Subject: [PATCH][next] hwtracing: stm: stm.h: Replace zero-length array with flexible-array member

From: "Gustavo A. R. Silva" <[email-masked]>
Date: Thu, 19 Mar 2020 16:40:07 -0500

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 <[email-masked]>
---
 drivers/hwtracing/stm/stm.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/hwtracing/stm/stm.h b/drivers/hwtracing/stm/stm.h
index 3569439d53bb..a9be49fc7a6b 100644
--- a/drivers/hwtracing/stm/stm.h
+++ b/drivers/hwtracing/stm/stm.h
@@ -23,7 +23,7 @@ void *stp_policy_node_priv(struct stp_policy_node *pn);
 
 struct stp_master {
 	unsigned int	nr_free;
-	unsigned long	chan_map[0];
+	unsigned long	chan_map[];
 };
 
 struct stm_device {
@@ -42,7 +42,7 @@ struct stm_device {
 	const struct config_item_type		*pdrv_node_type;
 	/* master allocation */
 	spinlock_t		mc_lock;
-	struct stp_master	*masters[0];
+	struct stp_master	*masters[];
 };
 
 #define to_stm_device(_d)				\
-- 
2.23.0

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