On Tue, Aug 17, 2021 at 04:33:25PM -0500, Gustavo A. R. Silva wrote:

On 8/17/21 16:17, Masahiro Yamada wrote:

quoted

On Wed, Aug 18, 2021 at 3:25 AM Nathan Chancellor [off-list ref] wrote:

quoted

On 8/17/2021 11:03 AM, Kees Cook wrote:

quoted

On Mon, Aug 16, 2021 at 09:55:28PM -0700, Nathan Chancellor wrote:

quoted

If you/Gustavo would prefer, I can upgrade that check to

ifneq ($(call cc-option, -Wunreachable-code-fallthrough),)

I was just trying to save a call to the compiler, as that is more expensive
than a shell test call.

I prefer the option test -- this means no changes are needed on the
kernel build side if it ever finds itself backported to earlier versions
(and it handles the current case of "14" not meaning "absolute latest").

More specifically, I think you want this (untested):

That should work but since -Wunreachable-code-fallthrough is off by
default, I did not really see a reason to include it in KBUILD_CFLAGS. I
do not have a strong opinion though, your version is smaller than mine
is so we can just go with that. I'll defer to Gustavo on it since he has
put in all of the work cleaning up the warnings.

https://github.com/llvm/llvm-project/commit/9ed4a94d6451046a51ef393cd62f00710820a7e8

   did two things:

 (1) Change the -Wimplicit-fallthrough behavior so that it fits
      to our use in the kernel

 (2) Add a new option -Wunreachable-code-fallthrough
      that works like the previous -Wimplicit-fallthrough of
      Clang <= 13.0.0


They are separate things.

Checking the presence of -Wunreachable-code-fallthrough
does not make sense since we are only interested in (1) here.

So, checking the Clang version is sensible and matches
the explanation in the comment block.

I thought one of the problems (which is quickly draining away) that
needed to be solved here is that some Clang trunk builds (that report
as version 14) don't yet have support for -Wunreachable-code-fallthrough
since they aren't new enough.

quoted

# Warn about unmarked fall-throughs in switch statement.
# Clang prior to 14.0.0 warned on unreachable fallthroughs with
# -Wimplicit-fallthrough, which is unacceptable due to IS_ENABLED().
# https://bugs.llvm.org/show_bug.cgi?id=51094
ifeq ($(firstword $(sort $(CONFIG_CLANG_VERSION) 140000)),140000)
KBUILD_CFLAGS += -Wimplicit-fallthrough
endif

The $(sort ...) is alphabetical sort, not numeric sort.
It works for us because the minimum Clang version is 10.0.1
(that is CONFIG_CLANG_VERSION is always 6-digit)

It will break when Clang version 100.0.0 is released.

But, before that, we will raise the minimum supported clang version,
and this conditional will go away.

If a version test is preferred, cool; this is a nice trick. :)

I like this. :)

I'm going to make the 0-day robot test it in my tree, first.

Sounds good to me!

-- 
Kees Cook