linux-hardening - RE: [PATCH][next] PCI: hv: Avoid multiple -Wflex-array-member-not-at-end warnings

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Message-ID:
 <SN6PR02MB4157EC72378905EE97CA0934D491A@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Wed, 14 May 2025 04:47:19 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Wei Liu <wei.liu@...nel.org>
CC: "Gustavo A. R. Silva" <gustavoars@...nel.org>, "K. Y. Srinivasan"
	<kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, Dexuan Cui
	<decui@...rosoft.com>, Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>, Manivannan Sadhasivam
	<manivannan.sadhasivam@...aro.org>, Rob Herring <robh@...nel.org>, Bjorn
 Helgaas <bhelgaas@...gle.com>, "linux-hyperv@...r.kernel.org"
	<linux-hyperv@...r.kernel.org>, "linux-pci@...r.kernel.org"
	<linux-pci@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-hardening@...r.kernel.org"
	<linux-hardening@...r.kernel.org>
Subject: RE: [PATCH][next] PCI: hv: Avoid multiple
 -Wflex-array-member-not-at-end warnings

From: Wei Liu <wei.liu@...nel.org> Sent: Tuesday, May 13, 2025 9:45 AM
> 
> On Tue, May 13, 2025 at 02:07:45AM +0000, Michael Kelley wrote:
> > From: Michael Kelley <mhklinux@...look.com> Sent: Sunday, April 27, 2025 8:22 AM
> > >
> > > From: Gustavo A. R. Silva <gustavoars@...nel.org> Sent: Friday, April 25, 2025 9:48 AM
> > > >
> > > > -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> > > > getting ready to enable it, globally.
> > > >
> > > > Use the `DEFINE_RAW_FLEX()` helper for a few on-stack definitions
> > > > of a flexible structure where the size of the flexible-array member
> > > > is known at compile-time, and refactor the rest of the code,
> > > > accordingly.
> > > >
> > > > So, with these changes, fix the following warnings:
> > > >
> > > > 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]
> > > > drivers/pci/controller/pci-hyperv.c:2831:35: warning: structure containing a flexible
> > > > array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > > > drivers/pci/controller/pci-hyperv.c:2468:35: warning: structure containing a flexible
> > > > array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > > > drivers/pci/controller/pci-hyperv.c:1830:35: warning: structure containing a flexible
> > > > array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > > > drivers/pci/controller/pci-hyperv.c:1593:35: warning: structure containing a flexible
> > > > array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > > > drivers/pci/controller/pci-hyperv.c:1504:35: warning: structure containing a flexible
> > > > array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > > > drivers/pci/controller/pci-hyperv.c:1424:35: warning: structure containing a flexible
> > > > array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > >
> > > I'm supportive of cleaning up these warnings. I've worked with the pci-hyperv.c
> > > code a fair amount over the years, but never had looked closely at the on-stack
> > > structs that are causing the warnings. The current code is a bit unusual and
> > > perhaps unnecessarily obtuse.
> > >
> > > Rather than the approach you've taken below, I tried removing the flex array
> > > entirely from struct pci_packet. In all cases except one, it was used only to
> > > locate the end of struct pci_packet, which is the beginning of the follow-on
> > > message. Locating that follow-on message can easily be done by just referencing
> > > the "buf" field in the on-stack structs, or as (pkt + 1) in the dynamically allocated
> > > case. In both cases, there's no need for the flex array. In the one exception, a
> > > couple of minor tweaks avoids the need for the flex array as well.
> > >
> > > So here's an alternate approach to solving the problem. This approach is
> > > 14 insertions and 15 deletions, so it's a lot less change than your approach.
> > > I still don't understand why the on-stack struct are declared as (for example):
> > >
> > > 	struct {
> > > 		struct pci_packet pkt;
> > > 		char buf[sizeof(struct pci_read_block)];
> > > 	} pkt;
> > >
> > > instead of just:
> > >
> > > 	struct {
> > > 		struct pci_packet pkt;
> > > 		struct pci_read_block msg;
> > > 	} pkt;
> > >
> > > but that's a topic for another time.  Anyway, here's my proposed diff, which I've
> > > compiled and smoke-tested in a VM in the Azure cloud:
> > >
> >
> > Gustavo -- Are you waiting for me to submit a patch with my alternate proposal?
> > I had not seen any follow up, so wanted to make sure we have clarity on who
> > has the next action. Thx.
> 
> Michael, I prefer your approach. Please send a patch.
> 

Patch posted here:  https://lore.kernel.org/linux-hyperv/20250514044440.48924-1-mhklinux@outlook.com/

Michael