runtime: avoid write barriers to uninitialized finalizer frame memory

[gostls13.git] / src / runtime / mfinal.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.

// Garbage collector: finalizers and block profiling.

package runtime

import (
        "runtime/internal/atomic"
        "runtime/internal/sys"
        "unsafe"
)

// finblock is allocated from non-GC'd memory, so any heap pointers
// must be specially handled.
//
//go:notinheap
type finblock struct {
        alllink *finblock
        next    *finblock
        cnt     int32
        _       int32
        fin     [(_FinBlockSize - 2*sys.PtrSize - 2*4) / unsafe.Sizeof(finalizer{})]finalizer
}

var finlock mutex  // protects the following variables
var fing *g        // goroutine that runs finalizers
var finq *finblock // list of finalizers that are to be executed
var finc *finblock // cache of free blocks
var finptrmask [_FinBlockSize / sys.PtrSize / 8]byte
var fingwait bool
var fingwake bool
var allfin *finblock // list of all blocks

// NOTE: Layout known to queuefinalizer.
type finalizer struct {
        fn   *funcval       // function to call (may be a heap pointer)
        arg  unsafe.Pointer // ptr to object (may be a heap pointer)
        nret uintptr        // bytes of return values from fn
        fint *_type         // type of first argument of fn
        ot   *ptrtype       // type of ptr to object (may be a heap pointer)
}

var finalizer1 = [...]byte{
        // Each Finalizer is 5 words, ptr ptr INT ptr ptr (INT = uintptr here)
        // Each byte describes 8 words.
        // Need 8 Finalizers described by 5 bytes before pattern repeats:
        //      ptr ptr INT ptr ptr
        //      ptr ptr INT ptr ptr
        //      ptr ptr INT ptr ptr
        //      ptr ptr INT ptr ptr
        //      ptr ptr INT ptr ptr
        //      ptr ptr INT ptr ptr
        //      ptr ptr INT ptr ptr
        //      ptr ptr INT ptr ptr
        // aka
        //
        //      ptr ptr INT ptr ptr ptr ptr INT
        //      ptr ptr ptr ptr INT ptr ptr ptr
        //      ptr INT ptr ptr ptr ptr INT ptr
        //      ptr ptr ptr INT ptr ptr ptr ptr
        //      INT ptr ptr ptr ptr INT ptr ptr
        //
        // Assumptions about Finalizer layout checked below.
        1<<0 | 1<<1 | 0<<2 | 1<<3 | 1<<4 | 1<<5 | 1<<6 | 0<<7,
        1<<0 | 1<<1 | 1<<2 | 1<<3 | 0<<4 | 1<<5 | 1<<6 | 1<<7,
        1<<0 | 0<<1 | 1<<2 | 1<<3 | 1<<4 | 1<<5 | 0<<6 | 1<<7,
        1<<0 | 1<<1 | 1<<2 | 0<<3 | 1<<4 | 1<<5 | 1<<6 | 1<<7,
        0<<0 | 1<<1 | 1<<2 | 1<<3 | 1<<4 | 0<<5 | 1<<6 | 1<<7,
}

func queuefinalizer(p unsafe.Pointer, fn *funcval, nret uintptr, fint *_type, ot *ptrtype) {
        lock(&finlock)
        if finq == nil || finq.cnt == int32(len(finq.fin)) {
                if finc == nil {
                        finc = (*finblock)(persistentalloc(_FinBlockSize, 0, &memstats.gc_sys))
                        finc.alllink = allfin
                        allfin = finc
                        if finptrmask[0] == 0 {
                                // Build pointer mask for Finalizer array in block.
                                // Check assumptions made in finalizer1 array above.
                                if (unsafe.Sizeof(finalizer{}) != 5*sys.PtrSize ||
                                        unsafe.Offsetof(finalizer{}.fn) != 0 ||
                                        unsafe.Offsetof(finalizer{}.arg) != sys.PtrSize ||
                                        unsafe.Offsetof(finalizer{}.nret) != 2*sys.PtrSize ||
                                        unsafe.Offsetof(finalizer{}.fint) != 3*sys.PtrSize ||
                                        unsafe.Offsetof(finalizer{}.ot) != 4*sys.PtrSize) {
                                        throw("finalizer out of sync")
                                }
                                for i := range finptrmask {
                                        finptrmask[i] = finalizer1[i%len(finalizer1)]
                                }
                        }
                }
                block := finc
                finc = block.next
                block.next = finq
                finq = block
        }
        f := &finq.fin[finq.cnt]
        finq.cnt++
        f.fn = fn
        f.nret = nret
        f.fint = fint
        f.ot = ot
        f.arg = p
        fingwake = true
        unlock(&finlock)
}

//go:nowritebarrier
func iterate_finq(callback func(*funcval, unsafe.Pointer, uintptr, *_type, *ptrtype)) {
        for fb := allfin; fb != nil; fb = fb.alllink {
                for i := int32(0); i < fb.cnt; i++ {
                        f := &fb.fin[i]
                        callback(f.fn, f.arg, f.nret, f.fint, f.ot)
                }
        }
}

func wakefing() *g {
        var res *g
        lock(&finlock)
        if fingwait && fingwake {
                fingwait = false
                fingwake = false
                res = fing
        }
        unlock(&finlock)
        return res
}

var (
        fingCreate  uint32
        fingRunning bool
)

func createfing() {
        // start the finalizer goroutine exactly once
        if fingCreate == 0 && atomic.Cas(&fingCreate, 0, 1) {
                go runfinq()
        }
}

// This is the goroutine that runs all of the finalizers
func runfinq() {
        var (
                frame    unsafe.Pointer
                framecap uintptr
        )

        for {
                lock(&finlock)
                fb := finq
                finq = nil
                if fb == nil {
                        gp := getg()
                        fing = gp
                        fingwait = true
                        goparkunlock(&finlock, "finalizer wait", traceEvGoBlock, 1)
                        continue
                }
                unlock(&finlock)
                if raceenabled {
                        racefingo()
                }
                for fb != nil {
                        for i := fb.cnt; i > 0; i-- {
                                f := &fb.fin[i-1]

                                framesz := unsafe.Sizeof((interface{})(nil)) + f.nret
                                if framecap < framesz {
                                        // The frame does not contain pointers interesting for GC,
                                        // all not yet finalized objects are stored in finq.
                                        // If we do not mark it as FlagNoScan,
                                        // the last finalized object is not collected.
                                        frame = mallocgc(framesz, nil, true)
                                        framecap = framesz
                                }

                                if f.fint == nil {
                                        throw("missing type in runfinq")
                                }
                                // frame is effectively uninitialized
                                // memory. That means we have to clear
                                // it before writing to it to avoid
                                // confusing the write barrier.
                                *(*[2]uintptr)(frame) = [2]uintptr{}
                                switch f.fint.kind & kindMask {
                                case kindPtr:
                                        // direct use of pointer
                                        *(*unsafe.Pointer)(frame) = f.arg
                                case kindInterface:
                                        ityp := (*interfacetype)(unsafe.Pointer(f.fint))
                                        // set up with empty interface
                                        (*eface)(frame)._type = &f.ot.typ
                                        (*eface)(frame).data = f.arg
                                        if len(ityp.mhdr) != 0 {
                                                // convert to interface with methods
                                                // this conversion is guaranteed to succeed - we checked in SetFinalizer
                                                assertE2I(ityp, *(*eface)(frame), (*iface)(frame))
                                        }
                                default:
                                        throw("bad kind in runfinq")
                                }
                                fingRunning = true
                                reflectcall(nil, unsafe.Pointer(f.fn), frame, uint32(framesz), uint32(framesz))
                                fingRunning = false

                                // drop finalizer queue references to finalized object
                                f.fn = nil
                                f.arg = nil
                                f.ot = nil
                                fb.cnt = i - 1
                        }
                        next := fb.next
                        lock(&finlock)
                        fb.next = finc
                        finc = fb
                        unlock(&finlock)
                        fb = next
                }
        }
}

// SetFinalizer sets the finalizer associated with obj to the provided
// finalizer function. When the garbage collector finds an unreachable block
// with an associated finalizer, it clears the association and runs
// finalizer(obj) in a separate goroutine. This makes obj reachable again,
// but now without an associated finalizer. Assuming that SetFinalizer
// is not called again, the next time the garbage collector sees
// that obj is unreachable, it will free obj.
//
// SetFinalizer(obj, nil) clears any finalizer associated with obj.
//
// The argument obj must be a pointer to an object allocated by calling
// new, by taking the address of a composite literal, or by taking the
// address of a local variable.
// The argument finalizer must be a function that takes a single argument
// to which obj's type can be assigned, and can have arbitrary ignored return
// values. If either of these is not true, SetFinalizer may abort the
// program.
//
// Finalizers are run in dependency order: if A points at B, both have
// finalizers, and they are otherwise unreachable, only the finalizer
// for A runs; once A is freed, the finalizer for B can run.
// If a cyclic structure includes a block with a finalizer, that
// cycle is not guaranteed to be garbage collected and the finalizer
// is not guaranteed to run, because there is no ordering that
// respects the dependencies.
//
// The finalizer for obj is scheduled to run at some arbitrary time after
// obj becomes unreachable.
// There is no guarantee that finalizers will run before a program exits,
// so typically they are useful only for releasing non-memory resources
// associated with an object during a long-running program.
// For example, an os.File object could use a finalizer to close the
// associated operating system file descriptor when a program discards
// an os.File without calling Close, but it would be a mistake
// to depend on a finalizer to flush an in-memory I/O buffer such as a
// bufio.Writer, because the buffer would not be flushed at program exit.
//
// It is not guaranteed that a finalizer will run if the size of *obj is
// zero bytes.
//
// It is not guaranteed that a finalizer will run for objects allocated
// in initializers for package-level variables. Such objects may be
// linker-allocated, not heap-allocated.
//
// A finalizer may run as soon as an object becomes unreachable.
// In order to use finalizers correctly, the program must ensure that
// the object is reachable until it is no longer required.
// Objects stored in global variables, or that can be found by tracing
// pointers from a global variable, are reachable. For other objects,
// pass the object to a call of the KeepAlive function to mark the
// last point in the function where the object must be reachable.
//
// For example, if p points to a struct that contains a file descriptor d,
// and p has a finalizer that closes that file descriptor, and if the last
// use of p in a function is a call to syscall.Write(p.d, buf, size), then
// p may be unreachable as soon as the program enters syscall.Write. The
// finalizer may run at that moment, closing p.d, causing syscall.Write
// to fail because it is writing to a closed file descriptor (or, worse,
// to an entirely different file descriptor opened by a different goroutine).
// To avoid this problem, call runtime.KeepAlive(p) after the call to
// syscall.Write.
//
// A single goroutine runs all finalizers for a program, sequentially.
// If a finalizer must run for a long time, it should do so by starting
// a new goroutine.
func SetFinalizer(obj interface{}, finalizer interface{}) {
        if debug.sbrk != 0 {
                // debug.sbrk never frees memory, so no finalizers run
                // (and we don't have the data structures to record them).
                return
        }
        e := efaceOf(&obj)
        etyp := e._type
        if etyp == nil {
                throw("runtime.SetFinalizer: first argument is nil")
        }
        if etyp.kind&kindMask != kindPtr {
                throw("runtime.SetFinalizer: first argument is " + etyp.string() + ", not pointer")
        }
        ot := (*ptrtype)(unsafe.Pointer(etyp))
        if ot.elem == nil {
                throw("nil elem type!")
        }

        // find the containing object
        _, base, _ := findObject(e.data)

        if base == nil {
                // 0-length objects are okay.
                if e.data == unsafe.Pointer(&zerobase) {
                        return
                }

                // Global initializers might be linker-allocated.
                //      var Foo = &Object{}
                //      func main() {
                //              runtime.SetFinalizer(Foo, nil)
                //      }
                // The relevant segments are: noptrdata, data, bss, noptrbss.
                // We cannot assume they are in any order or even contiguous,
                // due to external linking.
                for datap := &firstmoduledata; datap != nil; datap = datap.next {
                        if datap.noptrdata <= uintptr(e.data) && uintptr(e.data) < datap.enoptrdata ||
                                datap.data <= uintptr(e.data) && uintptr(e.data) < datap.edata ||
                                datap.bss <= uintptr(e.data) && uintptr(e.data) < datap.ebss ||
                                datap.noptrbss <= uintptr(e.data) && uintptr(e.data) < datap.enoptrbss {
                                return
                        }
                }
                throw("runtime.SetFinalizer: pointer not in allocated block")
        }

        if e.data != base {
                // As an implementation detail we allow to set finalizers for an inner byte
                // of an object if it could come from tiny alloc (see mallocgc for details).
                if ot.elem == nil || ot.elem.kind&kindNoPointers == 0 || ot.elem.size >= maxTinySize {
                        throw("runtime.SetFinalizer: pointer not at beginning of allocated block")
                }
        }

        f := efaceOf(&finalizer)
        ftyp := f._type
        if ftyp == nil {
                // switch to system stack and remove finalizer
                systemstack(func() {
                        removefinalizer(e.data)
                })
                return
        }

        if ftyp.kind&kindMask != kindFunc {
                throw("runtime.SetFinalizer: second argument is " + ftyp.string() + ", not a function")
        }
        ft := (*functype)(unsafe.Pointer(ftyp))
        if ft.dotdotdot() {
                throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string() + " because dotdotdot")
        }
        if ft.inCount != 1 {
                throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
        }
        fint := ft.in()[0]
        switch {
        case fint == etyp:
                // ok - same type
                goto okarg
        case fint.kind&kindMask == kindPtr:
                if (fint.uncommon() == nil || etyp.uncommon() == nil) && (*ptrtype)(unsafe.Pointer(fint)).elem == ot.elem {
                        // ok - not same type, but both pointers,
                        // one or the other is unnamed, and same element type, so assignable.
                        goto okarg
                }
        case fint.kind&kindMask == kindInterface:
                ityp := (*interfacetype)(unsafe.Pointer(fint))
                if len(ityp.mhdr) == 0 {
                        // ok - satisfies empty interface
                        goto okarg
                }
                if assertE2I2(ityp, *efaceOf(&obj), nil) {
                        goto okarg
                }
        }
        throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
okarg:
        // compute size needed for return parameters
        nret := uintptr(0)
        for _, t := range ft.out() {
                nret = round(nret, uintptr(t.align)) + uintptr(t.size)
        }
        nret = round(nret, sys.PtrSize)

        // make sure we have a finalizer goroutine
        createfing()

        systemstack(func() {
                if !addfinalizer(e.data, (*funcval)(f.data), nret, fint, ot) {
                        throw("runtime.SetFinalizer: finalizer already set")
                }
        })
}

// Look up pointer v in heap. Return the span containing the object,
// the start of the object, and the size of the object. If the object
// does not exist, return nil, nil, 0.
func findObject(v unsafe.Pointer) (s *mspan, x unsafe.Pointer, n uintptr) {
        c := gomcache()
        c.local_nlookup++
        if sys.PtrSize == 4 && c.local_nlookup >= 1<<30 {
                // purge cache stats to prevent overflow
                lock(&mheap_.lock)
                purgecachedstats(c)
                unlock(&mheap_.lock)
        }

        // find span
        arena_start := mheap_.arena_start
        arena_used := mheap_.arena_used
        if uintptr(v) < arena_start || uintptr(v) >= arena_used {
                return
        }
        p := uintptr(v) >> pageShift
        q := p - arena_start>>pageShift
        s = mheap_.spans[q]
        if s == nil {
                return
        }
        x = unsafe.Pointer(s.base())

        if uintptr(v) < uintptr(x) || uintptr(v) >= uintptr(unsafe.Pointer(s.limit)) || s.state != mSpanInUse {
                s = nil
                x = nil
                return
        }

        n = s.elemsize
        if s.sizeclass != 0 {
                x = add(x, (uintptr(v)-uintptr(x))/n*n)
        }
        return
}

// Mark KeepAlive as noinline so that the current compiler will ensure
// that the argument is alive at the point of the function call.
// If it were inlined, it would disappear, and there would be nothing
// keeping the argument alive. Perhaps a future compiler will recognize
// runtime.KeepAlive specially and do something more efficient.
//go:noinline

// KeepAlive marks its argument as currently reachable.
// This ensures that the object is not freed, and its finalizer is not run,
// before the point in the program where KeepAlive is called.
//
// A very simplified example showing where KeepAlive is required:
//      type File struct { d int }
//      d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
//      // ... do something if err != nil ...
//      p := &File{d}
//      runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) })
//      var buf [10]byte
//      n, err := syscall.Read(p.d, buf[:])
//      // Ensure p is not finalized until Read returns.
//      runtime.KeepAlive(p)
//      // No more uses of p after this point.
//
// Without the KeepAlive call, the finalizer could run at the start of
// syscall.Read, closing the file descriptor before syscall.Read makes
// the actual system call.
func KeepAlive(interface{}) {}