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dt _LIST_ENTRY 0x00080000+0x178+8
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This command dumps out the first node in the free list that corresponds to allocations of size 16 The 0x00080000 is the address of our heap We add an offset of 0x178 to get the start of the free list table The first entry in the free list table points to free list[0] Because our allocation is much smaller than the free list[0] size threshold, we simply skip this free list by adding an additional 8 bytes (the size of the _LIST_ENTRY structure), which puts us at free list[1] representing free blocks of size 16
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0:000> dt _LIST_ENTRY 0x00080000+0x178+8 [ 0x80180 - 0x80180 ] +0x000 Flink : 0x00080180 _LIST_ENTRY [ 0x80180 - 0x80180 ] +0x004 Blink : 0x00080180 _LIST_ENTRY [ 0x80180 - 0x80180 ]
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Remember that the free lists are doubly linked lists; hence the Flink and Blink fields of the _LIST_ENTRY structure are simply pointers to the next and previous allocations It is critical to note that the pointer listed in the free lists actually points to the user-accessible part of the heap block and not to the start of the heap block itself As such, if you want to look at the allocation metadata, you need to first subtract 8 bytes from the pointer Both of these pointers seem to point to 0x00080180, which in actuality is the address of the list node we were just dumping out (0x00080000+0x178+8=0x00080180) This implies that the free list corresponding to allocations of size 16 is empty Before we assume that the heap manager must commit more memory in the segment, remember that it will only do so as the absolute last resort Hence, the heap manager first tries to see if there are any other free blocks of sizes greater than 16 that it could split to satisfy the allocation In our particular case, free list[0] contains a free heap block:
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0:000> dt _LIST_ENTRY 0x00080000+0x178 [ 0x82ab0 - 0x82ab0 ] +0x000 Flink : 0x00082ab0 _LIST_ENTRY [ 0x80178 - 0x80178 ] +0x004 Blink : 0x00082ab0 _LIST_ENTRY [ 0x80178 - 0x80178 ]
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The Flink member points to the location in the heap block available to the caller In order to see the full heap block (including metadata), we must first subtract 8 bytes from the pointer (refer to Figure 68)
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0:000> dt +0x000 +0x002 +0x000 +0x004 +0x005 +0x006 +0x007 _HEAP_ENTRY 0x00082ab0-0x8 Size : 0xab PreviousSize : 0xb SubSegmentCode : 0x000b00ab SmallTagIndex : 0xee Flags : 0x14 UnusedBytes : 0xee SegmentIndex : 0
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It is important to note that the size reported is the true size of the heap block divided by the heap granularity The heap granularity is easily found by taking the size of the _HEAP_ENTY_STRUCTURE A heap block, the size of which is reported to be 0xab, is in reality 0xb8*8 = 0x558 (1368) bytes The free heap block we are looking at definitely seems to be big enough to fit our allocation request of size 16 In the debug session, step over the first instruction that calls HeapAlloc If successful, we can then check free list[0] again and see if the allocation we looked at prior to the call has changed:
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0:000> dt _LIST_ENTRY 0x00080000+0x178 [ 0x82ad8 - 0x82ad8 ] +0x000 Flink : 0x00082ad8 _LIST_ENTRY [ 0x80178 - 0x80178 ] +0x004 Blink : 0x00082ad8 _LIST_ENTRY [ 0x80178 - 0x80178 ] 0:000> dt _HEAP_ENTRY 0x00082ad8-0x8 +0x000 Size : 0xa6 +0x002 PreviousSize : 5 +0x000 SubSegmentCode : 0x000500a6 +0x004 SmallTagIndex : 0xee +0x005 Flags : 0x14 +0x006 UnusedBytes : 0xee +0x007 SegmentIndex : 0
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Sure enough, what used to be the first entry in free list[0] has now changed Instead of a free block of size 0xab, we now have a free block of size 0xa6 The difference in size (0x5) is due to our allocation request breaking up the larger free block we saw
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