The code below shows that it's possible to throw a purecall exception by
calling a function from a delete object.
I suspect this is what is happening in our code.
Nicolas
class Derived;
class Base {
public:
Base(Derived *derived): m_pDerived(derived) {};
~Base() {}; // Needed, dont know why.
virtual void function(void) = 0;
void bleh();
Derived * m_pDerived;
};
class Derived : public Base {
public:
Derived() : Base(this) {}; // C4355
virtual void function(void) {};
};
void Base::bleh() {
m_pDerived -> function();
}
void purecall(void) {
__debugbreak();
}
#include <windows.h>
int _tmain(int argc, _TCHAR* argv[]) {
_set_purecall_handler(purecall);
Base* base = NULL;
{
Derived myDerived;
myDerived.function();
base = &myDerived;
}
base->bleh();
}
On Fri, Apr 3, 2009 at 2:17 PM, Tommi <to...@chromium.org> wrote:
> 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
>> >> >>
>> >> >
>> >> >
>> >> > >
>> >> >
>> >>
>> >>
>> >
>> >
>> > >
>> >
>>
>
>
> >
>
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