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

[Bart Van Assche]

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 <keesc...@chromium.org>
---
  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)?

Thanks,

Bart.