(st.fields) != 1 { 530 throw("can't happen") 531 } 532 cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg) 533 return 534 } 535 for _, f := range st.fields { 536 if f.typ.ptrdata == 0 { 537 continue 538 } 539 cgoCheckArg(f.typ, add(p, f.offset), true, top, msg) 540 } 541 case kindPtr, kindUnsafePointer: 542 if indir { 543 p = *(*unsafe.Pointer)(p) 544 if p == nil { 545 return 546 } 547 } 548 549 if !cgoIsGoPointer(p) { 550 return 551 } 552 if !top { 553 panic(errorString(msg)) 554 } 555 556 cgoCheckUnknownPointer(p, msg) 557 } 558 } 559 560 // cgoCheckUnknownPointer is called for an arbitrary pointer into Go 561 // memory. It checks whether that Go memory contains any other 562 // pointer into Go memory. If it does, we panic. 563 // The return values are unused but useful to see in panic tracebacks. 564 func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) { 565 if inheap(uintptr(p)) { 566 b, span, _ := findObject(uintptr(p), 0, 0) 567 base = b 568 if base == 0 { 569 return 570 } 571 hbits := heapBitsForAddr(base) 572 n := span.elemsize 573 for i = uintptr(0); i < n; i += goarch.PtrSize { 574 if !hbits.morePointers() { 575 // No more possible pointers. 576 break 577 } 578 if hbits.isPointer() && cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(base + i))) { 579 panic(errorString(msg)) 580 } 581 hbits = hbits.next() 582 } 583 584 return 585 } 586 587 for _, datap := range activeModules() { 588 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) { 589 // We have no way to know the size of the object. 590 // We have to assume that it might contain a pointer. 591 panic(errorString(msg)) 592 } 593 // In the text or noptr sections, we know that the 594 // pointer does not point to a Go pointer. 595 } 596 597 return 598 } 599 600 // cgoIsGoPointer reports whether the pointer is a Go pointer--a 601 // pointer to Go memory. We only care about Go memory that might 602 // contain pointers. 603 // 604 //go:nosplit 605 //go:nowritebarrierrec 606 func cgoIsGoPointer(p unsafe.Pointer) bool { 607 if p == nil { 608 return false 609 } 610 611 if inHeapOrStack(uintptr(p)) { 612 return true 613 } 614 615 for _, datap := range activeModules() { 616 if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) { 617 return true 618 } 619 } 620 621 return false 622 } 623 624 // cgoInRange reports whether p is between start and end. 625 // 626 //go:nosplit 627 //go:nowritebarrierrec 628 func cgoInRange(p unsafe.Pointer, start, end uintptr) bool { 629 return start <= uintptr(p) && uintptr(p) < end 630 } 631 632 // cgoCheckResult is called to check the result parameter of an 633 // exported Go function. It panics if the result is or contains a Go 634 // pointer. 635 func cgoCheckResult(val any) { 636 if debug.cgocheck == 0 { 637 return 638 } 639 640 ep := efaceOf(&val) 641 t := ep._type 642 cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail) 643 } 644