[PATCH] can: peak_canfd: Replace zero-length array with flexible-array

Gustavo A. R. Silva
2020-05-07T18:46:44Z

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]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

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 <gustavoars@kernel.org>
---
drivers/net/can/peak_canfd/peak_pciefd_main.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/can/peak_canfd/peak_pciefd_main.c b/drivers/net/can/peak_canfd/peak_pciefd_main.c
index d08a3d559114..6ad83a881039 100644
--- a/drivers/net/can/peak_canfd/peak_pciefd_main.c
+++ b/drivers/net/can/peak_canfd/peak_pciefd_main.c
@@ -146,7 +146,7 @@ struct pciefd_rx_dma {
__le32 irq_status;
__le32 sys_time_low;
__le32 sys_time_high;
- struct pucan_rx_msg msg[0];
+ struct pucan_rx_msg msg[];
} __packed __aligned(4);

/* Tx Link record */
@@ -194,7 +194,7 @@ struct pciefd_board {
struct pci_dev *pci_dev;
int can_count;
spinlock_t cmd_lock; /* 64-bits cmds must be atomic */
- struct pciefd_can *can[0]; /* array of network devices */
+ struct pciefd_can *can[]; /* array of network devices */
};

/* supported device ids. */