Messages in this thread | | | Subject | Re: [PATCH v2 2/3] binder: do not initialize locals passed to copy_from_user() | From | Joe Perches <> | Date | Mon, 02 Mar 2020 05:58:33 -0800 |
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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?
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