On Thu, Jul 30, 2020 at 10:45:02PM +0200, Arnd Bergmann wrote:
> On Thu, Jul 30, 2020 at 8:10 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > On Thu, Jul 30, 2020 at 10:11:07AM +0200, Arnd Bergmann wrote:
> >
> > test_stackinit.c intended to use six cases (where "full" is in the sense
> > of "all members are named", this is intentionally testing the behavior
> > of padding hole initialization):
>
> Ok, so I read that correctly, thanks for confirming.
>
> > >
> > > struct test_big_hole var = *arg;
> >
> > So this one is a "whole structure copy" which I didn't have any tests
> > for, since I'd (perhaps inappropriately) assumed would be accomplished
> > with memcpy() internally, which means the incoming "*arg"'s padding holes
> > would be copied as-is. If the compiler is actually doing per-member copies
> > and leaving holes in "var" untouched, that's unexpected, so clearly that
> > needs to be added to test_stackinit.c! :)
>
> For some reason I remembered this not turning into a memcpy()
> somewhere, but I can't reproduce it in any of my recent attempts,
> just like what Denis found.
>
> > > or the a constructor like
> > >
> > > struct test_big_hole var;
> > > var = (struct test_big_hole){ .one = arg->one, .two=arg->two, .three
> > > = arg->three, .four = arg->four };
> > >
> > > Kees, do you know whether those two would behave differently?
> > > Would it make sense to also check for those, or am I perhaps
> > > misreading your code and it already gets checked?
> >
> > I *think* the above constructor would be covered under "full runtime
> > init", but it does also seem likely it would be handled similarly to
> > the "whole structure copy" in the previous example.
>
> I would assume that at least with C99 it is more like the
> "whole structure copy", based on the standard language of
>
> "The value of the compound literal is that of an unnamed
> object initialized by the initializer list. If the compound literal
> occurs outside the body of a function, the object has static
> storage duration; otherwise, it has automatic storage duration
> associated with the enclosing block."
>
> > I will go add more tests...
>
> Thanks!
Well, nearly exactly a year later, I've finally done this. :P
https://lore.kernel.org/lkml/20210723221933.3431999-1-keescook@xxxxxxxxxxxx
--
Kees Cook
