cgocheck.go

     1  // Copyright 2015 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  // Code to check that pointer writes follow the cgo rules.
     6  // These functions are invoked when GOEXPERIMENT=cgocheck2 is enabled.
     7  
     8  package runtime
     9  
    10  import (
    11  	"internal/goarch"
    12  	"unsafe"
    13  )
    14  
    15  const cgoWriteBarrierFail = "unpinned Go pointer stored into non-Go memory"
    16  
    17  // cgoCheckPtrWrite is called whenever a pointer is stored into memory.
    18  // It throws if the program is storing an unpinned Go pointer into non-Go
    19  // memory.
    20  //
    21  // This is called from generated code when GOEXPERIMENT=cgocheck2 is enabled.
    22  //
    23  //go:nosplit
    24  //go:nowritebarrier
    25  func cgoCheckPtrWrite(dst *unsafe.Pointer, src unsafe.Pointer) {
    26  	if !mainStarted {
    27  		// Something early in startup hates this function.
    28  		// Don't start doing any actual checking until the
    29  		// runtime has set itself up.
    30  		return
    31  	}
    32  	if !cgoIsGoPointer(src) {
    33  		return
    34  	}
    35  	if cgoIsGoPointer(unsafe.Pointer(dst)) {
    36  		return
    37  	}
    38  
    39  	// If we are running on the system stack then dst might be an
    40  	// address on the stack, which is OK.
    41  	gp := getg()
    42  	if gp == gp.m.g0 || gp == gp.m.gsignal {
    43  		return
    44  	}
    45  
    46  	// Allocating memory can write to various mfixalloc structs
    47  	// that look like they are non-Go memory.
    48  	if gp.m.mallocing != 0 {
    49  		return
    50  	}
    51  
    52  	// If the object is pinned, it's safe to store it in C memory. The GC
    53  	// ensures it will not be moved or freed.
    54  	if isPinned(src) {
    55  		return
    56  	}
    57  
    58  	// It's OK if writing to memory allocated by persistentalloc.
    59  	// Do this check last because it is more expensive and rarely true.
    60  	// If it is false the expense doesn't matter since we are crashing.
    61  	if inPersistentAlloc(uintptr(unsafe.Pointer(dst))) {
    62  		return
    63  	}
    64  
    65  	systemstack(func() {
    66  		println("write of unpinned Go pointer", hex(uintptr(src)), "to non-Go memory", hex(uintptr(unsafe.Pointer(dst))))
    67  		throw(cgoWriteBarrierFail)
    68  	})
    69  }
    70  
    71  // cgoCheckMemmove is called when moving a block of memory.
    72  // It throws if the program is copying a block that contains an unpinned Go
    73  // pointer into non-Go memory.
    74  //
    75  // This is called from generated code when GOEXPERIMENT=cgocheck2 is enabled.
    76  //
    77  //go:nosplit
    78  //go:nowritebarrier
    79  func cgoCheckMemmove(typ *_type, dst, src unsafe.Pointer) {
    80  	cgoCheckMemmove2(typ, dst, src, 0, typ.Size_)
    81  }
    82  
    83  // cgoCheckMemmove2 is called when moving a block of memory.
    84  // dst and src point off bytes into the value to copy.
    85  // size is the number of bytes to copy.
    86  // It throws if the program is copying a block that contains an unpinned Go
    87  // pointer into non-Go memory.
    88  //
    89  //go:nosplit
    90  //go:nowritebarrier
    91  func cgoCheckMemmove2(typ *_type, dst, src unsafe.Pointer, off, size uintptr) {
    92  	if typ.PtrBytes == 0 {
    93  		return
    94  	}
    95  	if !cgoIsGoPointer(src) {
    96  		return
    97  	}
    98  	if cgoIsGoPointer(dst) {
    99  		return
   100  	}
   101  	cgoCheckTypedBlock(typ, src, off, size)
   102  }
   103  
   104  // cgoCheckSliceCopy is called when copying n elements of a slice.
   105  // src and dst are pointers to the first element of the slice.
   106  // typ is the element type of the slice.
   107  // It throws if the program is copying slice elements that contain unpinned Go
   108  // pointers into non-Go memory.
   109  //
   110  //go:nosplit
   111  //go:nowritebarrier
   112  func cgoCheckSliceCopy(typ *_type, dst, src unsafe.Pointer, n int) {
   113  	if typ.PtrBytes == 0 {
   114  		return
   115  	}
   116  	if !cgoIsGoPointer(src) {
   117  		return
   118  	}
   119  	if cgoIsGoPointer(dst) {
   120  		return
   121  	}
   122  	p := src
   123  	for i := 0; i < n; i++ {
   124  		cgoCheckTypedBlock(typ, p, 0, typ.Size_)
   125  		p = add(p, typ.Size_)
   126  	}
   127  }
   128  
   129  // cgoCheckTypedBlock checks the block of memory at src, for up to size bytes,
   130  // and throws if it finds an unpinned Go pointer. The type of the memory is typ,
   131  // and src is off bytes into that type.
   132  //
   133  //go:nosplit
   134  //go:nowritebarrier
   135  func cgoCheckTypedBlock(typ *_type, src unsafe.Pointer, off, size uintptr) {
   136  	// Anything past typ.PtrBytes is not a pointer.
   137  	if typ.PtrBytes <= off {
   138  		return
   139  	}
   140  	if ptrdataSize := typ.PtrBytes - off; size > ptrdataSize {
   141  		size = ptrdataSize
   142  	}
   143  
   144  	if typ.Kind_&kindGCProg == 0 {
   145  		cgoCheckBits(src, typ.GCData, off, size)
   146  		return
   147  	}
   148  
   149  	// The type has a GC program. Try to find GC bits somewhere else.
   150  	for _, datap := range activeModules() {
   151  		if cgoInRange(src, datap.data, datap.edata) {
   152  			doff := uintptr(src) - datap.data
   153  			cgoCheckBits(add(src, -doff), datap.gcdatamask.bytedata, off+doff, size)
   154  			return
   155  		}
   156  		if cgoInRange(src, datap.bss, datap.ebss) {
   157  			boff := uintptr(src) - datap.bss
   158  			cgoCheckBits(add(src, -boff), datap.gcbssmask.bytedata, off+boff, size)
   159  			return
   160  		}
   161  	}
   162  
   163  	s := spanOfUnchecked(uintptr(src))
   164  	if s.state.get() == mSpanManual {
   165  		// There are no heap bits for value stored on the stack.
   166  		// For a channel receive src might be on the stack of some
   167  		// other goroutine, so we can't unwind the stack even if
   168  		// we wanted to.
   169  		// We can't expand the GC program without extra storage
   170  		// space we can't easily get.
   171  		// Fortunately we have the type information.
   172  		systemstack(func() {
   173  			cgoCheckUsingType(typ, src, off, size)
   174  		})
   175  		return
   176  	}
   177  
   178  	// src must be in the regular heap.
   179  
   180  	hbits := heapBitsForAddr(uintptr(src), size)
   181  	for {
   182  		var addr uintptr
   183  		if hbits, addr = hbits.next(); addr == 0 {
   184  			break
   185  		}
   186  		v := *(*unsafe.Pointer)(unsafe.Pointer(addr))
   187  		if cgoIsGoPointer(v) && !isPinned(v) {
   188  			throw(cgoWriteBarrierFail)
   189  		}
   190  	}
   191  }
   192  
   193  // cgoCheckBits checks the block of memory at src, for up to size
   194  // bytes, and throws if it finds an unpinned Go pointer. The gcbits mark each
   195  // pointer value. The src pointer is off bytes into the gcbits.
   196  //
   197  //go:nosplit
   198  //go:nowritebarrier
   199  func cgoCheckBits(src unsafe.Pointer, gcbits *byte, off, size uintptr) {
   200  	skipMask := off / goarch.PtrSize / 8
   201  	skipBytes := skipMask * goarch.PtrSize * 8
   202  	ptrmask := addb(gcbits, skipMask)
   203  	src = add(src, skipBytes)
   204  	off -= skipBytes
   205  	size += off
   206  	var bits uint32
   207  	for i := uintptr(0); i < size; i += goarch.PtrSize {
   208  		if i&(goarch.PtrSize*8-1) == 0 {
   209  			bits = uint32(*ptrmask)
   210  			ptrmask = addb(ptrmask, 1)
   211  		} else {
   212  			bits >>= 1
   213  		}
   214  		if off > 0 {
   215  			off -= goarch.PtrSize
   216  		} else {
   217  			if bits&1 != 0 {
   218  				v := *(*unsafe.Pointer)(add(src, i))
   219  				if cgoIsGoPointer(v) && !isPinned(v) {
   220  					throw(cgoWriteBarrierFail)
   221  				}
   222  			}
   223  		}
   224  	}
   225  }
   226  
   227  // cgoCheckUsingType is like cgoCheckTypedBlock, but is a last ditch
   228  // fall back to look for pointers in src using the type information.
   229  // We only use this when looking at a value on the stack when the type
   230  // uses a GC program, because otherwise it's more efficient to use the
   231  // GC bits. This is called on the system stack.
   232  //
   233  //go:nowritebarrier
   234  //go:systemstack
   235  func cgoCheckUsingType(typ *_type, src unsafe.Pointer, off, size uintptr) {
   236  	if typ.PtrBytes == 0 {
   237  		return
   238  	}
   239  
   240  	// Anything past typ.PtrBytes is not a pointer.
   241  	if typ.PtrBytes <= off {
   242  		return
   243  	}
   244  	if ptrdataSize := typ.PtrBytes - off; size > ptrdataSize {
   245  		size = ptrdataSize
   246  	}
   247  
   248  	if typ.Kind_&kindGCProg == 0 {
   249  		cgoCheckBits(src, typ.GCData, off, size)
   250  		return
   251  	}
   252  	switch typ.Kind_ & kindMask {
   253  	default:
   254  		throw("can't happen")
   255  	case kindArray:
   256  		at := (*arraytype)(unsafe.Pointer(typ))
   257  		for i := uintptr(0); i < at.Len; i++ {
   258  			if off < at.Elem.Size_ {
   259  				cgoCheckUsingType(at.Elem, src, off, size)
   260  			}
   261  			src = add(src, at.Elem.Size_)
   262  			skipped := off
   263  			if skipped > at.Elem.Size_ {
   264  				skipped = at.Elem.Size_
   265  			}
   266  			checked := at.Elem.Size_ - skipped
   267  			off -= skipped
   268  			if size <= checked {
   269  				return
   270  			}
   271  			size -= checked
   272  		}
   273  	case kindStruct:
   274  		st := (*structtype)(unsafe.Pointer(typ))
   275  		for _, f := range st.Fields {
   276  			if off < f.Typ.Size_ {
   277  				cgoCheckUsingType(f.Typ, src, off, size)
   278  			}
   279  			src = add(src, f.Typ.Size_)
   280  			skipped := off
   281  			if skipped > f.Typ.Size_ {
   282  				skipped = f.Typ.Size_
   283  			}
   284  			checked := f.Typ.Size_ - skipped
   285  			off -= skipped
   286  			if size <= checked {
   287  				return
   288  			}
   289  			size -= checked
   290  		}
   291  	}
   292  }
   293
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