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 via the write barrier when debug.cgocheck > 1.
     7  
     8  package runtime
     9  
    10  import (
    11  	"internal/goarch"
    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.ptrdata == 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.
    80  // src and dst are pointers to the first element of the slice.
    81  // typ is the element type of the slice.
    82  // It throws if the program is copying slice elements that contain Go pointers
    83  // into non-Go memory.
    84  //go:nosplit
    85  //go:nowritebarrier
    86  func cgoCheckSliceCopy(typ *_type, dst, src unsafe.Pointer, n int) {
    87  	if typ.ptrdata == 0 {
    88  		return
    89  	}
    90  	if !cgoIsGoPointer(src) {
    91  		return
    92  	}
    93  	if cgoIsGoPointer(dst) {
    94  		return
    95  	}
    96  	p := src
    97  	for i := 0; i < n; i++ {
    98  		cgoCheckTypedBlock(typ, p, 0, typ.size)
    99  		p = add(p, typ.size)
   100  	}
   101  }
   102  
   103  // cgoCheckTypedBlock checks the block of memory at src, for up to size bytes,
   104  // and throws if it finds a Go pointer. The type of the memory is typ,
   105  // and src is off bytes into that type.
   106  //go:nosplit
   107  //go:nowritebarrier
   108  func cgoCheckTypedBlock(typ *_type, src unsafe.Pointer, off, size uintptr) {
   109  	// Anything past typ.ptrdata is not a pointer.
   110  	if typ.ptrdata <= off {
   111  		return
   112  	}
   113  	if ptrdataSize := typ.ptrdata - off; size > ptrdataSize {
   114  		size = ptrdataSize
   115  	}
   116  
   117  	if typ.kind&kindGCProg == 0 {
   118  		cgoCheckBits(src, typ.gcdata, off, size)
   119  		return
   120  	}
   121  
   122  	// The type has a GC program. Try to find GC bits somewhere else.
   123  	for _, datap := range activeModules() {
   124  		if cgoInRange(src, datap.data, datap.edata) {
   125  			doff := uintptr(src) - datap.data
   126  			cgoCheckBits(add(src, -doff), datap.gcdatamask.bytedata, off+doff, size)
   127  			return
   128  		}
   129  		if cgoInRange(src, datap.bss, datap.ebss) {
   130  			boff := uintptr(src) - datap.bss
   131  			cgoCheckBits(add(src, -boff), datap.gcbssmask.bytedata, off+boff, size)
   132  			return
   133  		}
   134  	}
   135  
   136  	s := spanOfUnchecked(uintptr(src))
   137  	if s.state.get() == mSpanManual {
   138  		// There are no heap bits for value stored on the stack.
   139  		// For a channel receive src might be on the stack of some
   140  		// other goroutine, so we can't unwind the stack even if
   141  		// we wanted to.
   142  		// We can't expand the GC program without extra storage
   143  		// space we can't easily get.
   144  		// Fortunately we have the type information.
   145  		systemstack(func() {
   146  			cgoCheckUsingType(typ, src, off, size)
   147  		})
   148  		return
   149  	}
   150  
   151  	// src must be in the regular heap.
   152  
   153  	hbits := heapBitsForAddr(uintptr(src))
   154  	for i := uintptr(0); i < off+size; i += goarch.PtrSize {
   155  		bits := hbits.bits()
   156  		if i >= off && bits&bitPointer != 0 {
   157  			v := *(*unsafe.Pointer)(add(src, i))
   158  			if cgoIsGoPointer(v) {
   159  				throw(cgoWriteBarrierFail)
   160  			}
   161  		}
   162  		hbits = hbits.next()
   163  	}
   164  }
   165  
   166  // cgoCheckBits checks the block of memory at src, for up to size
   167  // bytes, and throws if it finds a Go pointer. The gcbits mark each
   168  // pointer value. The src pointer is off bytes into the gcbits.
   169  //go:nosplit
   170  //go:nowritebarrier
   171  func cgoCheckBits(src unsafe.Pointer, gcbits *byte, off, size uintptr) {
   172  	skipMask := off / goarch.PtrSize / 8
   173  	skipBytes := skipMask * goarch.PtrSize * 8
   174  	ptrmask := addb(gcbits, skipMask)
   175  	src = add(src, skipBytes)
   176  	off -= skipBytes
   177  	size += off
   178  	var bits uint32
   179  	for i := uintptr(0); i < size; i += goarch.PtrSize {
   180  		if i&(goarch.PtrSize*8-1) == 0 {
   181  			bits = uint32(*ptrmask)
   182  			ptrmask = addb(ptrmask, 1)
   183  		} else {
   184  			bits >>= 1
   185  		}
   186  		if off > 0 {
   187  			off -= goarch.PtrSize
   188  		} else {
   189  			if bits&1 != 0 {
   190  				v := *(*unsafe.Pointer)(add(src, i))
   191  				if cgoIsGoPointer(v) {
   192  					throw(cgoWriteBarrierFail)
   193  				}
   194  			}
   195  		}
   196  	}
   197  }
   198  
   199  // cgoCheckUsingType is like cgoCheckTypedBlock, but is a last ditch
   200  // fall back to look for pointers in src using the type information.
   201  // We only use this when looking at a value on the stack when the type
   202  // uses a GC program, because otherwise it's more efficient to use the
   203  // GC bits. This is called on the system stack.
   204  //go:nowritebarrier
   205  //go:systemstack
   206  func cgoCheckUsingType(typ *_type, src unsafe.Pointer, off, size uintptr) {
   207  	if typ.ptrdata == 0 {
   208  		return
   209  	}
   210  
   211  	// Anything past typ.ptrdata is not a pointer.
   212  	if typ.ptrdata <= off {
   213  		return
   214  	}
   215  	if ptrdataSize := typ.ptrdata - off; size > ptrdataSize {
   216  		size = ptrdataSize
   217  	}
   218  
   219  	if typ.kind&kindGCProg == 0 {
   220  		cgoCheckBits(src, typ.gcdata, off, size)
   221  		return
   222  	}
   223  	switch typ.kind & kindMask {
   224  	default:
   225  		throw("can't happen")
   226  	case kindArray:
   227  		at := (*arraytype)(unsafe.Pointer(typ))
   228  		for i := uintptr(0); i < at.len; i++ {
   229  			if off < at.elem.size {
   230  				cgoCheckUsingType(at.elem, src, off, size)
   231  			}
   232  			src = add(src, at.elem.size)
   233  			skipped := off
   234  			if skipped > at.elem.size {
   235  				skipped = at.elem.size
   236  			}
   237  			checked := at.elem.size - skipped
   238  			off -= skipped
   239  			if size <= checked {
   240  				return
   241  			}
   242  			size -= checked
   243  		}
   244  	case kindStruct:
   245  		st := (*structtype)(unsafe.Pointer(typ))
   246  		for _, f := range st.fields {
   247  			if off < f.typ.size {
   248  				cgoCheckUsingType(f.typ, src, off, size)
   249  			}
   250  			src = add(src, f.typ.size)
   251  			skipped := off
   252  			if skipped > f.typ.size {
   253  				skipped = f.typ.size
   254  			}
   255  			checked := f.typ.size - skipped
   256  			off -= skipped
   257  			if size <= checked {
   258  				return
   259  			}
   260  			size -= checked
   261  		}
   262  	}
   263  }
   264
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