// 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 $GOMAXPROCS limit,
// and then unlocks g from m.
//
// The above description skipped over the possibility of the gcc-compiled
// function f calling back into Go. If that happens, we continue down
// the rabbit hole during the execution of f.
//
// To make it possible for gcc-compiled C code to call a Go function p.GoF,
// cgo writes a gcc-compiled function named GoF (not p.GoF, since gcc doesn't
// know about packages). The gcc-compiled C function f calls GoF.
//
// GoF calls crosscall2(_cgoexp_GoF, frame, framesize). Crosscall2
// (in cgo/gcc_$GOARCH.S, a gcc-compiled assembly file) is a two-argument
// adapter from the gcc function call ABI to the 6c function call ABI.
// It is called from gcc to call 6c functions. In this case it calls
// _cgoexp_GoF(frame, framesize), still running on m->g0's stack
// and outside the $GOMAXPROCS limit. Thus, this code cannot yet
// call arbitrary Go code directly and must be careful not to allocate
// memory or use up m->g0's stack.
//
// _cgoexp_GoF calls runtime.cgocallback(p.GoF, frame, framesize, ctxt).
// (The reason for having _cgoexp_GoF instead of writing a crosscall3
// to make this call directly is that _cgoexp_GoF, because it is compiled
// with 6c instead of gcc, can refer to dotted names like
// runtime.cgocallback and p.GoF.)
//
// runtime.cgocallback (in asm_$GOARCH.s) switches from m->g0's
// stack to the original g (m->curg)'s stack, on which it calls
// runtime.cgocallbackg(p.GoF, frame, framesize).
// 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 p.GoF. 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 p.GoF. Finally it pops but does not execute the deferred
// function, 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 _cgoexp_GoF.
//
// _cgoexp_GoF immediately returns to crosscall2, which restores the
// callee-save registers for gcc and returns to GoF, which 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
// Call from Go to C.
//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()
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--
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(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
cgocallbackg1(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.
gp.m.incgo = true
// going back to cgo call
reentersyscall(savedpc, uintptr(savedsp))
gp.m.syscall = syscall
}
func cgocallbackg1(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))
}
type args struct {
fn *funcval
arg unsafe.Pointer
argsize uintptr
}
var cb *args
// Location of callback arguments depends on stack frame layout
// and size of stack frame of cgocallback_gofunc.
sp := gp.m.g0.sched.sp
switch GOARCH {
default:
throw("cgocallbackg is unimplemented on arch")
case "arm":
// On arm, stack frame is two words and there's a saved LR between
// SP and the stack frame and between the stack frame and the arguments.
cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
case "arm64":
// On arm64, stack frame is four words and there's a saved LR between
// SP and the stack frame and between the stack frame and the arguments.
// Additional two words (16-byte alignment) are for saving FP.
cb = (*args)(unsafe.Pointer(sp + 7*sys.PtrSize))
case "amd64":
// On amd64, stack frame is two words, plus caller PC.
if framepointer_enabled {
// In this case, there's also saved BP.
cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
break
}
cb = (*args)(unsafe.Pointer(sp + 3*sys.PtrSize))
case "386":
// On 386, stack frame is three words, plus caller PC.
cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
case "ppc64", "ppc64le", "s390x":
// On ppc64 and s390x, the callback arguments are in the arguments area of
// cgocallback's stack frame. The stack looks like this:
// +--------------------+------------------------------+
// | | ... |
// | cgoexp_$fn +------------------------------+
// | | fixed frame area |
// +--------------------+------------------------------+
// | | arguments area |
// | cgocallback +------------------------------+ <- sp + 2*minFrameSize + 2*ptrSize
// | | fixed frame area |
// +--------------------+------------------------------+ <- sp + minFrameSize + 2*ptrSize
// | | local variables (2 pointers) |
// | cgocallback_gofunc +------------------------------+ <- sp + minFrameSize
// | | fixed frame area |
// +--------------------+------------------------------+ <- sp
cb = (*args)(unsafe.Pointer(sp + 2*sys.MinFrameSize + 2*sys.PtrSize))
case "mips64", "mips64le":
// On mips64x, stack frame is two words and there's a saved LR between
// SP and the stack frame and between the stack frame and the arguments.
cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
case "mips", "mipsle":
// On mipsx, stack frame is two words and there's a saved LR between
// SP and the stack frame and between the stack frame and the arguments.
cb = (*args)(unsafe.Pointer(sp + 4*sys.PtrSize))
}
// Invoke callback.
// NOTE(rsc): passing nil for argtype means that the copying of the
// results back into cb.arg happens without any corresponding write barriers.
// For cgo, cb.arg points into a C stack frame and therefore doesn't
// hold any pointers that the GC can find anyway - the write barrier
// would be a no-op.
reflectcall(nil, unsafe.Pointer(cb.fn), cb.arg, uint32(cb.argsize), 0)
if raceenabled {
racereleasemerge(unsafe.Pointer(&racecgosync))
}
if msanenabled {
// Tell msan that we wrote to the entire argument block.
// This tells msan that we set the results.
// Since we have already called the function it doesn't
// matter that we are writing to the non-result parameters.
msanwrite(cb.arg, cb.argsize)
}
// 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":
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--
}
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{}, args ...interface{}) {
if debug.cgocheck == 0 {
return
}
ep := (*eface)(unsafe.Pointer(&ptr))
t := ep._type
top := true
if len(args) > 0 && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) {
p := ep.data
if t.kind&kindDirectIface == 0 {
p = *(*unsafe.Pointer)(p)
}
if !cgoIsGoPointer(p) {
return
}
aep := (*eface)(unsafe.Pointer(&args[0]))
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 {
// 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 !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 {
cgoCheckArg(f.typ, add(p, f.offset()), true, top, msg)
}
case kindPtr, kindUnsafePointer:
if indir {
p = *(*unsafe.Pointer)(p)
}
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 i != 1*sys.PtrSize && !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 := (*eface)(unsafe.Pointer(&val))
t := ep._type
cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail)
}
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