RFR(M): 8247515: OSX pc_to_symbol() lookup does not work with core files

[Chris Plummer]

Hi Kevin,

Thanks for the review. Yes, that lack of initialization of 
lowest_offset_from_sym is a bug. I'm real surprised the compiler didn't 
catch it as it will be uninitialized garbage the first time it is 
referenced. Fortunately usually the eventual offset is very small if not 
0, so probably this never prevented a proper match. I think there's also 
another bug:

  415       uintptr_t offset_from_sym = offset - sym->offset;

"offset" is the passed in offset, essentially the address of the symbol 
we are interested in, but given as an offset from the start of the DSO. 
"sym->offset" is also an offset from the start of the DSO. It could be 
located before or after "offset". This means the math could result in a 
negative number, which when converted to unsigned would be a very large 
positive number. This happens whenever you check a symbol that is 
actually located after the address you are looking up. The end result is 
harmless, because it just means there's no way we will match that 
symbol, which is what you want, but it would be good to clean this up.

I think what is best is to use ptrdiff_t and initialize 
lowest_offset_from_sym to -1. I've updated the webrev:

http://cr.openjdk.java.net/~cjplummer/8247515/webrev.01/index.html

thanks,

Chris

On 7/7/20 4:09 AM, Kevin Walls wrote:
> Hi Chris,
>
> Yes I think this looks good.
>
> Question: In nearest_symbol, do we need to initialize 
> lowest_offset_from_sym to something impossibly high, as if it defaults 
> to zero we never find a better/nearer result?
>
> Thanks
> Kevin
>
>
> On 07/07/2020 06:10, Chris Plummer wrote:
>> Hello,
>>
>> Please help review the following:
>>
>> http://cr.openjdk.java.net/~cjplummer/8247515/webrev.00/index.html
>> https://bugs.openjdk.java.net/browse/JDK-8247515
>>
>> The CR contains a description of the issues being addressed. There is 
>> also no test for this symbol lookup support yet. It will be there 
>> after I push JDK-8247516 and  JDK-8247514, which are both blocked by 
>> the CR.
>>
>> [1] https://bugs.openjdk.java.net/browse/JDK-8247516
>> [2] https://bugs.openjdk.java.net/browse/JDK-8247514
>>
>> thanks,
>>
>> Chris
>>