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

From: Lars Ellenberg <[email-masked]>
Date: Fri, 30 Jul 2021 11:25:57 +0200

On Thu, Jul 29, 2021 at 07:57:47PM -0700, Bart Van Assche wrote:
> On 7/29/21 7:31 PM, Kees Cook 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 <[email-masked]>
> >>> ---
> >>>   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 don't have a strong opinion about this. Lars, do you want to comment
> on this patch?

Fine either way. "no binary changes" sounds good ;-)

Thanks,
    Lars