Hi all,
I finished implementing main features of a special mechanism for
monitoring uesrmode heap allocations in ReactOS. This mechanism is
called Debug Page Heap, after the very similar mechanism present in
recent versions of Microsoft Windows NT.
Explaining the heap manager, the "usual" heap allocator (the one I
also developed earlier) also contains a heap corruption detector, but
it works post-factum, only after the respective block is freed its
patterns are checked (if such heap flags are specified) and if they
are damaged the block is reported. This makes debugging problematic,
because a lot of code is executed between the moment the corruption
occured and moment when the corrupted block is freed. Also it may be
so that block's patterns are untouched, but internal heap structures
are damaged. The usual heap allocator won't be able to catch this,
but will crash with undetermined exception.
Debug page heap (DPH) comes to solve this problem. Simply speaking it
guards every block with a no-access page either after or before the
block, so that when a badly written app wants to write beyond the
allowed area, an exception occurs showing exactly the faulty
instruction.
Not so simply speaking, it's a rather complicated debugging tool with
abilities to catch access-after-free cases also when they happen, and
do some other nice tricks too. If you want to read more about it,
look for "debug page heap" in MSDN and also visit URLs specified as
references in rtl/heappage.c.
How to use it? The best way is to use Microsoft's utility gflags.exe
which is part of the Debugging package of WDK. To make long things
short, just copy gflags.exe to C:\ReactOS\system32, boot your ReactOS
installation, fire up cmd prompt and type:
gflags /p /enable name.exe /full
Now, when you run your app called name.exe, it will use the DPH heap
allocator.
Detailed information about gflags.exe could be found here: http://
msdn.microsoft.com/en-us/library/ff549557%28VS.85%29.aspx
Also, I suggest looking in MSDN for page heap explanation (I provided
some references in heappage.c too).
Have fun!
Aleksey.
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