cmd/cgo: add #cgo noescape/nocallback annotations

[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) 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 (
        "internal/goarch"
        "internal/goexperiment"
        "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
}

var ncgocall uint64 // number of cgo calls in total for dead m

// 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++

        // 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
        // We use ncgo as a check during execution tracing for whether there is
        // any C on the call stack, which there will be after this point. If
        // there isn't, we can use frame pointer unwinding to collect call
        // stacks efficiently. This will be the case for the first Go-to-C call
        // on a stack, so it's preferable to update it here, after we emit a
        // trace event in entersyscall above.
        mp.ncgo++

        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. fn must point to an ABIInternal Go entry-point.
//
//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()

        checkm := gp.m

        // 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
        if gp.m.isextra {
                gp.m.isExtraInC = false
        }

        osPreemptExtExit(gp.m)

        if gp.nocgocallback {
                panic("runtime: function marked with #cgo nocallback called back into Go")
        }

        cgocallbackg1(fn, frame, ctxt) // will call unlockOSThread

        // 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
        if gp.m.isextra {
                gp.m.isExtraInC = true
        }

        if gp.m != checkm {
                throw("m changed unexpectedly in cgocallbackg")
        }

        osPreemptExtEnter(gp.m)

        // going back to cgo call
        reentersyscall(savedpc, uintptr(savedsp))

        gp.m.syscall = syscall
}

func cgocallbackg1(fn, frame unsafe.Pointer, ctxt uintptr) {
        gp := getg()

        // When we return, undo the call to lockOSThread in cgocallbackg.
        // We must still stay on the same m.
        defer unlockOSThread()

        if gp.m.needextram || extraMWaiters.Load() > 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
        }

        // Check whether the profiler needs to be turned on or off; this route to
        // run Go code does not use runtime.execute, so bypasses the check there.
        hz := sched.profilehz
        if gp.m.profilehz != hz {
                setThreadCPUProfiler(hz)
        }

        // 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
                sched.sp = *(*uintptr)(unsafe.Pointer(sched.sp + alignUp(sys.MinFrameSize, sys.StackAlign)))

                // 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)
        }
}

// 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 an unpinned 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 an unpinned 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 an unpinned Go pointer, and panics if it does.
func cgoCheckPointer(ptr any, arg any) {
        if !goexperiment.CgoCheck2 && 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 unpinned 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. Go pointers to pinned objects are
// always allowed.
func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) {
        if t.PtrBytes == 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, goarch.PtrSize))
                if !cgoIsGoPointer(p) {
                        return
                }
                if !top && !isPinned(p) {
                        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 && !isPinned(p) {
                        panic(errorString(msg))
                }
                if st.Elem.PtrBytes == 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 && !isPinned(ss.str) {
                        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.PtrBytes == 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 && !isPinned(p) {
                        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 unpinned 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
                }
                n := span.elemsize
                hbits := heapBitsForAddr(base, n)
                for {
                        var addr uintptr
                        if hbits, addr = hbits.next(); addr == 0 {
                                break
                        }
                        pp := *(*unsafe.Pointer)(unsafe.Pointer(addr))
                        if cgoIsGoPointer(pp) && !isPinned(pp) {
                                panic(errorString(msg))
                        }
                }

                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 any) {
        if !goexperiment.CgoCheck2 && debug.cgocheck == 0 {
                return
        }

        ep := efaceOf(&val)
        t := ep._type
        cgoCheckArg(t, ep.data, t.Kind_&kindDirectIface == 0, false, cgoResultFail)
}