checkptr.go

Documentation: runtime

     1  // Copyright 2019 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  package runtime
     6  
     7  import "unsafe"
     8  
     9  func checkptrAlignment(p unsafe.Pointer, elem *_type, n uintptr) {
    10  	// Check that (*[n]elem)(p) is appropriately aligned.
    11  	// Note that we allow unaligned pointers if the types they point to contain
    12  	// no pointers themselves. See issue 37298.
    13  	// TODO(mdempsky): What about fieldAlign?
    14  	if elem.ptrdata != 0 && uintptr(p)&(uintptr(elem.align)-1) != 0 {
    15  		throw("checkptr: unsafe pointer conversion")
    16  	}
    17  
    18  	// Check that (*[n]elem)(p) doesn't straddle multiple heap objects.
    19  	if size := n * elem.size; size > 1 && checkptrBase(p) != checkptrBase(add(p, size-1)) {
    20  		throw("checkptr: unsafe pointer conversion")
    21  	}
    22  }
    23  
    24  func checkptrArithmetic(p unsafe.Pointer, originals []unsafe.Pointer) {
    25  	if 0 < uintptr(p) && uintptr(p) < minLegalPointer {
    26  		throw("checkptr: unsafe pointer arithmetic")
    27  	}
    28  
    29  	// Check that if the computed pointer p points into a heap
    30  	// object, then one of the original pointers must have pointed
    31  	// into the same object.
    32  	base := checkptrBase(p)
    33  	if base == 0 {
    34  		return
    35  	}
    36  
    37  	for _, original := range originals {
    38  		if base == checkptrBase(original) {
    39  			return
    40  		}
    41  	}
    42  
    43  	throw("checkptr: unsafe pointer arithmetic")
    44  }
    45  
    46  // checkptrBase returns the base address for the allocation containing
    47  // the address p.
    48  //
    49  // Importantly, if p1 and p2 point into the same variable, then
    50  // checkptrBase(p1) == checkptrBase(p2). However, the converse/inverse
    51  // is not necessarily true as allocations can have trailing padding,
    52  // and multiple variables may be packed into a single allocation.
    53  func checkptrBase(p unsafe.Pointer) uintptr {
    54  	// stack
    55  	if gp := getg(); gp.stack.lo <= uintptr(p) && uintptr(p) < gp.stack.hi {
    56  		// TODO(mdempsky): Walk the stack to identify the
    57  		// specific stack frame or even stack object that p
    58  		// points into.
    59  		//
    60  		// In the mean time, use "1" as a pseudo-address to
    61  		// represent the stack. This is an invalid address on
    62  		// all platforms, so it's guaranteed to be distinct
    63  		// from any of the addresses we might return below.
    64  		return 1
    65  	}
    66  
    67  	// heap (must check after stack because of #35068)
    68  	if base, _, _ := findObject(uintptr(p), 0, 0); base != 0 {
    69  		return base
    70  	}
    71  
    72  	// data or bss
    73  	for _, datap := range activeModules() {
    74  		if datap.data <= uintptr(p) && uintptr(p) < datap.edata {
    75  			return datap.data
    76  		}
    77  		if datap.bss <= uintptr(p) && uintptr(p) < datap.ebss {
    78  			return datap.bss
    79  		}
    80  	}
    81  
    82  	return 0
    83  }
    84
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