[cfe-dev] Porting libcxxabi / Unwind to Windows / MingW 32 bit

[Yaron Keren]

Reviewing the code, the problem is not with processes. Without any Dwarf
support from Windows, MingW (Windows) libgcc load the EH frame by itself.
First it:

/* Stick a label at the beginning of the frame unwind info so we can
   register/deregister it with the exception handling library code.  */
static EH_FRAME_SECTION_CONST char __EH_FRAME_BEGIN__[]
  __attribute__((used, section(EH_FRAME_SECTION_NAME), aligned(4)))
  = { };

then the linker merges all EH frames together with the above being first so
that in runtime __EH_FRAME_BEGIN__ points to the start of the EH frame.

Finally at runtime the CRT init code registers the frame:

     register_frame_fn (__EH_FRAME_BEGIN__, &obj);

The root of our problem is that __EH_FRAME_BEGIN__ is registered with the
MingW (gcc) exception runtime library but not with our libunwind so it does
not know how to unwind its own functions. That's why the IP is out of range
the FDE tables.

So, findUnwindSections needs to know about __EH_FRAME_BEGIN__ . I'll try
this.

Yaron



2013/10/31 Yaron Keren <yaron.keren at gmail.com>

> I see. It is different than the gcc runtime libgcc.
>
> libgcc always uses the set of FDE which was registered by
> __register_frame. This data is private to every process since unwind runs
> in user space as part of the process.
>
> Here, the OS first locates which set of FDE is relevant.
> Does this mean that in OS-X unwind runs in system space and holds FDE data
> for all processes?
>
> Yaron
>
>
>
>
> 2013/10/31 Nick Kledzik <kledzik at apple.com>
>
>>
>> On Oct 30, 2013, at 3:22 PM, Yaron Keren <yaron.keren at gmail.com> wrote:
>>
>> Hi,
>>
>> You are exactly correct, LocalAddressSpace::findUnwindSections does
>> nothing #if !APPLE so the output value is random.
>>
>> LocalAddressSpace::findOtherFDE and LocalAddressSpace::findFunctionName
>> also currently do nothing. Are these also required for regular exceptions?
>>
>> No.
>>
>> findOtherFDE() is only need if the OS has an alternate way to locate FDEs
>> such as those for JITed code.
>>
>> findFunctionNam() is used in debug builds to print out the name of
>> functions as the unwinding happens.
>>
>> -Nick
>>
>>
>>
>> 2013/10/31 Nick Kledzik <kledzik at apple.com>
>>
>>> Yaron,
>>>
>>> Each separately linked image (program, DLLs, etc) has its own set of
>>> FDEs (in its .eh_frame) section.  The method
>>> LocalAddressSpace::findUnwindSections() needs to be updated in an OS
>>> specific way to: 1) find the image for a given pc value, and 2) find the
>>> .eh_frame section bounds for that image.
>>>
>>> -Nick
>>>
>>>
>>> On Oct 30, 2013, at 3:03 PM, Yaron Keren <yaron.keren at gmail.com> wrote:
>>>
>>> Hi Nick,
>>>
>>> I looked at the disassembly, unw_getcontext() gets the correct IP into
>>> the context and later setInfoBasedOnIPRegister gets it right into "pc".
>>> This is OK.
>>>
>>> The trouble is, the whole address space is nowhere near the fde values.
>>> For example,
>>>
>>> p =          0x66179070
>>> <libunwind::DwarfFDECache<libunwind::LocalAddressSpace>::_initialBuffer>
>>> pc =         0x6604e7d9
>>> fde =         0x40801c0
>>> p->ip_start = 0x4080010
>>> p->ip_end =   0x408010c
>>>
>>> p is consistent with pc and local vars and addresses in unwind.dll
>>> (0x66YYYYYY)
>>> fde is consistent with the ip ranges and with the caller of unwind.dll
>>> (the throwing function) address space (0x40800YY)
>>>
>>> they live in different memory ranges, the fde addresses are in caller
>>> range whereas the unwind addresses are in the DLL range so when it tries to
>>> find its IP in the fde tables it can't find.
>>>
>>> It's as if the DLL is supposed to load in other memory range, some
>>> linker option required?
>>>
>>> Yaron
>>>
>>>
>>>
>>> 2013/10/29 Nick Kledzik <kledzik at apple.com>
>>>
>>>>
>>>> On Oct 29, 2013, at 12:11 AM, Yaron Keren <yaron.keren at gmail.com>
>>>> wrote:
>>>>
>>>> > Hello,
>>>> >
>>>> > I'm trying to make libcxxabi / Unwind work on Windows / MingW 32 bit.
>>>> Attached is a patch to make it compile without warnings and run without
>>>> crashing.
>>>> >
>>>> > However the functionality isn't working yet: upon exception, the IP
>>>> seems to be completely out of range the FDE tables. This may be related to
>>>> the memory model / compilation options???
>>>> Take a look at UnwindRegisterRestore.s.  When an exception is thrown,
>>>> unw_getcontext() is called which saves all registers and the return address
>>>> into a register struct.  Then the FDE info is used to step through each
>>>> frame and modify the register set.  At which step is the IP register out of
>>>> bounds?
>>>>
>>>> It might be that the C files need to be compiled with -fexceptions so
>>>> that they have unwind tables.
>>>>
>>>> -Nick
>>>>
>>>>
>>>>
>>>
>>>
>>
>>
>
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