Re: [PATCH v2 5/9] mm: Initialize struct vm_unmapped_area_info

Christophe Leroy
2024-02-27T18:16:38Z

Le 27/02/2024 à 19:07, Kees Cook a écrit :
> On Tue, Feb 27, 2024 at 07:02:59AM +0000, Christophe Leroy wrote:
>>
>>
>> Le 26/02/2024 à 20:09, Rick Edgecombe a écrit :
>>> Future changes will need to add a field to struct vm_unmapped_area_info.
>>> This would cause trouble for any archs that don't initialize the
>>> struct. Currently every user sets each field, so if new fields are
>>> added, the core code parsing the struct will see garbage in the new
>>> field.
>>>
>>> It could be possible to initialize the new field for each arch to 0, but
>>> instead simply inialize the field with a C99 struct inializing syntax.
>>
>> Why doing a full init of the struct when all fields are re-written a few
>> lines after ?
>
> It's a nice change for robustness and makes future changes easier. It's
> not actually wasteful since the compiler will throw away all redundant
> stores.

Well, I tend to dislike default init at declaration because it often
hides missed real init. When a field is not initialized GCC should emit
a Warning, at least when built with W=2 which sets
-Wmissing-field-initializers ?

>
>> If I take the exemple of powerpc function slice_find_area_bottomup():
>>
>> struct vm_unmapped_area_info info;
>>
>> info.flags = 0;
>> info.length = len;
>> info.align_mask = PAGE_MASK & ((1ul << pshift) - 1);
>> info.align_offset = 0;
>
> But one cleanup that is possible from explicitly zero-initializing the
> whole structure would be dropping all the individual "= 0" assignments.
> :)
>

Sure if we decide to go that direction all those 0 assignments void.