Re: [PATCH 48/64] drbd: Use struct_group() to zero algs

[Nick Desaulniers]

On Thu, Jul 29, 2021 at 7:31 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Wed, Jul 28, 2021 at 02:45:55PM -0700, Bart Van Assche wrote:
> > On 7/27/21 1:58 PM, Kees Cook wrote:
> > > In preparation for FORTIFY_SOURCE performing compile-time and run-time
> > > field bounds checking for memset(), avoid intentionally writing across
> > > neighboring fields.
> > >
> > > Add a struct_group() for the algs so that memset() can correctly reason
> > > about the size.
> > >
> > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > ---
> > >   drivers/block/drbd/drbd_main.c     | 3 ++-
> > >   drivers/block/drbd/drbd_protocol.h | 6 ++++--
> > >   drivers/block/drbd/drbd_receiver.c | 3 ++-
> > >   3 files changed, 8 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/block/drbd/drbd_main.c b/drivers/block/drbd/drbd_main.c
> > > index 55234a558e98..b824679cfcb2 100644
> > > --- a/drivers/block/drbd/drbd_main.c
> > > +++ b/drivers/block/drbd/drbd_main.c
> > > @@ -729,7 +729,8 @@ int drbd_send_sync_param(struct drbd_peer_device *peer_device)
> > >     cmd = apv >= 89 ? P_SYNC_PARAM89 : P_SYNC_PARAM;
> > >     /* initialize verify_alg and csums_alg */
> > > -   memset(p->verify_alg, 0, 2 * SHARED_SECRET_MAX);
> > > +   BUILD_BUG_ON(sizeof(p->algs) != 2 * SHARED_SECRET_MAX);
> > > +   memset(&p->algs, 0, sizeof(p->algs));
> > >     if (get_ldev(peer_device->device)) {
> > >             dc = rcu_dereference(peer_device->device->ldev->disk_conf);
> > > diff --git a/drivers/block/drbd/drbd_protocol.h b/drivers/block/drbd/drbd_protocol.h
> > > index dea59c92ecc1..a882b65ab5d2 100644
> > > --- a/drivers/block/drbd/drbd_protocol.h
> > > +++ b/drivers/block/drbd/drbd_protocol.h
> > > @@ -283,8 +283,10 @@ struct p_rs_param_89 {
> > >   struct p_rs_param_95 {
> > >     u32 resync_rate;
> > > -   char verify_alg[SHARED_SECRET_MAX];
> > > -   char csums_alg[SHARED_SECRET_MAX];
> > > +   struct_group(algs,
> > > +           char verify_alg[SHARED_SECRET_MAX];
> > > +           char csums_alg[SHARED_SECRET_MAX];
> > > +   );
> > >     u32 c_plan_ahead;
> > >     u32 c_delay_target;
> > >     u32 c_fill_target;
> > > diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
> > > index 1f740e42e457..6df2539e215b 100644
> > > --- a/drivers/block/drbd/drbd_receiver.c
> > > +++ b/drivers/block/drbd/drbd_receiver.c
> > > @@ -3921,7 +3921,8 @@ static int receive_SyncParam(struct drbd_connection *connection, struct packet_i
> > >     /* initialize verify_alg and csums_alg */
> > >     p = pi->data;
> > > -   memset(p->verify_alg, 0, 2 * SHARED_SECRET_MAX);
> > > +   BUILD_BUG_ON(sizeof(p->algs) != 2 * SHARED_SECRET_MAX);
> > > +   memset(&p->algs, 0, sizeof(p->algs));
> >
> > Using struct_group() introduces complexity. Has it been considered not to
> > modify struct p_rs_param_95 and instead to use two memset() calls instead of
> > one (one memset() call per member)?
>
> I went this direction because using two memset()s (or memcpy()s in other
> patches) changes the machine code. It's not much of a change, but it
> seems easier to justify "no binary changes" via the use of struct_group().
>
> If splitting the memset() is preferred, I can totally do that instead.
> :)

I'm not sure that compilers can fold memsets of adjacent members. It
might not matter, but you could wrap these members in a _named_ struct
then simply use assignment for optimal codegen.


-- 
Thanks,
~Nick Desaulniers