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

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Date: Tue, 27 Feb 2024 10:07:26 -0800
From: Kees Cook <keescook@...omium.org>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>,
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Subject: Re: [PATCH v2 5/9] mm: Initialize struct vm_unmapped_area_info

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.

> 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.
:)

-- 
Kees Cook