linux-kernel - [PATCH] PCI: hv: Replace zero-length array with flexible-array member

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Message-ID: <20200213005048.GA9662@embeddedor.com>
Date:   Wed, 12 Feb 2020 18:50:48 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Andrew Murray <amurray@...goodpenguin.co.uk>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     linux-hyperv@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] PCI: hv: Replace zero-length array with flexible-array member

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@...eddedor.com>
---
 drivers/pci/controller/pci-hyperv.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
index 9977abff92fc..be957268f9d6 100644
--- a/drivers/pci/controller/pci-hyperv.c
+++ b/drivers/pci/controller/pci-hyperv.c
@@ -260,7 +260,7 @@ struct pci_packet {
 				int resp_packet_size);
 	void *compl_ctxt;
 
-	struct pci_message message[0];
+	struct pci_message message[];
 };
 
 /*
@@ -296,7 +296,7 @@ struct pci_bus_d0_entry {
 struct pci_bus_relations {
 	struct pci_incoming_message incoming;
 	u32 device_count;
-	struct pci_function_description func[0];
+	struct pci_function_description func[];
 } __packed;
 
 struct pci_q_res_req_response {
@@ -508,7 +508,7 @@ struct hv_dr_work {
 struct hv_dr_state {
 	struct list_head list_entry;
 	u32 device_count;
-	struct pci_function_description func[0];
+	struct pci_function_description func[];
 };
 
 enum hv_pcichild_state {
-- 
2.23.0