linux-kernel - Re: [PATCH v2 5/9] mm: Initialize struct vm_unmapped_area

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Date: Tue, 27 Feb 2024 18:16:35 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Kees Cook <keescook@...omium.org>
CC: Rick Edgecombe <rick.p.edgecombe@...el.com>, "Liam.Howlett@...cle.com"
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Subject: Re: [PATCH v2 5/9] mm: Initialize struct vm_unmapped_area_info



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.