Subject: [PATCH] wcn36xx: Replace zero-length array with flexible-array

From: "Gustavo A. R. Silva" <[email-masked]>
Date: Thu, 7 May 2020 10:17:58 -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]

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 <[email-masked]>
---
 drivers/net/wireless/ath/wcn36xx/hal.h      | 4 ++--
 drivers/net/wireless/ath/wcn36xx/testmode.h | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/wireless/ath/wcn36xx/hal.h b/drivers/net/wireless/ath/wcn36xx/hal.h
index 6ba0fd57c951..aab5a58616fc 100644
--- a/drivers/net/wireless/ath/wcn36xx/hal.h
+++ b/drivers/net/wireless/ath/wcn36xx/hal.h
@@ -2240,7 +2240,7 @@ struct wcn36xx_hal_process_ptt_msg_req_msg {
 	struct wcn36xx_hal_msg_header header;
 
 	/* Actual FTM Command body */
-	u8 ptt_msg[0];
+	u8 ptt_msg[];
 } __packed;
 
 struct wcn36xx_hal_process_ptt_msg_rsp_msg {
@@ -2249,7 +2249,7 @@ struct wcn36xx_hal_process_ptt_msg_rsp_msg {
 	/* FTM Command response status */
 	u32 ptt_msg_resp_status;
 	/* Actual FTM Command body */
-	u8 ptt_msg[0];
+	u8 ptt_msg[];
 } __packed;
 
 struct update_edca_params_req_msg {
diff --git a/drivers/net/wireless/ath/wcn36xx/testmode.h b/drivers/net/wireless/ath/wcn36xx/testmode.h
index 4c6cfdb46580..09d68fab9add 100644
--- a/drivers/net/wireless/ath/wcn36xx/testmode.h
+++ b/drivers/net/wireless/ath/wcn36xx/testmode.h
@@ -20,7 +20,7 @@ struct ftm_rsp_msg {
 	u16 msg_id;
 	u16 msg_body_length;
 	u32 resp_status;
-	u8 msg_response[0];
+	u8 msg_response[];
 } __packed;
 
 /* The request buffer of FTM which contains a byte of command and the request */
-- 
2.26.2

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