RFR: 8325095: C2: bailout message broken: ResourceArea allocated string used after free

[David Holmes]

On Mon, 5 Feb 2024 16:02:57 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> **Problem**
> Bailout `failure_reason` strings used to be either `nullptr` or a static string. But with [JDK-8303951](https://bugs.openjdk.org/browse/JDK-8303951) I had introduced dynamic strings (e.g. formatted using `stringStream::as_string()`). These dynamic strings were ResourceArea allocated, and have a very limited life-span, i.e. withing the most recent ResourceMark scope. The pointer is passed to `Compile::_failure_reason`, which already might leave that ResourceMark scope, and the pointer is invalid. And then the pointer is further passed to `ciEnv::_failure_reason`. And finally, it even gets passed up to `CompileBroker::invoke_compiler_on_method`. When the string is finally printed for the bailout message, we already left original `ResourceMark` scope, and also the `Compile` and `ciEnv` scopes. The pointer now points to basically random data.
> 
> **Solution**
> Whenever a string is passed to an outer scope, I make a CHeap copy, and the outer scope is the owner of that copy.
> This way every scope is the owner of its own copy, and can allocate and deallocate according to its own strategy safely.
> 
> I introduced a utility class `CHeapStringHolder`:
>  - `set`: make local copy on CHeap.
>  - `clear`: free local copy. We before `set`, and in the destructor. Thus, when the holder goes out of scope, the memory is automatically freed.
> 
> We have these 4 scopes:
> - `ResourceMark`: It allocates from `ResourcArea`, and deallocation is taken care at the end of the ResourceMark scope.
> - `Compile`: We turn the `_failure_reason` from a `char*` into a `CHeapStringHolder`. We set `_failure_reason` while the `ResourceMark` is live.
> - `ciEnv`: We turn the `_failure_reason` from a `char*` into a `CHeapStringHolder`. We set `_failure_reason` while `Compile` is live.
> - `CompileTask`: We used to just set `failure_reason`, which assumes that the string is static or elsewhere managed. Now I use pattern available in other places of `CompileBroker::invoke_compiler_on_method`: duplicate the string with `os::strdup` from some other scope, and set `reason_on_C_heap = true`, which means that we are the owner of that copy, and are responsible for freeing it later. Not sure if that is a nice pattern, but I don't want to refactor that code and it does what I need it to do.

src/hotspot/share/utilities/cHeapStringHolder.cpp line 36:

> 34:     _string[len] = 0; // terminating null
> 35:     // Make sure it is written before the pointer is used again
> 36:     OrderAccess::storestore();

Is this internal string being exposed to other threads ??? That seems dangerous in general given this is a stackobj.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17710#discussion_r1492016569