Re: [PATCH 11/14] KVM: selftests: Disable "gnu-variable-sized-type-not-at-end" warning

[Sean Christopherson]

On Mon, Dec 12, 2022, Nick Desaulniers wrote:
> > diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
> > index 2487db21b177..9cff99a1cb2e 100644
> > --- a/tools/testing/selftests/kvm/Makefile
> > +++ b/tools/testing/selftests/kvm/Makefile
> > @@ -196,6 +196,7 @@ else
> >  LINUX_TOOL_ARCH_INCLUDE = $(top_srcdir)/tools/arch/$(ARCH)/include
> >  endif
> >  CFLAGS += -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99 \
> > +       -Wno-gnu-variable-sized-type-not-at-end \
> 
> This is a clang-specific warning. This will need to be wrapped in a
> cc-option check.

Not that I'm against guarding this code, but I don't think cc-option() will do
anything in this case.

AFAICT, gcc stopped treating unknown "-Wno" flags as unconditional errors starting
with gcc-4.4, and the kernel's min supported version is 5.1.  gcc-4.4 through
gcc-9.5 all print a mild warning if there's a different error, but otherwise
silently ignore the uknown "-Wno".

  cc1: warning: unrecognized command line option '-Wno-gnu-variable-sized-type-not-at-end'

gcc-10.1 is even friendlier and notes that the unknown flag may have been related
to the error.

  cc1: note: unrecognized command-line option '-Wno-gnu-variable-sized-type-not-at-end'
  may have been intended to silence earlier diagnostics

Because cc-option() doesn't have errors in its probing code, it will return "true"
on gcc for literally any "-Wno-*" input that gcc deems syntacially valid, e.g.
gcc barfs on

  depends on $(cc-option,-Wno-)
  depends on $(cc-option,-Wno)

but happily succeeds with

  depends on $(cc-option,-Wno-lol-gcc)

Various man pages suggest -Wunknown-warnings is a thing, but no gcc version
supported by godbolt recognizes it.

So unless I'm missing something, trying to detect lack of support will be non-trivial,
and the worst case scenario is that users of older gcc version will see a potentially
confusing warning when the build fails.