diff options
Diffstat (limited to 'src/runtime/cgocall.go')
| -rw-r--r-- | src/runtime/cgocall.go | 628 |
1 files changed, 628 insertions, 0 deletions
diff --git a/src/runtime/cgocall.go b/src/runtime/cgocall.go new file mode 100644 index 0000000..20cacd6 --- /dev/null +++ b/src/runtime/cgocall.go @@ -0,0 +1,628 @@ +// Copyright 2009 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +// Cgo call and callback support. +// +// To call into the C function f from Go, the cgo-generated code calls +// runtime.cgocall(_cgo_Cfunc_f, frame), where _cgo_Cfunc_f is a +// gcc-compiled function written by cgo. +// +// runtime.cgocall (below) calls entersyscall so as not to block +// other goroutines or the garbage collector, and then calls +// runtime.asmcgocall(_cgo_Cfunc_f, frame). +// +// runtime.asmcgocall (in asm_$GOARCH.s) switches to the m->g0 stack +// (assumed to be an operating system-allocated stack, so safe to run +// gcc-compiled code on) and calls _cgo_Cfunc_f(frame). +// +// _cgo_Cfunc_f invokes the actual C function f with arguments +// taken from the frame structure, records the results in the frame, +// and returns to runtime.asmcgocall. +// +// After it regains control, runtime.asmcgocall switches back to the +// original g (m->curg)'s stack and returns to runtime.cgocall. +// +// After it regains control, runtime.cgocall calls exitsyscall, which blocks +// until this m can run Go code without violating the $GOMAXPROCS limit, +// and then unlocks g from m. +// +// The above description skipped over the possibility of the gcc-compiled +// function f calling back into Go. If that happens, we continue down +// the rabbit hole during the execution of f. +// +// To make it possible for gcc-compiled C code to call a Go function p.GoF, +// cgo writes a gcc-compiled function named GoF (not p.GoF, since gcc doesn't +// know about packages). The gcc-compiled C function f calls GoF. +// +// GoF initializes "frame", a structure containing all of its +// arguments and slots for p.GoF's results. It calls +// crosscall2(_cgoexp_GoF, frame, framesize, ctxt) using the gcc ABI. +// +// crosscall2 (in cgo/asm_$GOARCH.s) is a four-argument adapter from +// the gcc function call ABI to the gc function call ABI. At this +// point we're in the Go runtime, but we're still running on m.g0's +// stack and outside the $GOMAXPROCS limit. crosscall2 calls +// runtime.cgocallback(_cgoexp_GoF, frame, ctxt) using the gc ABI. +// (crosscall2's framesize argument is no longer used, but there's one +// case where SWIG calls crosscall2 directly and expects to pass this +// argument. See _cgo_panic.) +// +// runtime.cgocallback (in asm_$GOARCH.s) switches from m.g0's stack +// to the original g (m.curg)'s stack, on which it calls +// runtime.cgocallbackg(_cgoexp_GoF, frame, ctxt). As part of the +// stack switch, runtime.cgocallback saves the current SP as +// m.g0.sched.sp, so that any use of m.g0's stack during the execution +// of the callback will be done below the existing stack frames. +// Before overwriting m.g0.sched.sp, it pushes the old value on the +// m.g0 stack, so that it can be restored later. +// +// runtime.cgocallbackg (below) is now running on a real goroutine +// stack (not an m.g0 stack). First it calls runtime.exitsyscall, which will +// block until the $GOMAXPROCS limit allows running this goroutine. +// Once exitsyscall has returned, it is safe to do things like call the memory +// allocator or invoke the Go callback function. runtime.cgocallbackg +// first defers a function to unwind m.g0.sched.sp, so that if p.GoF +// panics, m.g0.sched.sp will be restored to its old value: the m.g0 stack +// and the m.curg stack will be unwound in lock step. +// Then it calls _cgoexp_GoF(frame). +// +// _cgoexp_GoF, which was generated by cmd/cgo, unpacks the arguments +// from frame, calls p.GoF, writes the results back to frame, and +// returns. Now we start unwinding this whole process. +// +// runtime.cgocallbackg pops but does not execute the deferred +// function to unwind m.g0.sched.sp, calls runtime.entersyscall, and +// returns to runtime.cgocallback. +// +// After it regains control, runtime.cgocallback switches back to +// m.g0's stack (the pointer is still in m.g0.sched.sp), restores the old +// m.g0.sched.sp value from the stack, and returns to crosscall2. +// +// crosscall2 restores the callee-save registers for gcc and returns +// to GoF, which unpacks any result values and returns to f. + +package runtime + +import ( + "runtime/internal/atomic" + "runtime/internal/sys" + "unsafe" +) + +// Addresses collected in a cgo backtrace when crashing. +// Length must match arg.Max in x_cgo_callers in runtime/cgo/gcc_traceback.c. +type cgoCallers [32]uintptr + +// argset matches runtime/cgo/linux_syscall.c:argset_t +type argset struct { + args unsafe.Pointer + retval uintptr +} + +// wrapper for syscall package to call cgocall for libc (cgo) calls. +//go:linkname syscall_cgocaller syscall.cgocaller +//go:nosplit +//go:uintptrescapes +func syscall_cgocaller(fn unsafe.Pointer, args ...uintptr) uintptr { + as := argset{args: unsafe.Pointer(&args[0])} + cgocall(fn, unsafe.Pointer(&as)) + return as.retval +} + +// Call from Go to C. +// +// This must be nosplit because it's used for syscalls on some +// platforms. Syscalls may have untyped arguments on the stack, so +// it's not safe to grow or scan the stack. +// +//go:nosplit +func cgocall(fn, arg unsafe.Pointer) int32 { + if !iscgo && GOOS != "solaris" && GOOS != "illumos" && GOOS != "windows" { + throw("cgocall unavailable") + } + + if fn == nil { + throw("cgocall nil") + } + + if raceenabled { + racereleasemerge(unsafe.Pointer(&racecgosync)) + } + + mp := getg().m + mp.ncgocall++ + mp.ncgo++ + + // Reset traceback. + mp.cgoCallers[0] = 0 + + // Announce we are entering a system call + // so that the scheduler knows to create another + // M to run goroutines while we are in the + // foreign code. + // + // The call to asmcgocall is guaranteed not to + // grow the stack and does not allocate memory, + // so it is safe to call while "in a system call", outside + // the $GOMAXPROCS accounting. + // + // fn may call back into Go code, in which case we'll exit the + // "system call", run the Go code (which may grow the stack), + // and then re-enter the "system call" reusing the PC and SP + // saved by entersyscall here. + entersyscall() + + // Tell asynchronous preemption that we're entering external + // code. We do this after entersyscall because this may block + // and cause an async preemption to fail, but at this point a + // sync preemption will succeed (though this is not a matter + // of correctness). + osPreemptExtEnter(mp) + + mp.incgo = true + errno := asmcgocall(fn, arg) + + // Update accounting before exitsyscall because exitsyscall may + // reschedule us on to a different M. + mp.incgo = false + mp.ncgo-- + + osPreemptExtExit(mp) + + exitsyscall() + + // Note that raceacquire must be called only after exitsyscall has + // wired this M to a P. + if raceenabled { + raceacquire(unsafe.Pointer(&racecgosync)) + } + + // From the garbage collector's perspective, time can move + // backwards in the sequence above. If there's a callback into + // Go code, GC will see this function at the call to + // asmcgocall. When the Go call later returns to C, the + // syscall PC/SP is rolled back and the GC sees this function + // back at the call to entersyscall. Normally, fn and arg + // would be live at entersyscall and dead at asmcgocall, so if + // time moved backwards, GC would see these arguments as dead + // and then live. Prevent these undead arguments from crashing + // GC by forcing them to stay live across this time warp. + KeepAlive(fn) + KeepAlive(arg) + KeepAlive(mp) + + return errno +} + +// Call from C back to Go. +//go:nosplit +func cgocallbackg(fn, frame unsafe.Pointer, ctxt uintptr) { + gp := getg() + if gp != gp.m.curg { + println("runtime: bad g in cgocallback") + exit(2) + } + + // The call from C is on gp.m's g0 stack, so we must ensure + // that we stay on that M. We have to do this before calling + // exitsyscall, since it would otherwise be free to move us to + // a different M. The call to unlockOSThread is in unwindm. + lockOSThread() + + // Save current syscall parameters, so m.syscall can be + // used again if callback decide to make syscall. + syscall := gp.m.syscall + + // entersyscall saves the caller's SP to allow the GC to trace the Go + // stack. However, since we're returning to an earlier stack frame and + // need to pair with the entersyscall() call made by cgocall, we must + // save syscall* and let reentersyscall restore them. + savedsp := unsafe.Pointer(gp.syscallsp) + savedpc := gp.syscallpc + exitsyscall() // coming out of cgo call + gp.m.incgo = false + + osPreemptExtExit(gp.m) + + cgocallbackg1(fn, frame, ctxt) + + // At this point unlockOSThread has been called. + // The following code must not change to a different m. + // This is enforced by checking incgo in the schedule function. + + osPreemptExtEnter(gp.m) + + gp.m.incgo = true + // going back to cgo call + reentersyscall(savedpc, uintptr(savedsp)) + + gp.m.syscall = syscall +} + +func cgocallbackg1(fn, frame unsafe.Pointer, ctxt uintptr) { + gp := getg() + if gp.m.needextram || atomic.Load(&extraMWaiters) > 0 { + gp.m.needextram = false + systemstack(newextram) + } + + if ctxt != 0 { + s := append(gp.cgoCtxt, ctxt) + + // Now we need to set gp.cgoCtxt = s, but we could get + // a SIGPROF signal while manipulating the slice, and + // the SIGPROF handler could pick up gp.cgoCtxt while + // tracing up the stack. We need to ensure that the + // handler always sees a valid slice, so set the + // values in an order such that it always does. + p := (*slice)(unsafe.Pointer(&gp.cgoCtxt)) + atomicstorep(unsafe.Pointer(&p.array), unsafe.Pointer(&s[0])) + p.cap = cap(s) + p.len = len(s) + + defer func(gp *g) { + // Decrease the length of the slice by one, safely. + p := (*slice)(unsafe.Pointer(&gp.cgoCtxt)) + p.len-- + }(gp) + } + + if gp.m.ncgo == 0 { + // The C call to Go came from a thread not currently running + // any Go. In the case of -buildmode=c-archive or c-shared, + // this call may be coming in before package initialization + // is complete. Wait until it is. + <-main_init_done + } + + // Add entry to defer stack in case of panic. + restore := true + defer unwindm(&restore) + + if raceenabled { + raceacquire(unsafe.Pointer(&racecgosync)) + } + + // Invoke callback. This function is generated by cmd/cgo and + // will unpack the argument frame and call the Go function. + var cb func(frame unsafe.Pointer) + cbFV := funcval{uintptr(fn)} + *(*unsafe.Pointer)(unsafe.Pointer(&cb)) = noescape(unsafe.Pointer(&cbFV)) + cb(frame) + + if raceenabled { + racereleasemerge(unsafe.Pointer(&racecgosync)) + } + + // Do not unwind m->g0->sched.sp. + // Our caller, cgocallback, will do that. + restore = false +} + +func unwindm(restore *bool) { + if *restore { + // Restore sp saved by cgocallback during + // unwind of g's stack (see comment at top of file). + mp := acquirem() + sched := &mp.g0.sched + switch GOARCH { + default: + throw("unwindm not implemented") + case "386", "amd64", "arm", "ppc64", "ppc64le", "mips64", "mips64le", "s390x", "mips", "mipsle", "riscv64": + sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + sys.MinFrameSize)) + case "arm64": + sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + 16)) + } + + // Do the accounting that cgocall will not have a chance to do + // during an unwind. + // + // In the case where a Go call originates from C, ncgo is 0 + // and there is no matching cgocall to end. + if mp.ncgo > 0 { + mp.incgo = false + mp.ncgo-- + osPreemptExtExit(mp) + } + + releasem(mp) + } + + // Undo the call to lockOSThread in cgocallbackg. + // We must still stay on the same m. + unlockOSThread() +} + +// called from assembly +func badcgocallback() { + throw("misaligned stack in cgocallback") +} + +// called from (incomplete) assembly +func cgounimpl() { + throw("cgo not implemented") +} + +var racecgosync uint64 // represents possible synchronization in C code + +// Pointer checking for cgo code. + +// We want to detect all cases where a program that does not use +// unsafe makes a cgo call passing a Go pointer to memory that +// contains a Go pointer. Here a Go pointer is defined as a pointer +// to memory allocated by the Go runtime. Programs that use unsafe +// can evade this restriction easily, so we don't try to catch them. +// The cgo program will rewrite all possibly bad pointer arguments to +// call cgoCheckPointer, where we can catch cases of a Go pointer +// pointing to a Go pointer. + +// Complicating matters, taking the address of a slice or array +// element permits the C program to access all elements of the slice +// or array. In that case we will see a pointer to a single element, +// but we need to check the entire data structure. + +// The cgoCheckPointer call takes additional arguments indicating that +// it was called on an address expression. An additional argument of +// true means that it only needs to check a single element. An +// additional argument of a slice or array means that it needs to +// check the entire slice/array, but nothing else. Otherwise, the +// pointer could be anything, and we check the entire heap object, +// which is conservative but safe. + +// When and if we implement a moving garbage collector, +// cgoCheckPointer will pin the pointer for the duration of the cgo +// call. (This is necessary but not sufficient; the cgo program will +// also have to change to pin Go pointers that cannot point to Go +// pointers.) + +// cgoCheckPointer checks if the argument contains a Go pointer that +// points to a Go pointer, and panics if it does. +func cgoCheckPointer(ptr interface{}, arg interface{}) { + if debug.cgocheck == 0 { + return + } + + ep := efaceOf(&ptr) + t := ep._type + + top := true + if arg != nil && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) { + p := ep.data + if t.kind&kindDirectIface == 0 { + p = *(*unsafe.Pointer)(p) + } + if p == nil || !cgoIsGoPointer(p) { + return + } + aep := efaceOf(&arg) + switch aep._type.kind & kindMask { + case kindBool: + if t.kind&kindMask == kindUnsafePointer { + // We don't know the type of the element. + break + } + pt := (*ptrtype)(unsafe.Pointer(t)) + cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail) + return + case kindSlice: + // Check the slice rather than the pointer. + ep = aep + t = ep._type + case kindArray: + // Check the array rather than the pointer. + // Pass top as false since we have a pointer + // to the array. + ep = aep + t = ep._type + top = false + default: + throw("can't happen") + } + } + + cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail) +} + +const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer" +const cgoResultFail = "cgo result has Go pointer" + +// cgoCheckArg is the real work of cgoCheckPointer. The argument p +// is either a pointer to the value (of type t), or the value itself, +// depending on indir. The top parameter is whether we are at the top +// level, where Go pointers are allowed. +func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) { + if t.ptrdata == 0 || p == nil { + // If the type has no pointers there is nothing to do. + return + } + + switch t.kind & kindMask { + default: + throw("can't happen") + case kindArray: + at := (*arraytype)(unsafe.Pointer(t)) + if !indir { + if at.len != 1 { + throw("can't happen") + } + cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg) + return + } + for i := uintptr(0); i < at.len; i++ { + cgoCheckArg(at.elem, p, true, top, msg) + p = add(p, at.elem.size) + } + case kindChan, kindMap: + // These types contain internal pointers that will + // always be allocated in the Go heap. It's never OK + // to pass them to C. + panic(errorString(msg)) + case kindFunc: + if indir { + p = *(*unsafe.Pointer)(p) + } + if !cgoIsGoPointer(p) { + return + } + panic(errorString(msg)) + case kindInterface: + it := *(**_type)(p) + if it == nil { + return + } + // A type known at compile time is OK since it's + // constant. A type not known at compile time will be + // in the heap and will not be OK. + if inheap(uintptr(unsafe.Pointer(it))) { + panic(errorString(msg)) + } + p = *(*unsafe.Pointer)(add(p, sys.PtrSize)) + if !cgoIsGoPointer(p) { + return + } + if !top { + panic(errorString(msg)) + } + cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg) + case kindSlice: + st := (*slicetype)(unsafe.Pointer(t)) + s := (*slice)(p) + p = s.array + if p == nil || !cgoIsGoPointer(p) { + return + } + if !top { + panic(errorString(msg)) + } + if st.elem.ptrdata == 0 { + return + } + for i := 0; i < s.cap; i++ { + cgoCheckArg(st.elem, p, true, false, msg) + p = add(p, st.elem.size) + } + case kindString: + ss := (*stringStruct)(p) + if !cgoIsGoPointer(ss.str) { + return + } + if !top { + panic(errorString(msg)) + } + case kindStruct: + st := (*structtype)(unsafe.Pointer(t)) + if !indir { + if len(st.fields) != 1 { + throw("can't happen") + } + cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg) + return + } + for _, f := range st.fields { + if f.typ.ptrdata == 0 { + continue + } + cgoCheckArg(f.typ, add(p, f.offset()), true, top, msg) + } + case kindPtr, kindUnsafePointer: + if indir { + p = *(*unsafe.Pointer)(p) + if p == nil { + return + } + } + + if !cgoIsGoPointer(p) { + return + } + if !top { + panic(errorString(msg)) + } + + cgoCheckUnknownPointer(p, msg) + } +} + +// cgoCheckUnknownPointer is called for an arbitrary pointer into Go +// memory. It checks whether that Go memory contains any other +// pointer into Go memory. If it does, we panic. +// The return values are unused but useful to see in panic tracebacks. +func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) { + if inheap(uintptr(p)) { + b, span, _ := findObject(uintptr(p), 0, 0) + base = b + if base == 0 { + return + } + hbits := heapBitsForAddr(base) + n := span.elemsize + for i = uintptr(0); i < n; i += sys.PtrSize { + if !hbits.morePointers() { + // No more possible pointers. + break + } + if hbits.isPointer() && cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(base + i))) { + panic(errorString(msg)) + } + hbits = hbits.next() + } + + return + } + + for _, datap := range activeModules() { + if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) { + // We have no way to know the size of the object. + // We have to assume that it might contain a pointer. + panic(errorString(msg)) + } + // In the text or noptr sections, we know that the + // pointer does not point to a Go pointer. + } + + return +} + +// cgoIsGoPointer reports whether the pointer is a Go pointer--a +// pointer to Go memory. We only care about Go memory that might +// contain pointers. +//go:nosplit +//go:nowritebarrierrec +func cgoIsGoPointer(p unsafe.Pointer) bool { + if p == nil { + return false + } + + if inHeapOrStack(uintptr(p)) { + return true + } + + for _, datap := range activeModules() { + if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) { + return true + } + } + + return false +} + +// cgoInRange reports whether p is between start and end. +//go:nosplit +//go:nowritebarrierrec +func cgoInRange(p unsafe.Pointer, start, end uintptr) bool { + return start <= uintptr(p) && uintptr(p) < end +} + +// cgoCheckResult is called to check the result parameter of an +// exported Go function. It panics if the result is or contains a Go +// pointer. +func cgoCheckResult(val interface{}) { + if debug.cgocheck == 0 { + return + } + + ep := efaceOf(&val) + t := ep._type + cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail) +} |