issue27518a.go

     1  // run
     2  
     3  // Copyright 2018 The Go Authors. All rights reserved.
     4  // Use of this source code is governed by a BSD-style
     5  // license that can be found in the LICENSE file.
     6  
     7  package main
     8  
     9  import (
    10  	"runtime"
    11  )
    12  
    13  var nilp *int
    14  var forceHeap interface{}
    15  
    16  func main() {
    17  	// x is a pointer on the stack to heap-allocated memory.
    18  	x := new([32]*int)
    19  	forceHeap = x
    20  	forceHeap = nil
    21  
    22  	// Push a defer to be run when we panic below.
    23  	defer func() {
    24  		// Ignore the panic.
    25  		recover()
    26  		// Force a stack walk. Go 1.11 will fail because x is now
    27  		// considered live again.
    28  		runtime.GC()
    29  	}()
    30  	// Make x live at the defer's PC.
    31  	runtime.KeepAlive(x)
    32  
    33  	// x is no longer live. Garbage collect the [32]*int on the
    34  	// heap.
    35  	runtime.GC()
    36  	// At this point x's dead stack slot points to dead memory.
    37  
    38  	// Trigger a sigpanic. Since this is an implicit panic, we
    39  	// don't have an explicit liveness map here.
    40  	// Traceback used to use the liveness map of the most recent defer,
    41  	// but in that liveness map, x will be live again even though
    42  	// it points to dead memory. The fix is to use the liveness
    43  	// map of a deferreturn call instead.
    44  	*nilp = 0
    45  }
    46
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