From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Subject: Re: [PATCH][next] treewide: uapi: Replace zero-length arrays with
 flexible-array members
Date: Tue, 28 Jun 2022 15:56:23 +0200
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To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Daniel Borkmann <daniel@iogearbox.net>, Kees Cook <keescook@chromium.org>, linux-kernel@vger.kernel.org, x86@kernel.org, dm-devel@redhat.com, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-s390@vger.kernel.org, kvm@vger.kernel.org, intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, netdev@vger.kernel.org, bpf@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-can@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux1394-devel@lists.sourceforge.net, io-uring@vger.kernel.org, lvs-devel@vger.kernel.org, linux-mtd@lists.infradead.org, kasan-dev@googlegroups.com, linux-mmc@vger.kernel.org, nvdimm@lists.linux.dev, netfilter-devel@vger.kernel.org, coreteam@netfilter.org, linux-perf-users@vger.kernel.org, linux-raid@vger.kernel.org, linux-sctp@vg

On Tue, Jun 28, 2022 at 10:36:51AM -0300, Jason Gunthorpe wrote:
> On Tue, Jun 28, 2022 at 04:21:29AM +0200, Gustavo A. R. Silva wrote:
> 
> > > > Though maybe we could just switch off -Wgnu-variable-sized-type-not-at-end  during configuration ?
> 
> > We need to think in a different strategy.
> 
> I think we will need to switch off the warning in userspace - this is
> doable for rdma-core.
> 
> On the other hand, if the goal is to enable the array size check
> compiler warning I would suggest focusing only on those structs that
> actually hit that warning in the kernel. IIRC infiniband doesn't
> trigger it because it just pointer casts the flex array to some other
> struct.

Yep; this is actually why I reverted those changes in rdma (before
sending out the patch) when 0-day reported the same problems you pointed
out[1].

Also, that's the strategy I'm following right now with the one-element
array into flex-array member transformations. I'm addressing those cases
in which the trailing array is actually being iterated over, first.

I just added the patch to my -next tree, so it can be build-tested by
other people, and let's see what else is reported this week. :)

--
Gustavo

[1] https://lore.kernel.org/lkml/620ca2a5.NkAEIDEfiYoxE9%2Fu%25lkp@intel.com/

> 
> It isn't actually an array it is a placeholder for a trailing
> structure, so it is never indexed.
> 
> This is also why we hit the warning because the convient way for
> userspace to compose the message is to squash the header and trailer
> structs together in a super struct on the stack, then invoke the
> ioctl.
> 
> Jason