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Re: [Xen-devel] [RFC PATCH 0/4] Add missing default labels to switch statements

On Fri, 22 Feb 2019, Julien Grall wrote:
> > > > BTW, I checked the series with -Wswitch-default:
> > > > -Wswitch-default
> > > > Warn whenever a switch statement does not have a default case.
> > > > > Furthermore, using BUG() is a pretty bad idea in switch. 
> > > > It is and not only in the switch. The reason I put BUG is that I tried
> > > > to follow
> > > > the existing "error handling" at those places.
> > > 
> > > It is not because BUG() is been used today in some places that we need to
> > > continue to spread it.
> > > 
> > > > Use of BUG() itself is another topic which will also need to be
> > > > addressed
> > > 
> > > So we should not add more of them...
> > Again, I see this as a dedicated change. So, in the current series I think
> > it is
> > acceptable to use the existing way of error handling if any at all.
> That's not how it works in upstream. If you know some constructs are wrong, it
> is best to try to address partially the problem directly then having so you
> reduce the amounts of change afterwards.
> So please try to not introduce more BUG() in the code base.

Hi Oleksandr, Julien,

Julien's right that we should not introduce any more BUG()s. In fact,
each of them makes the code less safe, not more safe! The purpose of
MISRAC 16.4 is "defensive programming": write the code in a way that is
more (not less!) resilient to failure.

So, I think it is a good idea to introduce a default label because it
can help us spot unexpected issues. Instead of calling BUG() in the
default handler, which is detrimental, we should return an error when
possible, or just print a warning.

As 16.4 clearly state, even a simple comment would be enough to address
the rule. We just need to explain why a default label is not needed.
Such as:

  /* unreachable because blah and blah */

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