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

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

[Gustavo A. R. Silva]

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@embeddedor.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

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

[Jason Gunthorpe]

On Thu, Feb 13, 2020 at 12:37:15PM -0600, Gustavo A. R. Silva wrote:
> 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@embeddedor.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(-)

Any reason to skip these two?

drivers/infiniband/core/mad_priv.h:     u8 mad[0];
drivers/infiniband/core/mad_priv.h:     u8 data[0];

And may as well touch these in the subsystem headers too:

include/rdma/ib_fmr_pool.h:     u64                 page_list[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/opa_vnic.h:        char *dev_priv[0];
include/rdma/rdmavt_mr.h:       struct rvt_segarray *map[0];    /* the segments */
include/rdma/rdmavt_qp.h:       struct rvt_sge sg_list[0];

?

Jason

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

[Gustavo A. R. Silva]

On 2/13/20 12:38, Jason Gunthorpe wrote:

> 
> Any reason to skip these two?
> 
> drivers/infiniband/core/mad_priv.h:     u8 mad[0];
> drivers/infiniband/core/mad_priv.h:     u8 data[0];
> 
> And may as well touch these in the subsystem headers too:
> 
> include/rdma/ib_fmr_pool.h:     u64                 page_list[0];
> include/rdma/ib_verbs.h:        u8      real_sz[0];
> include/rdma/ib_verbs.h:        u8      real_sz[0];
> include/rdma/ib_verbs.h:        u8      real_sz[0];
> include/rdma/ib_verbs.h:        u8      real_sz[0];
> include/rdma/ib_verbs.h:        u8      real_sz[0];
> include/rdma/ib_verbs.h:        u8      real_sz[0];
> include/rdma/ib_verbs.h:        u8      real_sz[0];
> include/rdma/ib_verbs.h:        u8      real_sz[0];
> include/rdma/ib_verbs.h:        u8      real_sz[0];
> include/rdma/opa_vnic.h:        char *dev_priv[0];
> include/rdma/rdmavt_mr.h:       struct rvt_segarray *map[0];    /* the segments */
> include/rdma/rdmavt_qp.h:       struct rvt_sge sg_list[0];
> 
> ?

OK. Let me tweak my script a bit to catch all of those.

Thanks for the feedback.
--
Gustavo