Source file src/runtime/cgocheck.go

Documentation: runtime

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

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