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

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Message-ID: <HK0P153MB0148FB68FCBAE908CA5991C3BF1A0@HK0P153MB0148.APCP153.PROD.OUTLOOK.COM>
Date:   Thu, 13 Feb 2020 03:43:40 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        KY 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-hyperv@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] PCI: hv: Replace zero-length array with flexible-array
 member

> From: linux-hyperv-owner@...r.kernel.org
> <linux-hyperv-owner@...r.kernel.org> On Behalf Of Gustavo A. R. Silva
> Sent: Wednesday, February 12, 2020 4:51 PM
>  ...
> 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.

Looks good to me. Thanks, Gustavo!
 
Reviewed-by: Dexuan Cui <decui@...rosoft.com>

FWIW, it looks there are a lot of more to fix in the kernel tree: the below
commands return 1373 for me:

grep -nr '\[0\];$' * | grep '\.h:' | grep -v = | wc -l

Running the commands against the kernel/ directory returns 3.

Thanks,
-- Dexuan