runtime: break out system-specific constants into package sys

[gostls13.git] / src / runtime / cgocall.go

// 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) locks g to m, 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 $GOMAXPROCS limit.  Thus, this code cannot yet
// call arbitrary Go code directly and must be careful not to allocate
// memory or use up m->g0's stack.
//
// _cgoexp_GoF calls runtime.cgocallback(p.GoF, frame, framesize).
// (The reason for having _cgoexp_GoF instead of writing a crosscall3
// to make this call directly is that _cgoexp_GoF, because it is compiled
// with 6c instead of gcc, can refer to dotted names like
// runtime.cgocallback and p.GoF.)
//
// runtime.cgocallback (in asm_$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/sys"
        "unsafe"
)

// Call from Go to C.
//go:nosplit
func cgocall(fn, arg unsafe.Pointer) int32 {
        if !iscgo && GOOS != "solaris" && GOOS != "windows" {
                throw("cgocall unavailable")
        }

        if fn == nil {
                throw("cgocall nil")
        }

        if raceenabled {
                racereleasemerge(unsafe.Pointer(&racecgosync))
        }

        /*
         * Lock g to m to ensure we stay on the same stack if we do a
         * cgo callback. Add entry to defer stack in case of panic.
         */
        lockOSThread()
        mp := getg().m
        mp.ncgocall++
        mp.ncgo++
        defer endcgo(mp)

        /*
         * 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
         * split the stack and does not allocate memory,
         * so it is safe to call while "in a system call", outside
         * the $GOMAXPROCS accounting.
         */
        entersyscall(0)
        errno := asmcgocall(fn, arg)
        exitsyscall(0)

        return errno
}

//go:nosplit
func endcgo(mp *m) {
        mp.ncgo--

        if raceenabled {
                raceacquire(unsafe.Pointer(&racecgosync))
        }

        unlockOSThread() // invalidates mp
}

// Helper functions for cgo code.

func cmalloc(n uintptr) unsafe.Pointer {
        var args struct {
                n   uint64
                ret unsafe.Pointer
        }
        args.n = uint64(n)
        cgocall(_cgo_malloc, unsafe.Pointer(&args))
        if args.ret == nil {
                throw("C malloc failed")
        }
        return args.ret
}

func cfree(p unsafe.Pointer) {
        cgocall(_cgo_free, p)
}

// Call from C back to Go.
//go:nosplit
func cgocallbackg() {
        gp := getg()
        if gp != gp.m.curg {
                println("runtime: bad g in cgocallback")
                exit(2)
        }

        // 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(0) // coming out of cgo call
        cgocallbackg1()
        // going back to cgo call
        reentersyscall(savedpc, uintptr(savedsp))

        gp.m.syscall = syscall
}

func cgocallbackg1() {
        gp := getg()
        if gp.m.needextram {
                gp.m.needextram = false
                systemstack(newextram)
        }

        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.
                cb = (*args)(unsafe.Pointer(sp + 5*sys.PtrSize))
        case "amd64":
                // On amd64, stack frame is one word, plus caller PC.
                if framepointer_enabled {
                        // In this case, there's also saved BP.
                        cb = (*args)(unsafe.Pointer(sp + 3*sys.PtrSize))
                        break
                }
                cb = (*args)(unsafe.Pointer(sp + 2*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":
                // On ppc64, 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))
        }

        // 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), unsafe.Pointer(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 {
                return
        }
        // 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":
                sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + sys.MinFrameSize))
        case "arm64":
                sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + 16))
        }
        releasem(mp)
}

// 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 can not point to Go
// pointers.)

// cgoCheckPointer checks if the argument contains a Go pointer that
// points to a Go pointer, and panics if it does.  It returns the pointer.
func cgoCheckPointer(ptr interface{}, args ...interface{}) interface{} {
        if debug.cgocheck == 0 {
                return ptr
        }

        ep := (*eface)(unsafe.Pointer(&ptr))
        t := ep._type

        top := true
        if len(args) > 0 && t.kind&kindMask == kindPtr {
                p := ep.data
                if t.kind&kindDirectIface == 0 {
                        p = *(*unsafe.Pointer)(p)
                }
                if !cgoIsGoPointer(p) {
                        return ptr
                }
                aep := (*eface)(unsafe.Pointer(&args[0]))
                switch aep._type.kind & kindMask {
                case kindBool:
                        pt := (*ptrtype)(unsafe.Pointer(t))
                        cgoCheckArg(pt.elem, p, true, false)
                        return ptr
                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)
        return ptr
}

const cgoCheckPointerFail = "cgo argument has Go pointer to 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) {
        if t.kind&kindNoPointers != 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)
                        return
                }
                for i := uintptr(0); i < at.len; i++ {
                        cgoCheckArg(at.elem, p, true, top)
                        p = unsafe.Pointer(uintptr(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(cgoCheckPointerFail))
        case kindFunc:
                if indir {
                        p = *(*unsafe.Pointer)(p)
                }
                if !cgoIsGoPointer(p) {
                        return
                }
                panic(errorString(cgoCheckPointerFail))
        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(cgoCheckPointerFail))
                }
                p = *(*unsafe.Pointer)(unsafe.Pointer(uintptr(p) + sys.PtrSize))
                if !cgoIsGoPointer(p) {
                        return
                }
                if !top {
                        panic(errorString(cgoCheckPointerFail))
                }
                cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false)
        case kindSlice:
                st := (*slicetype)(unsafe.Pointer(t))
                s := (*slice)(p)
                p = s.array
                if !cgoIsGoPointer(p) {
                        return
                }
                if !top {
                        panic(errorString(cgoCheckPointerFail))
                }
                for i := 0; i < s.cap; i++ {
                        cgoCheckArg(st.elem, p, true, false)
                        p = unsafe.Pointer(uintptr(p) + st.elem.size)
                }
        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)
                        return
                }
                for _, f := range st.fields {
                        cgoCheckArg(f.typ, unsafe.Pointer(uintptr(p)+f.offset), true, top)
                }
        case kindPtr, kindUnsafePointer:
                if indir {
                        p = *(*unsafe.Pointer)(p)
                }

                if !cgoIsGoPointer(p) {
                        return
                }
                if !top {
                        panic(errorString(cgoCheckPointerFail))
                }

                cgoCheckUnknownPointer(p)
        }
}

// 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.
func cgoCheckUnknownPointer(p unsafe.Pointer) {
        if cgoInRange(p, mheap_.arena_start, mheap_.arena_used) {
                if !inheap(uintptr(p)) {
                        // This pointer is either to a stack or to an
                        // unused span.  Escape analysis should
                        // prevent the former and the latter should
                        // not happen.
                        panic(errorString("cgo argument has invalid Go pointer"))
                }

                base, hbits, span := heapBitsForObject(uintptr(p), 0, 0)
                if base == 0 {
                        return
                }
                n := span.elemsize
                for i := uintptr(0); i < n; i += sys.PtrSize {
                        bits := hbits.bits()
                        if i >= 2*sys.PtrSize && bits&bitMarked == 0 {
                                // No more possible pointers.
                                break
                        }
                        if bits&bitPointer != 0 {
                                if cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(base + i))) {
                                        panic(errorString(cgoCheckPointerFail))
                                }
                        }
                        hbits = hbits.next()
                }

                return
        }

        for datap := &firstmoduledata; datap != nil; datap = datap.next {
                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(cgoCheckPointerFail))
                }
                // In the text or noptr sections, we know that the
                // pointer does not point to a Go pointer.
        }
}

// cgoIsGoPointer returns whether the pointer is a Go pointer--a
// pointer to Go memory.  We only care about Go memory that might
// contain pointers.
func cgoIsGoPointer(p unsafe.Pointer) bool {
        if p == nil {
                return false
        }

        if cgoInRange(p, mheap_.arena_start, mheap_.arena_used) {
                return true
        }

        for datap := &firstmoduledata; datap != nil; datap = datap.next {
                if cgoInRange(p, datap.data, datap.edata) || cgoInRange(p, datap.bss, datap.ebss) {
                        return true
                }
        }

        return false
}

// cgoInRange returns whether p is between start and end.
func cgoInRange(p unsafe.Pointer, start, end uintptr) bool {
        return start <= uintptr(p) && uintptr(p) < end
}