Re: [PATCH] Makefile: Globally enable fall-through warning

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 1/10/26 12:55, Mohit Mishra wrote:
> Please accept my apologies for the unintended email. It was sent
> accidentally due to a configuration error while setting up git-send-email
> for a Linux fundamentals course.

Okay, best of luck with the course!

-Gustavo

> 
> I am sorry for any inconvenience this caused.
> 
> On Sat, Jan 10, 2026 at 8:43 AM Gustavo A. R. Silva <gustavo@embeddedor.com>
> wrote:
> 
>> Are you intentionally spamming us?
>>
>> -Gustavo
>>
>> On 1/10/26 10:34, MIshraMohit21-LE wrote:
>>> From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
>>>
>>> Now that all the fall-through warnings have been addressed in the
>>> kernel, enable the fall-through warning globally.
>>>
>>> Also, update the deprecated.rst file to include implicit fall-through
>>> as 'deprecated' so people can be pointed to a single location for
>>> justification.
>>>
>>> Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: Michal Marek <michal.lkml@markovi.net>
>>> Cc: Kees Cook <keescook@chromium.org>
>>> Cc: linux-kbuild@vger.kernel.org
>>> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
>>> ---
>>>    Documentation/process/deprecated.rst | 14 ++++++++++++++
>>>    Makefile                             |  3 +++
>>>    2 files changed, 17 insertions(+)
>>>
>>> diff --git a/Documentation/process/deprecated.rst
>> b/Documentation/process/deprecated.rst
>>> index 49e0f64a3427..053b24a6dd38 100644
>>> --- a/Documentation/process/deprecated.rst
>>> +++ b/Documentation/process/deprecated.rst
>>> @@ -119,3 +119,17 @@ array may exceed the remaining memory in the stack
>> segment. This could
>>>    lead to a crash, possible overwriting sensitive contents at the end of
>> the
>>>    stack (when built without `CONFIG_THREAD_INFO_IN_TASK=y`), or
>> overwriting
>>>    memory adjacent to the stack (when built without `CONFIG_VMAP_STACK=y`)
>>> +
>>> +Implicit switch case fall-through
>>> +---------------------------------
>>> +The C language allows switch cases to "fall through" when
>>> +a "break" statement is missing at the end of a case. This,
>>> +however, introduces ambiguity in the code, as it's not always
>>> +clear if the missing break is intentional or a bug. As there
>>> +have been a long list of flaws `due to missing "break" statements
>>> +<https://cwe.mitre.org/data/definitions/484.html>`_, we no longer allow
>>> +"implicit fall-through". In order to identify an intentional
>> fall-through
>>> +case, we have adopted the marking used by static analyzers: a comment
>>> +saying `/* Fall through */`. Once the C++17
>> `__attribute__((fallthrough))`
>>> +is more widely handled by C compilers, static analyzers, and IDEs, we
>> can
>>> +switch to using that instead.
>>> diff --git a/Makefile b/Makefile
>>> index 9be5834073f8..bdf8eac51b07 100644
>>> --- a/Makefile
>>> +++ b/Makefile
>>> @@ -843,6 +843,9 @@ NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC)
>> -print-file-name=include)
>>>    # warn about C99 declaration after statement
>>>    KBUILD_CFLAGS += -Wdeclaration-after-statement
>>>
>>> +# Warn about unmarked fall-throughs in switch statement.
>>> +KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough=3,)
>>> +
>>>    # Variable Length Arrays (VLAs) should not be used anywhere in the
>> kernel
>>>    KBUILD_CFLAGS += -Wvla
>>>
>>
>>
>