linux-hardening - Re: [PATCH 000/141] Fix fall-through warnings for Clang

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Message-ID: <CANiq72=z+tmuey9wj3Kk7wX5s0hTHpsQdLhAqcOVNrHon6xn5Q@mail.gmail.com>
Date:   Mon, 23 Nov 2020 15:05:31 +0100
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Finn Thain <fthain@...egraphics.com.au>
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Subject: Re: [PATCH 000/141] Fix fall-through warnings for Clang

On Sun, Nov 22, 2020 at 11:54 PM Finn Thain <fthain@...egraphics.com.au> wrote:
>
> We should also take into account optimisim about future improvements in
> tooling.

Not sure what you mean here. There is no reliable way to guess what
the intention was with a missing fallthrough, even if you parsed
whitespace and indentation.

> It is if you want to spin it that way.

How is that a "spin"? It is a fact that we won't get *implicit*
fallthrough mistakes anymore (in particular if we make it a hard
error).

> But what we inevitably get is changes like this:
>
>  case 3:
>         this();
> +       break;
>  case 4:
>         hmmm();
>
> Why? Mainly to silence the compiler. Also because the patch author argued
> successfully that they had found a theoretical bug, often in mature code.

If someone changes control flow, that is on them. Every kernel
developer knows what `break` does.

> But is anyone keeping score of the regressions? If unreported bugs count,
> what about unreported regressions?

Introducing `fallthrough` does not change semantics. If you are really
keen, you can always compare the objects because the generated code
shouldn't change.

Cheers,
Miguel