purecall isn't called when an exception occurs. purecall actually throws the exception - or exits the program (by default the crt throws up a dialog and then abort()s). in addition to cpu's email, raymond chen's article is a good (and short) read :) http://blogs.msdn.com/oldnewthing/archive/2004/04/28/122037.aspx
On Fri, Apr 3, 2009 at 3:15 PM, Huan Ren <hu...@google.com> wrote: > Based on what I saw in the bug, it looks like an exception happening > during CALL instruction may lead to PureCall(). > > For example, an object obj has been freed and later on someone calls > obj->func(). Then the assembly code looks like this: > > // ecx: pointer to obj which is in memory > // [ecx]: supposed to be pointer to vtable, it has invalid value since > obj is freed > // edx: now has pointer to vtable, which is invalid > mov edx,dword ptr [ecx] > > // deref the vtable and make the call > call dword ptr [edx+4] > > When a (hardware) exception happens during the call instruction, the > control will be eventually transfered to the routine handling this > type of exception which I *think* is PureCall(). > > Huan > > On Fri, Apr 3, 2009 at 11:26 AM, Ricardo Vargas <rvar...@chromium.org> > wrote: > > I certainly don't want to imply that it is the case with this particular > > bug, but I have seen crashes when the cause of the problem is using an > > object that was previously deleted (and only end up with this exception > when > > all the planets are properly aligned). I guess that it depends on the > actual > > class hierarchy of the objects in question, but I'd think that "simple" > > examples end up on a lot of crashes right after the cl that exposes the > > problem. > > > > On Fri, Apr 3, 2009 at 12:52 AM, Dean McNamee <de...@chromium.org> > wrote: > >> > >> You could, however, corrupt the vtable pointer (not the vtable). Say > >> somehow 32 was added to it, now the table is misaligned, and you might > >> get a purecall, etc. Not sure that's likely at all though. > >> > >> Since the vtable pointer is the first field, it seems ripe for > >> problems w/ use after free, etc. I kinda doubt that's what's > >> happening here though. Anyone who is working on one of these can bug > >> me and I'll look at the crash dump. > >> > >> On Fri, Apr 3, 2009 at 7:24 AM, Tommi <to...@chromium.org> wrote: > >> > On Thu, Apr 2, 2009 at 7:09 PM, cpu <c...@chromium.org> wrote: > >> >> > >> >> > >> >> > >> >> On Apr 2, 3:53 pm, Nicolas Sylvain <nsylv...@chromium.org> wrote: > >> >> > Another simple(r) example > >> >> > :http://msdn.microsoft.com/en-us/library/t296ys27(VS.80).aspx > >> >> > > >> >> > <http://msdn.microsoft.com/en-us/library/t296ys27(VS.80).aspx>But, > as > >> >> > discussed in bug 8544, we've see many purecall crashes that happens > >> >> > and > >> >> > we > >> >> > don't > >> >> > think it's related to virtual functions. The only thing I can think > >> >> > of > >> >> > is > >> >> > that the vtable is corrupted. (overwritten or freed) > >> >> > > >> >> > Does it not make sense? > >> >> > >> >> I don't think you can overwrite a vtables because they should be in > >> >> the code section of the executable (the pages marked as > read-execute), > >> >> they are known at compile time and it would not make sense to > >> >> construct them on the fly. > >> >> > >> >> But if you know of a case then that would be very interesting. > >> > > >> > > >> > yes they should be protected with read/execute and besides, you'd have > >> > to > >> > overwrite entries in the vtable with a pointer to __purecall for that > to > >> > happen > >> >> > >> >> > >> >> > >> >> > >> >> > > >> >> > Nicolas > >> >> > > >> >> > > >> >> > > >> >> > On Thu, Apr 2, 2009 at 1:54 PM, cpu <c...@chromium.org> wrote: > >> >> > > >> >> > > After reading some speculation in bugs such as > >> >> > >http://code.google.com/p/chromium/issues/detail?id=8544I felt > >> >> > > compelled to dispel some myths and misunderstandings about the > >> >> > > origin > >> >> > > and meaning of the mythical _purecall_ exception. My hope is that > >> >> > > then > >> >> > > you can spot the problems in our source code and fix them. Sorry > >> >> > > for > >> >> > > the long post. > >> >> > > >> >> > > So first of all, what do you see when you get this error? if you > >> >> > > are > >> >> > > in a debug build and you are not eating the exceptions via some > >> >> > > custom > >> >> > > handler you see this dialog: > >> >> > > >> >> > > --------------------------- > >> >> > > Debug Error! > >> >> > > R6025 > >> >> > > - pure virtual function call > >> >> > > (Press Retry to debug the application) > >> >> > > --------------------------- > >> >> > > Abort Retry Ignore > >> >> > > --------------------------- > >> >> > > >> >> > > For chrome/chromium we install a special handler, which forces a > >> >> > > crash > >> >> > > dump in which case you'll see in in the debugger analysis > something > >> >> > > like this: > >> >> > > >> >> > > [chrome_dll_main.cc:100] - `anonymous namespace'::PureCall() > >> >> > > [purevirt.c:47] - _purecall > >> >> > > >> >> > > Before going into too much detail, let me show you a small > program > >> >> > > that causes this exception: > >> >> > > >> >> > > ================================= > >> >> > > class Base { > >> >> > > public: > >> >> > > virtual ~Base() { > >> >> > > ThreeFn(); > >> >> > > } > >> >> > > >> >> > > virtual void OneFn() = 0; > >> >> > > virtual void TwoFn() = 0; > >> >> > > >> >> > > void ThreeFn() { > >> >> > > OneFn(); > >> >> > > TwoFn(); > >> >> > > } > >> >> > > }; > >> >> > > >> >> > > class Concrete : public Base { > >> >> > > public: > >> >> > > Concrete() : state_(0) { > >> >> > > } > >> >> > > >> >> > > virtual void OneFn() { > >> >> > > state_ += 1; > >> >> > > } > >> >> > > virtual void TwoFn() { > >> >> > > state_ += 2; > >> >> > > } > >> >> > > private: > >> >> > > int state_; > >> >> > > }; > >> >> > > >> >> > > int _tmain(int argc, _TCHAR* argv[]) { > >> >> > > >> >> > > Concrete* obj = new Concrete(); > >> >> > > obj->OneFn(); > >> >> > > obj->TwoFn(); > >> >> > > obj->ThreeFn(); > >> >> > > >> >> > > delete obj; > >> >> > > >> >> > > return 0; > >> >> > > } > >> >> > > ================================= > >> >> > > >> >> > > Can you spot the problem? do you know at which line it crashes, > do > >> >> > > you > >> >> > > know why? if so I have wasted your time, apologies. If you are > >> >> > > unsure > >> >> > > then read on. > >> >> > > >> >> > > This program crashes when trying to call OneFn() with a purecall > >> >> > > exception on debug build. On release build it exits with no > error, > >> >> > > but > >> >> > > your mileage might vary depending on what optimizations are > active. > >> >> > > >> >> > > The call stack for the crash is: > >> >> > > >> >> > > msvcr80d.dll!__purecall() + 0x25 > >> >> > > <------ > >> >> > > shows the > >> >> > > dialog (debug only) > >> >> > > app.exe!Base::ThreeFn() Line 16 + 0xfc <----- > error > >> >> > > here > >> >> > > app.exe!Base::~Base() Line 10 C++ > >> >> > > app.exe!Concrete::~Concrete() + 0x2b > >> >> > > app.exe!Concrete::`scalar deleting destructor'() + 0x2b > >> >> > > <----- > >> >> > > delete obj > >> >> > > >> >> > > So as you have guessed it has to do with calling virtual > functions > >> >> > > from a destructor. > >> >> > > >> >> > > What happens is that during construction an object evolves from > the > >> >> > > earliest base class to the actual type and during destruction the > >> >> > > object devolves (is that a word?) from the actual object to the > >> >> > > earliest base class; when we reach ~Base() body the object is no > >> >> > > longer of type Concrete but of type Base and thus the call > >> >> > > Base::OneFn > >> >> > > () is an error because that class does not in fact have any > >> >> > > implementation. > >> >> > > >> >> > > What the compiler does is create two vtables, the vtable of > >> >> > > Concrete > >> >> > > looks like this: > >> >> > > >> >> > > vtable 1: > >> >> > > [ 0 ] -> Concrete::OneFn() > >> >> > > [ 1 ] -> Concrete::TwoFn() > >> >> > > >> >> > > vtable 2: > >> >> > > [ 0 ]-> msvcr80d.dll!__purecall() > >> >> > > [ 1 ]-> msvcr80d.dll!__purecall() > >> >> > > >> >> > > The dtor of Concrete is the default dtor which does nothing > except > >> >> > > calling Base::~Base(), but the dtor of base does: > >> >> > > >> >> > > this->vtbl_ptr = vtable2 > >> >> > > call ThreeFn() > >> >> > > >> >> > > Now, why doesn't the release build crash? > >> >> > > >> >> > > That's because the compiler does not bother with generating the > >> >> > > second > >> >> > > vtable, after all is not going to be used and thus also > eliminates > >> >> > > the > >> >> > > related lines such as this->vtbl_ptr = vtable2. Therefore the > >> >> > > object > >> >> > > reaches the base dtor with the vtbl_ptr pointing to vtable1 which > >> >> > > makes the call ThreeFn() just work. > >> >> > > >> >> > > But that was just luck. If you ever modify the base class, such > as > >> >> > > introducing a new virtual function that is not pure, like this: > >> >> > > >> >> > > class Base { > >> >> > > public: > >> >> > > virtual ~Base() { > >> >> > > ThreeFn(); > >> >> > > } > >> >> > > >> >> > > virtual void OneFn() = 0; > >> >> > > virtual void TwoFn() = 0; > >> >> > > >> >> > > virtual void FourFn() { <--- new function, not pure > >> >> > > virtual > >> >> > > wprintf(L"aw snap"); > >> >> > > } > >> >> > > >> >> > > void ThreeFn() { > >> >> > > OneFn(); > >> >> > > TwoFn(); > >> >> > > } > >> >> > > }; > >> >> > > >> >> > > // Same program below. > >> >> > > // ....... > >> >> > > // ======================== > >> >> > > >> >> > > Then you are forcing the compiler to generate vtable 2, which > >> >> > > looks: > >> >> > > >> >> > > vtable 2: > >> >> > > [ 0 ]-> msvcr80d.dll!__purecall() > >> >> > > [ 1 ]-> msvcr80d.dll!__purecall() > >> >> > > [ 2 [-> Base::FourFn() > >> >> > > >> >> > > And now the purecall crash magically happens (on the same spot) > on > >> >> > > release builds, which is quite surprising since the trigger was > the > >> >> > > introduction of FourFn() which has _nothing_ to do with the crash > >> >> > > or > >> >> > > the problem and is many commits after the introduction of the > >> >> > > problem. > >> >> > > >> >> > > So the moral of the story? beware of virtual calls on dtors and > >> >> > > ctors. > >> >> > > Note that in practice this is quite tricky because of layers of > >> >> > > indirection / complexity of the code base. > >> >> > > >> >> > > ... so and what about the manbearpig ? Ah, yes no longer a myth: > >> >> > > >> >> > > >> >> > > > >> >> > > > > > http://www.thinkgene.com/scientists-successfully-create-human-bear-pi... > >> >> > > >> >> > > -cpu > >> >> > >> > > >> > > >> > > > >> > > >> > >> > > > > > > > > > > > --~--~---------~--~----~------------~-------~--~----~ Chromium Developers mailing list: chromium-dev@googlegroups.com View archives, change email options, or unsubscribe: http://groups.google.com/group/chromium-dev -~----------~----~----~----~------~----~------~--~---
