Strange issue with recv in Launchpad Enhanced

Damjan Jovanovic damjan.jov at gmail.com
Fri Jun 26 12:13:16 CDT 2009


On Fri, Jun 26, 2009 at 2:19 AM, Erich Hoover<ehoover at mines.edu> wrote:
> On Sun, Jun 21, 2009 at 1:03 PM, Erich Hoover <ehoover at mines.edu> wrote:
>>
>> On Sun, Jun 21, 2009 at 10:23 AM, Erich Hoover <ehoover at mines.edu> wrote:
>>>
>>> On Sun, Jun 21, 2009 at 1:36 AM, Damjan Jovanovic <damjan.jov at gmail.com>
>>> wrote:
>>>>
>>>> ...
>>>>
>>>> Maybe the memory is writable but not readable, and
>>>> WSARecvFrom()/recv() is reading it while memcpy() is not?
>>>>
>>>> Maybe the memory is from a DIB section which Wine lazily mprotects and
>>>> the kernel isn't raising SIGSEGV for the protection to be reapplied?
>>>> Does simply zero-filling buf before calling WSARecvFrom() help?
>>>
>>> The memory should be a buffer from the calling application that it is
>>> using temporarily to store update data before saving it to the hard-disk.
>>> Yes, oddly enough zero-filling the buffer before calling WSARecvFrom() also
>>> fixes the problem.
>>>
>>> So, where exactly should I be looking to find the real problem?  As far
>>> as I can tell the memory for the buffer is being allocated immediate prior
>>> to the call and the request is for read/write access:
>>> 0009:Call KERNEL32.VirtualAlloc(01b85000,00040000,00001000,00000004)
>>> ret=79e74a2b
>>> 0009:Ret  KERNEL32.VirtualAlloc() retval=01b85000 ret=79e74a2b
>>> 0009:Call ws2_32.recv(00000380,01ba4fc1,000178d0,00000000) ret=0036a287
>>> ...
>>
>> After looking over the documentation for VirtualAlloc, it appears that
>> Wine should be zeroing the returned memory if MEM_COMMIT is specified.
>> Making this change (rather than playing around in the socket code) also
>> fixes the problem (see attached patch).  Do you think this step occurs if
>> write access isn't specified?
>>
>> Erich Hoover
>> ehoover at mines.edu
>
> Any thoughts on this?  Is there anything a "+relay" wouldn't catch that
> could occur between the call to KERNEL32.VirtualAlloc and the call to
> ws2_32.recv?  This behavior (the memory area having to be cleared before the
> recv call) seems a tad on the odd side.

All that zeroing the memory area does is commit the memory, so the
future recv() works.

But is the problem that VirtualAlloc() doesn't already do this, or
that WSARecvFrom() doesn't do it? Who does it in Windows?

The fact MEM_COMMIT is specified seems to imply that VirtualAlloc()
should do the committing, but some tests on Windows with that exact
same VirtualAlloc() and a VirtualQuery() after to check would help.

I really hope that it's a VirtualAlloc() bug in Wine - we make tonnes
of system calls all over the place assuming the memory passed is
already committed...

> Erich Hoover
> ehoover at mines.edu
>

Damjan Jovanovic



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