Date:   Thu, 26 Nov 2020 15:53:27 +0100
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Edward Cree <ecree.xilinx@...il.com>
Cc:     James Bottomley <James.Bottomley@...senpartnership.com>,
        Kees Cook <keescook@...omium.org>,
        Jakub Kicinski <kuba@...nel.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        alsa-devel@...a-project.org,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        bridge@...ts.linux-foundation.org, ceph-devel@...r.kernel.org,
        cluster-devel@...hat.com, coreteam@...filter.org,
        devel@...verdev.osuosl.org, dm-devel@...hat.com,
        drbd-dev@...ts.linbit.com,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        GR-everest-linux-l2@...vell.com, GR-Linux-NIC-Dev@...vell.com,
        intel-gfx@...ts.freedesktop.org, intel-wired-lan@...ts.osuosl.org,
        keyrings@...r.kernel.org, linux1394-devel@...ts.sourceforge.net,
        linux-acpi@...r.kernel.org, linux-afs@...ts.infradead.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        linux-atm-general@...ts.sourceforge.net,
        linux-block@...r.kernel.org, linux-can@...r.kernel.org,
        linux-cifs@...r.kernel.org,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        linux-decnet-user@...ts.sourceforge.net,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-fbdev@...r.kernel.org, linux-geode@...ts.infradead.org,
        linux-gpio@...r.kernel.org, linux-hams@...r.kernel.org,
        linux-hwmon@...r.kernel.org, linux-i3c@...ts.infradead.org,
        linux-ide@...r.kernel.org, linux-iio@...r.kernel.org,
        linux-input <linux-input@...r.kernel.org>,
        linux-integrity@...r.kernel.org,
        linux-mediatek@...ts.infradead.org,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        linux-mmc@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        linux-mtd@...ts.infradead.org, linux-nfs@...r.kernel.org,
        linux-rdma@...r.kernel.org,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        linux-scsi@...r.kernel.org, linux-sctp@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-usb@...r.kernel.org, linux-watchdog@...r.kernel.org,
        linux-wireless <linux-wireless@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        netfilter-devel@...r.kernel.org, nouveau@...ts.freedesktop.org,
        op-tee@...ts.trustedfirmware.org, oss-drivers@...ronome.com,
        patches@...nsource.cirrus.com, rds-devel@....oracle.com,
        reiserfs-devel@...r.kernel.org, samba-technical@...ts.samba.org,
        selinux@...r.kernel.org, target-devel@...r.kernel.org,
        tipc-discussion@...ts.sourceforge.net,
        usb-storage@...ts.one-eyed-alien.net,
        virtualization@...ts.linux-foundation.org,
        wcn36xx@...ts.infradead.org,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        xen-devel@...ts.xenproject.org, linux-hardening@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Miguel Ojeda <ojeda@...nel.org>, Joe Perches <joe@...ches.com>
Subject: Re: [PATCH 000/141] Fix fall-through warnings for Clang

On Wed, Nov 25, 2020 at 11:44 PM Edward Cree <ecree.xilinx@...il.com> wrote:
>
> To make the intent clear, you have to first be certain that you
>  understand the intent; otherwise by adding either a break or a
>  fallthrough to suppress the warning you are just destroying the
>  information that "the intent of this code is unknown".

If you don't know what the intent of your own code is, then you
*already* have a problem in your hands.

> Figuring out the intent of a piece of unfamiliar code takes more
>  than 1 minute; just because
>     case foo:
>         thing;
>     case bar:
>         break;
>  produces identical code to
>     case foo:
>         thing;
>         break;
>     case bar:
>         break;
>  doesn't mean that *either* is correct — maybe the author meant

What takes 1 minute is adding it *mechanically* by the author, i.e. so
that you later compare whether codegen is the same.

>  to write
>     case foo:
>         return thing;
>     case bar:
>         break;
>  and by inserting that break you've destroyed the marker that
>  would direct someone who knew what the code was about to look
>  at that point in the code and spot the problem.

Then it means you already have a bug. This patchset gives the
maintainer a chance to notice it, which is a good thing. The "you've
destroyed the market" claim is bogus, because:
  1. you were not looking into it
  2. you didn't notice the bug so far
  3. is implicit -- harder to spot
  4. is only useful if you explicitly take a look at this kind of bug.
So why don't you do it now?

> Thus, you *always* have to look at more than just the immediate
>  mechanical context of the code, to make a proper judgement that
>  yes, this was the intent.

I find that is the responsibility of the maintainers and reviewers for
tree-wide patches like this, assuming they want. They can also keep
the behavior (and the bugs) without spending time. Their choice.

> If you think that that sort of thing
>  can be done in an *average* time of one minute, then I hope you
>  stay away from code I'm responsible for!

Please don't accuse others of recklessness or incompetence, especially
if you didn't understand what they said.

> A warning is only useful because it makes you *think* about the
>  code.  If you suppress the warning without doing that thinking,
>  then you made the warning useless; and if the warning made you
>  think about code that didn't *need* it, then the warning was
>  useless from the start.

We are not suppressing the warning. Quite the opposite, in fact.

> So make your mind up: does Clang's stricter -Wimplicit-fallthrough
>  flag up code that needs thought (in which case the fixes take
>  effort both to author and to review)

As I said several times already, it does take time to review if the
maintainer wants to take the chance to see if they had a bug to begin
with, but it does not require thought for the author if they just go
for equivalent codegen.

> or does it flag up code
>  that can be mindlessly "fixed" (in which case the warning is
>  worthless)?  Proponents in this thread seem to be trying to
>  have it both ways.

A warning is not worthless just because you can mindlessly fix it.
There are many counterexamples, e.g. many
checkpatch/lint/lang-format/indentation warnings, functional ones like
the `if (a = b)` warning...

Cheers,
Miguel

Powered by blists - more mailing lists