Message-Id: <20250514044440.48924-1-mhklinux@outlook.com>
Date: Tue, 13 May 2025 21:44:40 -0700
From: mhkelley58@...il.com
To: kys@...rosoft.com,
	haiyangz@...rosoft.com,
	wei.liu@...nel.org,
	decui@...rosoft.com,
	lpieralisi@...nel.org,
	kw@...ux.com,
	manivannan.sadhasivam@...aro.org,
	robh@...nel.org,
	bhelgaas@...gle.com,
	gustavoars@...nel.org
Cc: linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-hyperv@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: [PATCH 1/1] PCI: hv: Remove unnecessary flex array in struct pci_packet

From: Michael Kelley <mhklinux@...look.com>

struct pci_packet contains a "message" field that is a flex array
of struct pci_message. struct pci_packet is usually followed by a
second struct in a containing struct that is defined locally in
individual functions in pci-hyperv.c. As such, the compiler
flag -Wflex-array-member-not-at-end (introduced in gcc-14) generates
multiple warnings such as:

drivers/pci/controller/pci-hyperv.c:3809:35: warning: structure
    containing a flexible array member is not at the end of another
    structure [-Wflex-array-member-not-at-end]

The Linux kernel intends to introduce this compiler flag in standard
builds, so the current code is problematic in generating these warnings.

The "message" field is used only to locate the start of the second
struct, and not as an array. Because the second struct can be
addressed directly, the "message" field is not really necessary.
Rather than try to fix its usage to meet the requirements of
-Wflex-array-member-not-at-end, just eliminate the field and
either directly reference the second struct, or use "pkt + 1"
when "pkt" is dynamically allocated.

Reported-by: Gustavo A. R. Silva <gustavoars@...nel.org>
Signed-off-by: Michael Kelley <mhklinux@...look.com>
---
Original discussion of the issue is here:

https://lore.kernel.org/linux-hyperv/aAu8qsMQlbgH82iN@kspp/

 drivers/pci/controller/pci-hyperv.c | 29 ++++++++++++++---------------
 1 file changed, 14 insertions(+), 15 deletions(-)

diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
index e1eaa24559a2..ca5459e0dfcb 100644
--- a/drivers/pci/controller/pci-hyperv.c
+++ b/drivers/pci/controller/pci-hyperv.c
@@ -309,8 +309,6 @@ struct pci_packet {
 	void (*completion_func)(void *context, struct pci_response *resp,
 				int resp_packet_size);
 	void *compl_ctxt;
-
-	struct pci_message message[];
 };
 
 /*
@@ -1438,7 +1436,7 @@ static int hv_read_config_block(struct pci_dev *pdev, void *buf,
 	memset(&pkt, 0, sizeof(pkt));
 	pkt.pkt.completion_func = hv_pci_read_config_compl;
 	pkt.pkt.compl_ctxt = &comp_pkt;
-	read_blk = (struct pci_read_block *)&pkt.pkt.message;
+	read_blk = (struct pci_read_block *)pkt.buf;
 	read_blk->message_type.type = PCI_READ_BLOCK;
 	read_blk->wslot.slot = devfn_to_wslot(pdev->devfn);
 	read_blk->block_id = block_id;
@@ -1518,7 +1516,7 @@ static int hv_write_config_block(struct pci_dev *pdev, void *buf,
 	memset(&pkt, 0, sizeof(pkt));
 	pkt.pkt.completion_func = hv_pci_write_config_compl;
 	pkt.pkt.compl_ctxt = &comp_pkt;
-	write_blk = (struct pci_write_block *)&pkt.pkt.message;
+	write_blk = (struct pci_write_block *)pkt.buf;
 	write_blk->message_type.type = PCI_WRITE_BLOCK;
 	write_blk->wslot.slot = devfn_to_wslot(pdev->devfn);
 	write_blk->block_id = block_id;
@@ -1599,7 +1597,7 @@ static void hv_int_desc_free(struct hv_pci_dev *hpdev,
 		return;
 	}
 	memset(&ctxt, 0, sizeof(ctxt));
-	int_pkt = (struct pci_delete_interrupt *)&ctxt.pkt.message;
+	int_pkt = (struct pci_delete_interrupt *)ctxt.buffer;
 	int_pkt->message_type.type =
 		PCI_DELETE_INTERRUPT_MESSAGE;
 	int_pkt->wslot.slot = hpdev->desc.win_slot.slot;
@@ -2482,7 +2480,7 @@ static struct hv_pci_dev *new_pcichild_device(struct hv_pcibus_device *hbus,
 	comp_pkt.hpdev = hpdev;
 	pkt.init_packet.compl_ctxt = &comp_pkt;
 	pkt.init_packet.completion_func = q_resource_requirements;
-	res_req = (struct pci_child_message *)&pkt.init_packet.message;
+	res_req = (struct pci_child_message *)pkt.buffer;
 	res_req->message_type.type = PCI_QUERY_RESOURCE_REQUIREMENTS;
 	res_req->wslot.slot = desc->win_slot.slot;
 
@@ -2860,7 +2858,7 @@ static void hv_eject_device_work(struct work_struct *work)
 		pci_destroy_slot(hpdev->pci_slot);
 
 	memset(&ctxt, 0, sizeof(ctxt));
-	ejct_pkt = (struct pci_eject_response *)&ctxt.pkt.message;
+	ejct_pkt = (struct pci_eject_response *)ctxt.buffer;
 	ejct_pkt->message_type.type = PCI_EJECTION_COMPLETE;
 	ejct_pkt->wslot.slot = hpdev->desc.win_slot.slot;
 	vmbus_sendpacket(hbus->hdev->channel, ejct_pkt,
@@ -3118,7 +3116,7 @@ static int hv_pci_protocol_negotiation(struct hv_device *hdev,
 	init_completion(&comp_pkt.host_event);
 	pkt->completion_func = hv_pci_generic_compl;
 	pkt->compl_ctxt = &comp_pkt;
-	version_req = (struct pci_version_request *)&pkt->message;
+	version_req = (struct pci_version_request *)(pkt + 1);
 	version_req->message_type.type = PCI_QUERY_PROTOCOL_VERSION;
 
 	for (i = 0; i < num_version; i++) {
@@ -3340,7 +3338,7 @@ static int hv_pci_enter_d0(struct hv_device *hdev)
 	init_completion(&comp_pkt.host_event);
 	pkt->completion_func = hv_pci_generic_compl;
 	pkt->compl_ctxt = &comp_pkt;
-	d0_entry = (struct pci_bus_d0_entry *)&pkt->message;
+	d0_entry = (struct pci_bus_d0_entry *)(pkt + 1);
 	d0_entry->message_type.type = PCI_BUS_D0ENTRY;
 	d0_entry->mmio_base = hbus->mem_config->start;
 
@@ -3498,20 +3496,20 @@ static int hv_send_resources_allocated(struct hv_device *hdev)
 
 		if (hbus->protocol_version < PCI_PROTOCOL_VERSION_1_2) {
 			res_assigned =
-				(struct pci_resources_assigned *)&pkt->message;
+				(struct pci_resources_assigned *)(pkt + 1);
 			res_assigned->message_type.type =
 				PCI_RESOURCES_ASSIGNED;
 			res_assigned->wslot.slot = hpdev->desc.win_slot.slot;
 		} else {
 			res_assigned2 =
-				(struct pci_resources_assigned2 *)&pkt->message;
+				(struct pci_resources_assigned2 *)(pkt + 1);
 			res_assigned2->message_type.type =
 				PCI_RESOURCES_ASSIGNED2;
 			res_assigned2->wslot.slot = hpdev->desc.win_slot.slot;
 		}
 		put_pcichild(hpdev);
 
-		ret = vmbus_sendpacket(hdev->channel, &pkt->message,
+		ret = vmbus_sendpacket(hdev->channel, pkt + 1,
 				size_res, (unsigned long)pkt,
 				VM_PKT_DATA_INBAND,
 				VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED);
@@ -3809,6 +3807,7 @@ static int hv_pci_bus_exit(struct hv_device *hdev, bool keep_devs)
 		struct pci_packet teardown_packet;
 		u8 buffer[sizeof(struct pci_message)];
 	} pkt;
+	struct pci_message *msg;
 	struct hv_pci_compl comp_pkt;
 	struct hv_pci_dev *hpdev, *tmp;
 	unsigned long flags;
@@ -3854,10 +3853,10 @@ static int hv_pci_bus_exit(struct hv_device *hdev, bool keep_devs)
 	init_completion(&comp_pkt.host_event);
 	pkt.teardown_packet.completion_func = hv_pci_generic_compl;
 	pkt.teardown_packet.compl_ctxt = &comp_pkt;
-	pkt.teardown_packet.message[0].type = PCI_BUS_D0EXIT;
+	msg = (struct pci_message *)pkt.buffer;
+	msg->type = PCI_BUS_D0EXIT;
 
-	ret = vmbus_sendpacket_getid(chan, &pkt.teardown_packet.message,
-				     sizeof(struct pci_message),
+	ret = vmbus_sendpacket_getid(chan, msg, sizeof(*msg),
 				     (unsigned long)&pkt.teardown_packet,
 				     &trans_id, VM_PKT_DATA_INBAND,
 				     VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED);
-- 
2.25.1

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