On Sep 13, 2012, at 12:56 PM, ext Konstantin Tokarev <annu...@yandex.ru> wrote:
> 03.09.2012, 15:21, "Olivier Goffart" <oliv...@woboq.com>:
>
>> On Sunday 02 September 2012 23:10:18 Konstantin Tokarev wrote:
>>> Hi all,
>>>
>>> When building Qt 5 on big endian host (PPC) I've found moc breaking on Qt
>>> classes containing Q_PLUGIN_METADATA with
>>>
>>> ASSERT: "idx >= 0 && idx < s" in file
>>> ../../../include/QtCore/../../src/corelib/tools/qvarlengtharray.h, line
>>> 111
>>>
>>> It turned out to be a fault of QJsonPrivate::Parser::parseObject which has
>>> different code for handling of big endian and little endian cases:
>>>
>>> if (parsedObject.offsets.size()) {
>>> int tableSize = parsedObject.offsets.size()*sizeof(uint);
>> ^^^^^^^^^^^^^
>>
>> The error is there: one should multiply tableSize by sizeof(uint) only if
>> one
>> do a memcpy.
>>> table = reserveSpace(tableSize);
>>> #if Q_BYTE_ORDER == Q_LITTLE_ENDIAN
>>> memcpy(data + table, parsedObject.offsets.constData(), tableSize);
>>> #else
>>> offset *o = (offset *)(data + table);
>>> for (int i = 0; i < tableSize; ++i)
>>> o[i] = parsedObject.offsets[i];
>>>
>>> #endif
>>> }
>>>
>>> Could anyone explain why memcpy cannot be used for big endian case here?
>> I guess that's because the offsets needs to be stored in little endian.
>> offset is a typedef to a class that has an assignement operator which swap
>> the
>> bytes.
>
> It turned out to be not the only place where memcpy is working incorrectly.
> Unit test segfaults on memcpy in Value::copyData
>
> 404 case QJsonValue::Array:
> 405 case QJsonValue::Object: {
> 406 const QJsonPrivate::Base *b = v.base;
> 407 if (!b)
> 408 b = (v.t == QJsonValue::Array ? &emptyArray : &emptyObject);
> 409 memcpy(dest, b, b->size);
> 410 break;
> 411 }
>
> I tried to understand why memcpy fails here and how to replace it for BE case,
> but still don't have a clue. I'm also quite disappointed to see so heavily
> endian-
> dependent code written nowadays.
That's what you have to do when doing binary formats unfortunately. I think the
main problem is that we currently don't have a big endian machine at hand for
testing. I've tried to make it as endian independent as possible already, but
without being able to test you're bound to have bugs in such code.
As to why this would fail: The only reason I can see is if either dest is
invalid, or b->size contains garbage.
Cheers,
Lars
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