linux-kernel - [PATCH] IB/core: Replace zero-length array with flexible-array member

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Message-ID: <20200213183715.GA19636@embeddedor>
Date:   Thu, 13 Feb 2020 12:37:15 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Doug Ledford <dledford@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>
Cc:     linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] IB/core: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
---
 drivers/infiniband/core/cache.c     | 2 +-
 drivers/infiniband/core/cm.c        | 4 ++--
 drivers/infiniband/core/multicast.c | 2 +-
 drivers/infiniband/core/sa_query.c  | 2 +-
 4 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/infiniband/core/cache.c b/drivers/infiniband/core/cache.c
index 17bfedd24cc3..dedfbe27916f 100644
--- a/drivers/infiniband/core/cache.c
+++ b/drivers/infiniband/core/cache.c
@@ -46,7 +46,7 @@
 
 struct ib_pkey_cache {
 	int             table_len;
-	u16             table[0];
+	u16             table[];
 };
 
 struct ib_update_work {
diff --git a/drivers/infiniband/core/cm.c b/drivers/infiniband/core/cm.c
index 68cc1b2d6824..8424abe83607 100644
--- a/drivers/infiniband/core/cm.c
+++ b/drivers/infiniband/core/cm.c
@@ -197,7 +197,7 @@ struct cm_device {
 	struct ib_device *ib_device;
 	u8 ack_delay;
 	int going_down;
-	struct cm_port *port[0];
+	struct cm_port *port[];
 };
 
 struct cm_av {
@@ -216,7 +216,7 @@ struct cm_work {
 	__be32 local_id;			/* Established / timewait */
 	__be32 remote_id;
 	struct ib_cm_event cm_event;
-	struct sa_path_rec path[0];
+	struct sa_path_rec path[];
 };
 
 struct cm_timewait_info {
diff --git a/drivers/infiniband/core/multicast.c b/drivers/infiniband/core/multicast.c
index cd338ddc4a39..9c2d8b7f1af9 100644
--- a/drivers/infiniband/core/multicast.c
+++ b/drivers/infiniband/core/multicast.c
@@ -71,7 +71,7 @@ struct mcast_device {
 	struct ib_event_handler	event_handler;
 	int			start_port;
 	int			end_port;
-	struct mcast_port	port[0];
+	struct mcast_port	port[];
 };
 
 enum mcast_state {
diff --git a/drivers/infiniband/core/sa_query.c b/drivers/infiniband/core/sa_query.c
index 30d4c126a2db..74e0058fcf9e 100644
--- a/drivers/infiniband/core/sa_query.c
+++ b/drivers/infiniband/core/sa_query.c
@@ -101,7 +101,7 @@ struct ib_sa_port {
 struct ib_sa_device {
 	int                     start_port, end_port;
 	struct ib_event_handler event_handler;
-	struct ib_sa_port port[0];
+	struct ib_sa_port port[];
 };
 
 struct ib_sa_query {
-- 
2.25.0