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

On Fri, Nov 20, 2020 at 11:51:42AM -0800, Jakub Kicinski wrote:

On Fri, 20 Nov 2020 11:30:40 -0800 Kees Cook wrote:

quoted

On Fri, Nov 20, 2020 at 10:53:44AM -0800, Jakub Kicinski wrote:

quoted

On Fri, 20 Nov 2020 12:21:39 -0600 Gustavo A. R. Silva wrote:  

quoted

This series aims to fix almost all remaining fall-through warnings in
order to enable -Wimplicit-fallthrough for Clang.

In preparation to enable -Wimplicit-fallthrough for Clang, explicitly
add multiple break/goto/return/fallthrough statements instead of just
letting the code fall through to the next case.

Notice that in order to enable -Wimplicit-fallthrough for Clang, this
change[1] is meant to be reverted at some point. So, this patch helps
to move in that direction.

Something important to mention is that there is currently a discrepancy
between GCC and Clang when dealing with switch fall-through to empty case
statements or to cases that only contain a break/continue/return
statement[2][3][4].  

Are we sure we want to make this change? Was it discussed before?

Are there any bugs Clangs puritanical definition of fallthrough helped
find?

IMVHO compiler warnings are supposed to warn about issues that could
be bugs. Falling through to default: break; can hardly be a bug?!  

It's certainly a place where the intent is not always clear. I think
this makes all the cases unambiguous, and doesn't impact the machine
code, since the compiler will happily optimize away any behavioral
redundancy.

If none of the 140 patches here fix a real bug, and there is no change
to machine code then it sounds to me like a W=2 kind of a warning.

FWIW, this series has found at least one bug so far:
https://lore.kernel.org/lkml/CAFCwf11izHF=g1mGry1fE5kvFFFrxzhPSM6qKAO8gxSp=Kr_CQ@mail.gmail.com/ (local)

-- 
Kees Cook