[PATCH 0/5] MR201: ntdll: Implement RtlSetUserValueHeap.

[Zebediah Figura]

On 6/7/22 14:46, Rémi Bernon (@rbernon) wrote:
> On Tue Jun  7 19:33:23 2022 +0000, **** wrote:
>> Zebediah Figura replied on the mailing list:
>> ```
>> On 6/7/22 13:33, Rémi Bernon wrote:
>>>    typedef struct
>>>    {
>>> +    SIZE_T __pad[sizeof(SIZE_T) / sizeof(DWORD)];
>>>        struct list           entry;      /* entry in heap large blocks
>> list */
>>>        SIZE_T                data_size;  /* size of user data */
>>>        SIZE_T                block_size; /* total size of virtual
>> memory block */
>>> -    DWORD                 pad[2];     /* padding to ensure 16-byte
>> alignment of data */
>>> +    void                 *user_value;
>>>        DWORD                 size;       /* fields for compatibility
>> with normal arenas */
>>>        DWORD                 magic;      /* these must remain at the
>> end of the structure */
>>>    } ARENA_LARGE;
>> Doesn't this end up wasting space on 64 bits? I.e. the structure is
>> already aligned to a multiple of 16 bytes without any padding.
>> (Also, somewhat out of curiosity, why move the member?)
>> ```
> It makes the large block header size match native size, of `8 * sizeof(void *)`.
> 
> Moving the member makes the struct layout closer to SUBHEAP, and I think the same struct could be use for both later. Native has a concept of heap region, which seems to correspond to both large blocks and SUBHEAP at the same time. For instance only growable heap let you allocate large blocks.
> 

Okay, makes sense.

In the future I guess it'd be nice to see that spelled out in the patch, 
or perhaps ideally as separate patches; that'd help make things clearer.