[PATCH v2 2/3] binder: do not initialize locals passed to copy_from_user()

[Alexander Potapenko]

On Mon, Mar 2, 2020 at 2:11 PM Joe Perches <joe at perches.com> wrote:
>
> On Mon, 2020-03-02 at 14:04 +0100, glider at google.com wrote:
> > Certain copy_from_user() invocations in binder.c are known to
> > unconditionally initialize locals before their first use, like e.g. in
> > the following case:
> []
> > diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> []
> > @@ -3788,7 +3788,7 @@ static int binder_thread_write(struct binder_proc *proc,
> >
> >               case BC_TRANSACTION_SG:
> >               case BC_REPLY_SG: {
> > -                     struct binder_transaction_data_sg tr;
> > +                     struct binder_transaction_data_sg tr __no_initialize;
> >
> >                       if (copy_from_user(&tr, ptr, sizeof(tr)))
>
> I fail to see any value in marking tr with __no_initialize
> when it's immediately written to by copy_from_user.

This is being done exactly because it's immediately written to by copy_to_user()
Clang is currently unable to figure out that copy_to_user() initializes memory.
So building the kernel with CONFIG_INIT_STACK_ALL=y basically leads to
the following code:

  struct binder_transaction_data_sg tr;
  memset(&tr, 0xAA, sizeof(tr));
  if (copy_from_user(&tr, ptr, sizeof(tr))) {...}

This unnecessarily slows the code down, so we add __no_initialize to
prevent the compiler from emitting the redundant initialization.