libgo/go/runtime/cgocall.go

   1 // Copyright 2009 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 // Cgo call and callback support.
   6
   7 package runtime
   8
   9 import (
  10         "runtime/internal/sys"
  11         "unsafe"
  12 )
  13
  14 // Functions called by cgo-generated code.
  15 //go:linkname cgoCheckPointer
  16 //go:linkname cgoCheckResult
  17
  18 // Pointer checking for cgo code.
  19
  20 // We want to detect all cases where a program that does not use
  21 // unsafe makes a cgo call passing a Go pointer to memory that
  22 // contains a Go pointer. Here a Go pointer is defined as a pointer
  23 // to memory allocated by the Go runtime. Programs that use unsafe
  24 // can evade this restriction easily, so we don't try to catch them.
  25 // The cgo program will rewrite all possibly bad pointer arguments to
  26 // call cgoCheckPointer, where we can catch cases of a Go pointer
  27 // pointing to a Go pointer.
  28
  29 // Complicating matters, taking the address of a slice or array
  30 // element permits the C program to access all elements of the slice
  31 // or array. In that case we will see a pointer to a single element,
  32 // but we need to check the entire data structure.
  33
  34 // The cgoCheckPointer call takes additional arguments indicating that
  35 // it was called on an address expression. An additional argument of
  36 // true means that it only needs to check a single element. An
  37 // additional argument of a slice or array means that it needs to
  38 // check the entire slice/array, but nothing else. Otherwise, the
  39 // pointer could be anything, and we check the entire heap object,
  40 // which is conservative but safe.
  41
  42 // When and if we implement a moving garbage collector,
  43 // cgoCheckPointer will pin the pointer for the duration of the cgo
  44 // call.  (This is necessary but not sufficient; the cgo program will
  45 // also have to change to pin Go pointers that cannot point to Go
  46 // pointers.)
  47
  48 // cgoCheckPointer checks if the argument contains a Go pointer that
  49 // points to a Go pointer, and panics if it does.
  50 func cgoCheckPointer(ptr interface{}, arg interface{}) {
  51         if debug.cgocheck == 0 {
  52                 return
  53         }
  54
  55         ep := efaceOf(&ptr)
  56         t := ep._type
  57
  58         top := true
  59         if arg != nil && (t.kind&kindMask == kindPtr || t.kind&kindMask == kindUnsafePointer) {
  60                 p := ep.data
  61                 if t.kind&kindDirectIface == 0 {
  62                         p = *(*unsafe.Pointer)(p)
  63                 }
  64                 if p == nil || !cgoIsGoPointer(p) {
  65                         return
  66                 }
  67                 aep := efaceOf(&arg)
  68                 switch aep._type.kind & kindMask {
  69                 case kindBool:
  70                         if t.kind&kindMask == kindUnsafePointer {
  71                                 // We don't know the type of the element.
  72                                 break
  73                         }
  74                         pt := (*ptrtype)(unsafe.Pointer(t))
  75                         cgoCheckArg(pt.elem, p, true, false, cgoCheckPointerFail)
  76                         return
  77                 case kindSlice:
  78                         // Check the slice rather than the pointer.
  79                         ep = aep
  80                         t = ep._type
  81                 case kindArray:
  82                         // Check the array rather than the pointer.
  83                         // Pass top as false since we have a pointer
  84                         // to the array.
  85                         ep = aep
  86                         t = ep._type
  87                         top = false
  88                 default:
  89                         throw("can't happen")
  90                 }
  91         }
  92
  93         cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, top, cgoCheckPointerFail)
  94 }
  95
  96 const cgoCheckPointerFail = "cgo argument has Go pointer to Go pointer"
  97 const cgoResultFail = "cgo result has Go pointer"
  98
  99 // cgoCheckArg is the real work of cgoCheckPointer. The argument p
 100 // is either a pointer to the value (of type t), or the value itself,
 101 // depending on indir. The top parameter is whether we are at the top
 102 // level, where Go pointers are allowed.
 103 func cgoCheckArg(t *_type, p unsafe.Pointer, indir, top bool, msg string) {
 104         if t.ptrdata == 0 || p == nil {
 105                 // If the type has no pointers there is nothing to do.
 106                 return
 107         }
 108
 109         switch t.kind & kindMask {
 110         default:
 111                 throw("can't happen")
 112         case kindArray:
 113                 at := (*arraytype)(unsafe.Pointer(t))
 114                 if !indir {
 115                         if at.len != 1 {
 116                                 throw("can't happen")
 117                         }
 118                         cgoCheckArg(at.elem, p, at.elem.kind&kindDirectIface == 0, top, msg)
 119                         return
 120                 }
 121                 for i := uintptr(0); i < at.len; i++ {
 122                         cgoCheckArg(at.elem, p, true, top, msg)
 123                         p = add(p, at.elem.size)
 124                 }
 125         case kindChan, kindMap:
 126                 // These types contain internal pointers that will
 127                 // always be allocated in the Go heap. It's never OK
 128                 // to pass them to C.
 129                 panic(errorString(msg))
 130         case kindFunc:
 131                 if indir {
 132                         p = *(*unsafe.Pointer)(p)
 133                 }
 134                 if !cgoIsGoPointer(p) {
 135                         return
 136                 }
 137                 panic(errorString(msg))
 138         case kindInterface:
 139                 it := *(**_type)(p)
 140                 if it == nil {
 141                         return
 142                 }
 143                 // A type known at compile time is OK since it's
 144                 // constant. A type not known at compile time will be
 145                 // in the heap and will not be OK.
 146                 if inheap(uintptr(unsafe.Pointer(it))) {
 147                         panic(errorString(msg))
 148                 }
 149                 p = *(*unsafe.Pointer)(add(p, sys.PtrSize))
 150                 if !cgoIsGoPointer(p) {
 151                         return
 152                 }
 153                 if !top {
 154                         panic(errorString(msg))
 155                 }
 156                 cgoCheckArg(it, p, it.kind&kindDirectIface == 0, false, msg)
 157         case kindSlice:
 158                 st := (*slicetype)(unsafe.Pointer(t))
 159                 s := (*slice)(p)
 160                 p = s.array
 161                 if p == nil || !cgoIsGoPointer(p) {
 162                         return
 163                 }
 164                 if !top {
 165                         panic(errorString(msg))
 166                 }
 167                 if st.elem.ptrdata == 0 {
 168                         return
 169                 }
 170                 for i := 0; i < s.cap; i++ {
 171                         cgoCheckArg(st.elem, p, true, false, msg)
 172                         p = add(p, st.elem.size)
 173                 }
 174         case kindString:
 175                 ss := (*stringStruct)(p)
 176                 if !cgoIsGoPointer(ss.str) {
 177                         return
 178                 }
 179                 if !top {
 180                         panic(errorString(msg))
 181                 }
 182         case kindStruct:
 183                 st := (*structtype)(unsafe.Pointer(t))
 184                 if !indir {
 185                         if len(st.fields) != 1 {
 186                                 throw("can't happen")
 187                         }
 188                         cgoCheckArg(st.fields[0].typ, p, st.fields[0].typ.kind&kindDirectIface == 0, top, msg)
 189                         return
 190                 }
 191                 for _, f := range st.fields {
 192                         if f.typ.ptrdata == 0 {
 193                                 continue
 194                         }
 195                         cgoCheckArg(f.typ, add(p, f.offset()), true, top, msg)
 196                 }
 197         case kindPtr, kindUnsafePointer:
 198                 if indir {
 199                         p = *(*unsafe.Pointer)(p)
 200                         if p == nil {
 201                                 return
 202                         }
 203                 }
 204
 205                 if !cgoIsGoPointer(p) {
 206                         return
 207                 }
 208                 if !top {
 209                         panic(errorString(msg))
 210                 }
 211
 212                 cgoCheckUnknownPointer(p, msg)
 213         }
 214 }
 215
 216 // cgoCheckUnknownPointer is called for an arbitrary pointer into Go
 217 // memory. It checks whether that Go memory contains any other
 218 // pointer into Go memory. If it does, we panic.
 219 // The return values are unused but useful to see in panic tracebacks.
 220 func cgoCheckUnknownPointer(p unsafe.Pointer, msg string) (base, i uintptr) {
 221         if inheap(uintptr(p)) {
 222                 b, span, _ := findObject(uintptr(p), 0, 0, false)
 223                 base = b
 224                 if base == 0 {
 225                         return
 226                 }
 227                 hbits := heapBitsForAddr(base)
 228                 n := span.elemsize
 229                 for i = uintptr(0); i < n; i += sys.PtrSize {
 230                         if i != 1*sys.PtrSize && !hbits.morePointers() {
 231                                 // No more possible pointers.
 232                                 break
 233                         }
 234                         if hbits.isPointer() && cgoIsGoPointer(*(*unsafe.Pointer)(unsafe.Pointer(base + i))) {
 235                                 panic(errorString(msg))
 236                         }
 237                         hbits = hbits.next()
 238                 }
 239
 240                 return
 241         }
 242
 243         lo := 0
 244         hi := len(gcRootsIndex)
 245         for lo < hi {
 246                 m := lo + (hi-lo)/2
 247                 pr := gcRootsIndex[m]
 248                 addr := uintptr(pr.decl)
 249                 if cgoInRange(p, addr, addr+pr.size) {
 250                         cgoCheckBits(pr.decl, pr.gcdata, 0, pr.ptrdata)
 251                         return
 252                 }
 253                 if uintptr(p) < addr {
 254                         hi = m
 255                 } else {
 256                         lo = m + 1
 257                 }
 258         }
 259
 260         return
 261 }
 262
 263 // cgoIsGoPointer reports whether the pointer is a Go pointer--a
 264 // pointer to Go memory. We only care about Go memory that might
 265 // contain pointers.
 266 //go:nosplit
 267 //go:nowritebarrierrec
 268 func cgoIsGoPointer(p unsafe.Pointer) bool {
 269         if p == nil {
 270                 return false
 271         }
 272
 273         if inHeapOrStack(uintptr(p)) {
 274                 return true
 275         }
 276
 277         roots := gcRoots
 278         for roots != nil {
 279                 for i := 0; i < roots.count; i++ {
 280                         pr := roots.roots[i]
 281                         addr := uintptr(pr.decl)
 282                         if cgoInRange(p, addr, addr+pr.size) {
 283                                 return true
 284                         }
 285                 }
 286                 roots = roots.next
 287         }
 288
 289         return false
 290 }
 291
 292 // cgoInRange reports whether p is between start and end.
 293 //go:nosplit
 294 //go:nowritebarrierrec
 295 func cgoInRange(p unsafe.Pointer, start, end uintptr) bool {
 296         return start <= uintptr(p) && uintptr(p) < end
 297 }
 298
 299 // cgoCheckResult is called to check the result parameter of an
 300 // exported Go function. It panics if the result is or contains a Go
 301 // pointer.
 302 func cgoCheckResult(val interface{}) {
 303         if debug.cgocheck == 0 {
 304                 return
 305         }
 306
 307         ep := efaceOf(&val)
 308         t := ep._type
 309         cgoCheckArg(t, ep.data, t.kind&kindDirectIface == 0, false, cgoResultFail)
 310 }