linux-kernel - [PATCH] firewire: ohci: Replace zero-length array with flexible-array

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Date:   Thu, 7 May 2020 13:52:51 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Stefan Richter <stefanr@...6.in-berlin.de>
Cc:     linux1394-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: [PATCH] firewire: ohci: Replace zero-length array with flexible-array

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@...nel.org>
---
 drivers/firewire/core-cdev.c        |    2 +-
 drivers/firewire/core-transaction.c |    2 +-
 drivers/firewire/core.h             |    2 +-
 drivers/firewire/nosy.c             |    2 +-
 drivers/firewire/ohci.c             |    2 +-
 5 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/firewire/core-cdev.c b/drivers/firewire/core-cdev.c
index 6e291d8f3a27..e6fc20dff687 100644
--- a/drivers/firewire/core-cdev.c
+++ b/drivers/firewire/core-cdev.c
@@ -117,7 +117,7 @@ struct inbound_transaction_resource {
 struct descriptor_resource {
 	struct client_resource resource;
 	struct fw_descriptor descriptor;
-	u32 data[0];
+	u32 data[];
 };

 struct iso_resource {
diff --git a/drivers/firewire/core-transaction.c b/drivers/firewire/core-transaction.c
index 404a035f104d..439d918bbaaf 100644
--- a/drivers/firewire/core-transaction.c
+++ b/drivers/firewire/core-transaction.c
@@ -620,7 +620,7 @@ struct fw_request {
 	u32 request_header[4];
 	int ack;
 	u32 length;
-	u32 data[0];
+	u32 data[];
 };

 static void free_response_callback(struct fw_packet *packet,
diff --git a/drivers/firewire/core.h b/drivers/firewire/core.h
index 4b0e4ee655a1..71d5f16f311c 100644
--- a/drivers/firewire/core.h
+++ b/drivers/firewire/core.h
@@ -191,7 +191,7 @@ struct fw_node {
 	/* Upper layer specific data. */
 	void *data;

-	struct fw_node *ports[0];
+	struct fw_node *ports[];
 };

 static inline struct fw_node *fw_node_get(struct fw_node *node)
diff --git a/drivers/firewire/nosy.c b/drivers/firewire/nosy.c
index 6ca2f5ab6c57..5fd6a60b6741 100644
--- a/drivers/firewire/nosy.c
+++ b/drivers/firewire/nosy.c
@@ -52,7 +52,7 @@ struct pcl {

 struct packet {
 	unsigned int length;
-	char data[0];
+	char data[];
 };

 struct packet_buffer {
diff --git a/drivers/firewire/ohci.c b/drivers/firewire/ohci.c
index 33269316f111..54fdc39cd0bc 100644
--- a/drivers/firewire/ohci.c
+++ b/drivers/firewire/ohci.c
@@ -111,7 +111,7 @@ struct descriptor_buffer {
 	dma_addr_t buffer_bus;
 	size_t buffer_size;
 	size_t used;
-	struct descriptor buffer[0];
+	struct descriptor buffer[];
 };

 struct context {