RFR: 8187578: BitMap::reallocate should check if old_map is NULL

[David Holmes]

Hi Erik,

On 15/09/2017 11:04 PM, Erik Helin wrote:
> Hi all,
> 
> I'm still trying to compile with gcc 7.1.1 and run into another small 
> issue. BitMap::reallocate calls Copy::disjoint_words and there is a case 
> in Copy::disjoint_words that might result in call to memcpy. The problem 
> is that BitMap::reallocate does not check that the "from" argument to 
> Copy::disjoint_words differs from NULL, and a call to memcpy with a NULL 
> argument is undefined behavior.

Shouldn't this whole function be a no-op if old_map is NULL? Wouldn't 
calling it with NULL be a programming error that we should be checking 
via an assert?

Thanks,
David

> Webrev:
> http://cr.openjdk.java.net/~ehelin/8187578/00/
> 
> Patch:
> --- old/src/hotspot/share/utilities/bitMap.cpp    2017-09-15 
> 14:47:21.471113699 +0200
> +++ new/src/hotspot/share/utilities/bitMap.cpp    2017-09-15 
> 14:47:21.179112252 +0200
> @@ -81,8 +81,10 @@
>     if (new_size_in_words > 0) {
>       map = allocator.allocate(new_size_in_words);
> 
> -    Copy::disjoint_words((HeapWord*)old_map, (HeapWord*) map,
> -                         MIN2(old_size_in_words, new_size_in_words));
> +    if (old_map != NULL) {
> +      Copy::disjoint_words((HeapWord*)old_map, (HeapWord*) map,
> +                           MIN2(old_size_in_words, new_size_in_words));
> +    }
> 
>       if (new_size_in_words > old_size_in_words) {
>         clear_range_of_words(map, old_size_in_words, new_size_in_words);
> 
> Thanks,
> Erik