Re: [RFC PATCH] compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use

From: Joe Perches
Date: Sun Aug 04 2019 - 20:39:38 EST

On Sun, 2019-08-04 at 11:09 -0700, Linus Torvalds wrote:
> On Sun, Aug 4, 2019 at 11:01 AM Joe Perches <joe@xxxxxxxxxxx> wrote:
> > Linus? Do you have an opinion about this RFC/patch?
> So my only real concern is that the comment approach has always been
> the really traditional one, going back all the way to 'lint' days.
> And you obviously cannot use a #define to create a comment, so this
> whole keyword model will never be able to do that.
> At the same time, all the modern tools we care about do seem to be
> happy with it, either through the gcc attribute, the clang
> [[clang:fallthrough]] or the (eventual) standard C [[fallthrough]]
> model.

(adding Nick Desaulniers and clang-built-linux to cc's)

As far as I can tell, clang 10 (and it took hours to compile
and link the most current version here) does not support
and using just -Wimplicit-fallthrough with clang 10 does not emit
a fallthrough warning even with -Wextra and -Wimplicit-fallthrough
using switch / case code blocks like:
lib/test_module.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)

diff --git a/lib/test_module.c b/lib/test_module.c
index debd19e35198..30c835178c7c 100644
--- a/lib/test_module.c
+++ b/lib/test_module.c
@@ -14,6 +14,21 @@
#include <linux/module.h>
#include <linux/printk.h>

+static int switch_case(int val)
+ int i = 0;
+ switch (val) {
+ case 1:
+ i |= 1;
+ case 2:
+ i |= 2;
+ break;
+ }
+ return i;
static int __init test_module_init(void)
pr_warn("Hello, world\n");


$ clang -v
clang version 10.0.0 (git:// 305b961f64b75e73110e309341535f6d5a48ed72)

and the compilation command line:
$ clang -Wp,-MD,lib/.test_module.o.d -nostdinc -isystem /usr/local/lib/clang/10.0.0/include -I./arch/x86/include -I./arch/x86/include/generated -I./include -I./arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I./include/uapi -I./include/generated/uapi -include ./include/linux/kconfig.h -include ./include/linux/compiler_types.h -D__KERNEL__ -Qunused-arguments -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE -Werror=implicit-function-declaration -Werror=implicit-int -Wno-format-security -std=gnu89 -no-integrated-as -Werror=unknown-warning-option -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -m64 -mno-80387 -mstack-alignment=8 -mtune=generic -mno-red-zone -mcmodel=kernel -DCONFIG_X86_X32_ABI -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1 -DCONFIG_AS_SSSE3=1 -DCONFIG_AS_AVX=1 -DCONFIG_AS_AVX2=1 -DCONFIG_AS_AVX512=1 -DCONFIG_AS_SHA1_NI=1 -DCONFIG_AS_SHA256_NI=1 -Wno-sign-compare -fno-asynchronous-unwind-tables -mretpoline-external-thunk -fno-delete-null-pointer-checks -Wno-address-of-packed-member -O2 -Wframe-larger-than=2048 -fstack-protector-strong -Wno-format-invalid-specifier -Wno-gnu -Wno-tautological-compare -mno-global-merge -Wno-unused-const-variable -DCC_USING_FENTRY -Wdeclaration-after-statement -Wvla -Wno-pointer-sign -fno-strict-overflow -fno-merge-all-constants -fno-stack-check -Werror=date-time -Werror=incompatible-pointer-types -fcf-protection=none -Wno-initializer-overrides -Wno-format -Wno-sign-compare -Wno-format-zero-length -fsanitize=kernel-address -mllvm -asan-mapping-offset=0xdffffc0000000000 -mllvm -asan-globals=1 -mllvm -asan-instrumentation-with-call-threshold=0 -mllvm -asan-stack=0 --param asan-instrument-allocas=1 -fsanitize-coverage=trace-pc -fsanitize-coverage=trace-cmp -DMODULE -DKBUILD_BASENAME='"test_module"' -DKBUILD_MODNAME='"test_module"' -Wextra -Wimplicit-fallthrough -c -o lib/test_module.o lib/test_module.c

> So I'm ok with just saying "the comment model may be traditional, but
> it's not very good".
> I didn't look at all the patches, but the one I *did* see had a few issues:
> - it didn't seem to handle clang

The __has_attribute use is at least clang compatible.
even if it doesn't (seem to?) work.

> - we'd need to make -Wimplicit-fallthrough be dependent on the
> compiler actually supporting the attribute, not just on supporting the
> flag.

I believe that also needs work if ever clang works,

Makefile:KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough=3,)

this will have to be changed for clang as the =<val> isn't (yet?) supported.

> without those changes, nobody can actually start doing any
> conversions. But I assume such patches exist somewhere, and I've just
> missed them.

I haven't sent any patches for any comment conversions.
nor would I until the RFC is acceptable.

Just this RFC and the necessary conversion of the one use
of fallthrough as a label (which David Miller did not apply)

Some people reasonably feel that Coverity should recognize
fallthrough; style annotations before changing the existing
/* fallthrough */ comment uses. I think lint doesn't matter

I do have a script that does a reasonable job of converting
most of the /* fallthrough */ style comments to fallthrough;
while realigning to the last indentation.

That script still needs more work before I will post it.


I think using the pseudo-keyword
reads better than
to end case blocks.

Do you have an opinion here?