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

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

[Gustavo A. R. Silva]

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@embeddedor.com>
---
 drivers/nfc/fdp/fdp.c      |  2 +-
 drivers/nfc/st21nfca/dep.c |  4 ++--
 include/net/nfc/nci.h      | 14 +++++++-------
 include/net/nfc/nfc.h      |  2 +-
 4 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/drivers/nfc/fdp/fdp.c b/drivers/nfc/fdp/fdp.c
index 062535c8251b..d4ff9e6056c2 100644
--- a/drivers/nfc/fdp/fdp.c
+++ b/drivers/nfc/fdp/fdp.c
@@ -76,7 +76,7 @@ static u8 nci_core_get_config_otp_ram_version[5] = {
 struct nci_core_get_config_rsp {
 	u8 status;
 	u8 count;
-	u8 data[0];
+	u8 data[];
 };
 
 static int fdp_nci_create_conn(struct nci_dev *ndev)
diff --git a/drivers/nfc/st21nfca/dep.c b/drivers/nfc/st21nfca/dep.c
index 137eb3f7dabd..98e73e047e02 100644
--- a/drivers/nfc/st21nfca/dep.c
+++ b/drivers/nfc/st21nfca/dep.c
@@ -66,7 +66,7 @@ struct st21nfca_atr_req {
 	u8 bsi;
 	u8 bri;
 	u8 ppi;
-	u8 gbi[0];
+	u8 gbi[];
 } __packed;
 
 struct st21nfca_atr_res {
@@ -79,7 +79,7 @@ struct st21nfca_atr_res {
 	u8 bri;
 	u8 to;
 	u8 ppi;
-	u8 gbi[0];
+	u8 gbi[];
 } __packed;
 
 struct st21nfca_psl_req {
diff --git a/include/net/nfc/nci.h b/include/net/nfc/nci.h
index 6ab5a83f597c..0550e0380b8d 100644
--- a/include/net/nfc/nci.h
+++ b/include/net/nfc/nci.h
@@ -244,13 +244,13 @@ struct dest_spec_params {
 struct core_conn_create_dest_spec_params {
 	__u8    type;
 	__u8    length;
-	__u8    value[0];
+	__u8    value[];
 } __packed;
 
 struct nci_core_conn_create_cmd {
 	__u8    destination_type;
 	__u8    number_destination_params;
-	struct core_conn_create_dest_spec_params params[0];
+	struct core_conn_create_dest_spec_params params[];
 } __packed;
 
 #define NCI_OP_CORE_CONN_CLOSE_CMD	nci_opcode_pack(NCI_GID_CORE, 0x05)
@@ -321,7 +321,7 @@ struct nci_core_init_rsp_1 {
 	__u8	status;
 	__le32	nfcc_features;
 	__u8	num_supported_rf_interfaces;
-	__u8	supported_rf_interfaces[0];	/* variable size array */
+	__u8	supported_rf_interfaces[];	/* variable size array */
 	/* continuted in nci_core_init_rsp_2 */
 } __packed;
 
@@ -338,7 +338,7 @@ struct nci_core_init_rsp_2 {
 struct nci_core_set_config_rsp {
 	__u8	status;
 	__u8	num_params;
-	__u8	params_id[0];	/* variable size array */
+	__u8	params_id[];	/* variable size array */
 } __packed;
 
 #define NCI_OP_CORE_CONN_CREATE_RSP	nci_opcode_pack(NCI_GID_CORE, 0x04)
@@ -501,18 +501,18 @@ struct nci_rf_nfcee_action_ntf {
 	__u8 nfcee_id;
 	__u8 trigger;
 	__u8 supported_data_length;
-	__u8 supported_data[0];
+	__u8 supported_data[];
 } __packed;
 
 #define NCI_OP_NFCEE_DISCOVER_NTF nci_opcode_pack(NCI_GID_NFCEE_MGMT, 0x00)
 struct nci_nfcee_supported_protocol {
 	__u8	num_protocol;
-	__u8	supported_protocol[0];
+	__u8	supported_protocol[];
 } __packed;
 
 struct nci_nfcee_information_tlv {
 	__u8	num_tlv;
-	__u8	information_tlv[0];
+	__u8	information_tlv[];
 } __packed;
 
 struct nci_nfcee_discover_ntf {
diff --git a/include/net/nfc/nfc.h b/include/net/nfc/nfc.h
index 5d277d68fd8d..2cd3a261bcbc 100644
--- a/include/net/nfc/nfc.h
+++ b/include/net/nfc/nfc.h
@@ -146,7 +146,7 @@ struct nfc_evt_transaction {
 	u32 aid_len;
 	u8 aid[NFC_MAX_AID_LENGTH];
 	u8 params_len;
-	u8 params[0];
+	u8 params[];
 } __packed;
 
 struct nfc_genl_data {
-- 
2.25.0

Re: [PATCH][next] NFC: Replace zero-length array with flexible-array member

[David Miller]

From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Date: Wed, 26 Feb 2020 15:20:00 -0600

> 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@embeddedor.com>

Applied.