Re: [PATCH v2] overflow.h: Add flex_array_size() helper

Kees Cook
2020-06-17T03:48:54Z

On Tue, Jun 16, 2020 at 03:48:51PM -0500, Gustavo A. R. Silva wrote:
>
>
> On 6/10/20 16:38, Kees Cook wrote:
>
> >> -#define struct_size(p, member, n) \
> >> - __ab_c_size(n, \
> >> +#define struct_size(p, member, count) \
> >> + __ab_c_size(count, \
> >> sizeof(*(p)->member) + __must_be_array((p)->member),\
> >> sizeof(*(p)))
> >
> > I like the "count" change -- "n" can be seen as either count or bytes,
> > so I like this being distinctly "count".
> >
>
> Yep. :)
>
> >>
> >> +/**
> >> + * flex_array_size() - Calculate size, in bytes, of a flexible array member
> >> + * within an enclosing structure. Read on for more details.
> >> + *
> >> + * @p: Pointer to the structure.
> >> + * @member: Name of the flexible array member.
> >> + * @count: Number of elements in the array.
> >> + *
> >> + * Calculates size, in bytes, of a flexible array @member of @count elements
> >> + * within structure @p.
> >> + *
> >> + * Return: number of bytes needed or SIZE_MAX on overflow.
> >> + */
> >> +#define flex_array_size(p, member, count) \
> >> + array_size(count, \
> >> + sizeof(*(p)->member) + __must_be_array((p)->member))
> >> +
> >> #endif /* __LINUX_OVERFLOW_H */
> >
> > I like it! You mentioned off-list that maybe this could be named
> > sizeof_flex_array() (like sizeof_field(), etc), and that does seem
> > attractive. As you also mentioned, it begs the question of renaming
> > struct_size() to sizeof_struct().
> >
> > Looking back through the thread[1], it seems the name came from Linus[2],
> > and was more related to the existing array_size() helper.
> >
> > So, how about this, as a convention we can use to make a choice:
> >
> > For things that are strictly constant in size, we can use sizeof_*. For
> > things that have a dynamic component, we'll use *_size(). So, this patch
> > is correct as-is.
> >
>
> I like the idea. I haven't thought it in terms of dynamic and constant size,
> but it sounds sensible.
>
> > Acked-by: Kees Cook <keescook@chromium.org>
> >
> > (I wonder who's tree this should go via?)
> >
>
> Yours? :)

Done; I'll see if Linus will take this for -rc2 so you'll be able to use
it for v5.9 patches...

Thanks!

--
Kees Cook