linux-kernel - Re: [PATCH] usb: ch9: Replace 1-element array with flexible array

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Date:   Wed, 14 Jun 2023 10:47:48 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        kernel test robot <oliver.sang@...el.com>,
        kernel test robot <lkp@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jó Ágila Bitsch <jgilab@...il.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] usb: ch9: Replace 1-element array with flexible array

On Tue, Jun 13, 2023 at 04:52:08PM -0600, Gustavo A. R. Silva wrote:
> On Tue, Jun 13, 2023 at 02:04:04PM -0700, Kees Cook wrote:
> > With "-fstrict-flex-arrays=3" enabled, UBSAN_BOUNDS no longer pretends
> > 1-element arrays are unbounded. Walking wData will trigger a warning,
> > so make it a proper flexible array. Add a union to keep the struct size
> > identical for userspace in case anything was depending on the old size.
> > 
> > Reported-by: kernel test robot <oliver.sang@...el.com>
> > Closes: https://lore.kernel.org/oe-lkp/202306102333.8f5a7443-oliver.sang@intel.com
> > Fixes: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
> 
> I always have mixed feelings about a 'Fixes' tag applied to a commit
> like this (one that enables a compiler option that avoids the introduction
> of buggy code), when we are addressing the potentially buggy code that
> the option is inteded to prevent. (thinkingface)

Yeah, I've been on the fence about this too. Since it's fixing a
(modern) coding style issue, there's nothing wrong technically. i.e. I
can't say "Fixes: ...usb commit..." since this isn't a bug. But it's
fixing a warning introduced by the fstrict-flex-arrays=3, and tracking
those issues is useful. But, it's not really fixing _that_ commit, as
it's doing exactly what it should be doing. So, perhaps, in the future I
can just mention it more directly in the commit log without a Fixes tag.
For example, this should probably have been written as:

 Since commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3"),
 UBSAN_BOUNDS no longer pretends 1-element arrays are unbounded. Walking
 wData will trigger a warning, so make it a proper flexible array. Add a
 union to keep the struct size identical for userspace in case anything
 was depending on the old size.

 Reported-by: kernel test robot <oliver.sang@...el.com> Closes:
 https://lore.kernel.org/oe-lkp/202306102333.8f5a7443-oliver.sang@intel.com

-Kees

-- 
Kees Cook