[PATCH] pcdp: Replace zero-length array with flexible-array member

From: Gustavo A. R. Silva
Date: Fri Feb 21 2020 - 10:37:42 EST

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@xxxxxxxxxxxxxx>
drivers/firmware/pcdp.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/firmware/pcdp.h b/drivers/firmware/pcdp.h
index ce75d1da9e84..e02540571c52 100644
--- a/drivers/firmware/pcdp.h
+++ b/drivers/firmware/pcdp.h
@@ -103,6 +103,6 @@ struct pcdp {
u8 creator_id[4];
u32 creator_rev;
u32 num_uarts;
- struct pcdp_uart uart[0]; /* actual size is num_uarts */
+ struct pcdp_uart uart[]; /* actual size is num_uarts */
/* remainder of table is pcdp_device structures */
} __attribute__((packed));