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

[Joe Perches <joe@perches.com>]

On Mon, 2020-03-02 at 14:25 +0100, Alexander Potapenko wrote:
> On Mon, Mar 2, 2020 at 2:11 PM Joe Perches <joe@perches.com> wrote:
> > On Mon, 2020-03-02 at 14:04 +0100, glider@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.

So?  CONFIG_INIT_STACK_ALL by design slows down code.

This marking would likely need to be done for nearly all
3000+ copy_from_user entries.

Why not try to get something done on the compiler side
to mark the function itself rather than the uses?