[Vm-dev] corruption of PC in context objects or not (?)

[Andrei Chis]

Hi,

We are getting often crashes on our CI when calling `Context>copyTo:` in a
GT image and a vm build from https://github.com/feenkcom/opensmalltalk-vm.

To sum up during `Context>copyTo:`, `Object>>#copy` is called on a
context leading to a segmentation fault crash. Looking at that context in
lldb the pc looks off.  It has the value `0xfffffffffea7f6e1`.

 (lldb) call (void *) printOop(0x1206b6990)
    0x1206b6990: a(n) Context
     0x1206b6a48 0xfffffffffea7f6e1                0x9
0x1146b2e08        0x1206b6b00
     0x1206b6b28        0x1206b6b50


Can this indicate some corruption or is it expected to have such values?
`CoInterpreter>>ensureContextHasBytecodePC:` has code that also handles
negative values for the pc which suggests that this might be expected.

Changing `Context>copyTo:` by adding a `self pc` before calling `self copy`
leads to no more crashes. Not sure if there is a reason for that or just
plain luck.

A simple reduced stack is below (more details in this issue [1]). The crash
happens always with contexts reified as objects (in this case 0x1206b6990 s
[] in GtExamplesCommandLineHandler>runPackages).
Could this suggest some kind of issue in the vm when reifying contexts, or
just some other problem with memory corruption?


 0x7ffeefbb4380 M Context(Object)>copy 0x1206b6990: a(n) Context
    0x7ffeefbb43b8 M Context>copyTo: 0x1206b6990: a(n) Context
    0x7ffeefbb4400 M Context>copyTo: 0x1206b5ae0: a(n) Context
  ...
    0x7ffeefba6078 M Context>copyTo: 0x110548b28: a(n) Context
    0x7ffeefba60d0 I Context>copyTo: 0x110548a70: a(n) Context
    0x7ffeefba6118 I MessageNotUnderstood(Exception)>freezeUpTo:
0x110548a20: a(n) MessageNotUnderstood
    0x7ffeefba6160 I MessageNotUnderstood(Exception)>freeze
0x110548a20: a(n) MessageNotUnderstood
    0x7ffeefba6190 M [] in GtExampleEvaluator>result 0x110544fb8: a(n)
GtExampleEvaluator
    0x7ffeefba61c8 M BlockClosure>cull: 0x110545188: a(n) BlockClosure
    0x7ffeefba6208 M Context>evaluateSignal: 0x110548c98: a(n) Context
    0x7ffeefba6240 M Context>handleSignal: 0x110548c98: a(n) Context
    0x7ffeefba6278 M Context>handleSignal: 0x110548be0: a(n) Context
    0x7ffeefba62b0 M MessageNotUnderstood(Exception)>signal
0x110548a20: a(n) MessageNotUnderstood
    0x7ffeefba62f0 M
GtDummyExamplesWithInheritanceSubclassB(Object)>doesNotUnderstand:
exampleH 0x1105487d8: a(n) GtDummyExamplesWithInheritanceSubclassB
    0x7ffeefba6328 M
GtExampleEvaluator>primitiveProcessExample:withEvaluationContext:
0x110544fb8: a(n) GtExampleEvaluator
 ...
    0x7ffeefbe64d0 M [] in GtExamplesHDReport class(HDReport
class)>runPackages: 0x1145e41c8: a(n) GtExamplesHDReport class
    0x7ffeefbe6520 M [] in Set>collect: 0x1206b5ab0: a(n) Set
    0x7ffeefbe6568 M Array(SequenceableCollection)>do: 0x1206b5c50: a(n) Array
       0x1206b5b98 s Set>collect:
       0x1206b5ae0 s GtExamplesHDReport class(HDReport class)>runPackages:
       0x1206b6990 s [] in GtExamplesCommandLineHandler>runPackages
       0x1206b6a48 s BlockClosure>ensure:
       0x1206b6b68 s UIManager class>nonInteractiveDuring:
       0x1206b6c48 s GtExamplesCommandLineHandler>runPackages
       0x1206b6d98 s GtExamplesCommandLineHandler>activate
       0x1206b75d0 s GtExamplesCommandLineHandler
class(CommandLineHandler class)>activateWith:
       0x1207d2f00 s [] in
PharoCommandLineHandler(BasicCommandLineHandler)>activateSubCommand:
       0x1207e6620 s BlockClosure>on:do:
       0x1207f7ab8 s
PharoCommandLineHandler(BasicCommandLineHandler)>activateSubCommand:
       0x120809d40 s
PharoCommandLineHandler(BasicCommandLineHandler)>handleSubcommand
       0x12082ca60 s
PharoCommandLineHandler(BasicCommandLineHandler)>handleArgument:
       0x120789938 s [] in
PharoCommandLineHandler(BasicCommandLineHandler)>activate
       0x1207a83e0 s BlockClosure>on:do:
       0x1207b57a0 s [] in
PharoCommandLineHandler(BasicCommandLineHandler)>activate
       0x1207bf830 s [] in BlockClosure>newProcess

Cheers,
Andrei


[1] https://github.com/feenkcom/gtoolkit/issues/1440
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