linux-hardening - Re: [PATCH][next] treewide: uapi: Replace zero-length arrays with flexible-array members

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Date:   Tue, 28 Jun 2022 10:36:51 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Kees Cook <keescook@...omium.org>,
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Subject: Re: [PATCH][next] treewide: uapi: Replace zero-length arrays with
 flexible-array members

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.

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