linux-hardening - Re: [PATCH RESEND][next] rtl8xxxu: Fix fall-through warnings for Clang

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Date:   Wed, 10 Mar 2021 11:45:57 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Jes Sorensen <jes.sorensen@...il.com>
Cc:     Kalle Valo <kvalo@...eaurora.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH RESEND][next] rtl8xxxu: Fix fall-through warnings for
 Clang

On Wed, Mar 10, 2021 at 02:31:57PM -0500, Jes Sorensen wrote:
> On 3/10/21 2:14 PM, Kees Cook wrote:
> > On Fri, Mar 05, 2021 at 03:40:33PM +0200, Kalle Valo wrote:
> >> "Gustavo A. R. Silva" <gustavoars@...nel.org> writes:
> >>
> >>> In preparation to enable -Wimplicit-fallthrough for Clang, fix
> >>> multiple warnings by replacing /* fall through */ comments with
> >>> the new pseudo-keyword macro fallthrough; instead of letting the
> >>> code fall through to the next case.
> >>>
> >>> Notice that Clang doesn't recognize /* fall through */ comments as
> >>> implicit fall-through markings.
> >>>
> >>> Link: https://github.com/KSPP/linux/issues/115
> >>> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> >>
> >> It's not cool that you ignore the comments you got in [1], then after a
> >> while mark the patch as "RESEND" and not even include a changelog why it
> >> was resent.
> >>
> >> [1] https://patchwork.kernel.org/project/linux-wireless/patch/d522f387b2d0dde774785c7169c1f25aa529989d.1605896060.git.gustavoars@kernel.org/
> > 
> > Hm, this conversation looks like a miscommunication, mainly? I see
> > Gustavo, as requested by many others[1], replacing the fallthrough
> > comments with the "fallthrough" statement. (This is more than just a
> > "Clang doesn't parse comments" issue.)
> > 
> > This could be a tree-wide patch and not bother you, but Greg KH has
> > generally advised us to send these changes broken out. Anyway, this
> > change still needs to land, so what would be the preferred path? I think
> > Gustavo could just carry it for Linus to merge without bothering you if
> > that'd be preferred?
> 
> I'll respond with the same I did last time, fallthrough is not C and
> it's ugly.

I understand your point of view, but this is not the consensus[1] of
the community. "fallthrough" is a macro, using the GCC fallthrough
attribute, with the expectation that we can move to the C17/C18
"[[fallthrough]]" statement once it is finalized by the C standards
body.

-Kees

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#implicit-switch-case-fall-through

-- 
Kees Cook