On Thu, Jul 29, 2021 at 7:31 PM Kees Cook [off-list ref] wrote:

On Wed, Jul 28, 2021 at 02:45:55PM -0700, Bart Van Assche wrote:

quoted

On 7/27/21 1:58 PM, Kees Cook wrote:

quoted

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 <redacted>
---
  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