[PATCH v3 3/3] net: mana: Avoid open coded arithmetic

[Erick Archer <erick.archer@outlook.com>]

This is an effort to get rid of all multiplications from allocation
functions in order to prevent integer overflows [1][2].

As the "req" variable is a pointer to "struct mana_cfg_rx_steer_req_v2"
and this structure ends in a flexible array:

struct mana_cfg_rx_steer_req_v2 {
        [...]
        mana_handle_t indir_tab[] __counted_by(num_indir_entries);
};

the preferred way in the kernel is to use the struct_size() helper to
do the arithmetic instead of the calculation "size + size * count" in
the kzalloc() function.

Moreover, use the "offsetof" helper to get the indirect table offset
instead of the "sizeof" operator and avoid the open-coded arithmetic in
pointers using the new flex member. This new structure member also allow
us to remove the "req_indir_tab" variable since it is no longer needed.

Now, it is also possible to use the "flex_array_size" helper to compute
the size of these trailing elements in the "memcpy" function.

This way, the code is more readable and safer.

This code was detected with the help of Coccinelle, and audited and
modified manually.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
Link: https://github.com/KSPP/linux/issues/160 [2]
Signed-off-by: Erick Archer <erick.archer@outlook.com>
---
 drivers/net/ethernet/microsoft/mana/mana_en.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/drivers/net/ethernet/microsoft/mana/mana_en.c b/drivers/net/ethernet/microsoft/mana/mana_en.c
index d8af5e7e15b4..f2fae659bf3b 100644
--- a/drivers/net/ethernet/microsoft/mana/mana_en.c
+++ b/drivers/net/ethernet/microsoft/mana/mana_en.c
@@ -1058,11 +1058,10 @@ static int mana_cfg_vport_steering(struct mana_port_context *apc,
 	struct mana_cfg_rx_steer_req_v2 *req;
 	struct mana_cfg_rx_steer_resp resp = {};
 	struct net_device *ndev = apc->ndev;
-	mana_handle_t *req_indir_tab;
 	u32 req_buf_size;
 	int err;
 
-	req_buf_size = sizeof(*req) + sizeof(mana_handle_t) * num_entries;
+	req_buf_size = struct_size(req, indir_tab, num_entries);
 	req = kzalloc(req_buf_size, GFP_KERNEL);
 	if (!req)
 		return -ENOMEM;
@@ -1074,7 +1073,8 @@ static int mana_cfg_vport_steering(struct mana_port_context *apc,
 
 	req->vport = apc->port_handle;
 	req->num_indir_entries = num_entries;
-	req->indir_tab_offset = sizeof(*req);
+	req->indir_tab_offset = offsetof(struct mana_cfg_rx_steer_req_v2,
+					 indir_tab);
 	req->rx_enable = rx;
 	req->rss_enable = apc->rss_state;
 	req->update_default_rxobj = update_default_rxobj;
@@ -1086,11 +1086,9 @@ static int mana_cfg_vport_steering(struct mana_port_context *apc,
 	if (update_key)
 		memcpy(&req->hashkey, apc->hashkey, MANA_HASH_KEY_SIZE);
 
-	if (update_tab) {
-		req_indir_tab = (mana_handle_t *)(req + 1);
-		memcpy(req_indir_tab, apc->rxobj_table,
-		       req->num_indir_entries * sizeof(mana_handle_t));
-	}
+	if (update_tab)
+		memcpy(req->indir_tab, apc->rxobj_table,
+		       flex_array_size(req, indir_tab, req->num_indir_entries));
 
 	err = mana_send_request(apc->ac, req, req_buf_size, &resp,
 				sizeof(resp));
-- 
2.25.1