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

Kees Cook
2024-02-27T18:07:28Z

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