[PATCH] Makefile: Globally enable fall-through warning

[PATCH] Makefile: Globally enable fall-through warning

[ALOK JHA]

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
 
-- 
2.34.1

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

[Andrew Morton]

On Tue, 17 May 2022 23:05:34 +0530 ALOK JHA <alok08jha@gmail.com> 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>

Since you sent the patch, it should have your signoff as well as
Gustavo's.  Please resend?

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

[Andrew Morton]

On Tue, 17 May 2022 23:05:34 +0530 ALOK JHA <alok08jha@gmail.com> 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.
> 
> ...
>
> --- 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
> +---------------------------------
>
> ...
>

Documentation/process/deprecated.rst already has a section "Implicit
switch case fall-through".  Maybe you're working against an old kernel.
Please update when resending.

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

[Jeff Johnson]

On 5/17/2022 11:46 AM, Andrew Morton wrote:
> On Tue, 17 May 2022 23:05:34 +0530 ALOK JHA <alok08jha@gmail.com> 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.
>>
>> ...
>>
>> --- 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
>> +---------------------------------
>>
>> ...
>>
> 
> Documentation/process/deprecated.rst already has a section "Implicit
> switch case fall-through".  Maybe you're working against an old kernel.
> Please update when resending.
> 

shouldn't we now just be referencing the fallthrough() macro?

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

[Gustavo A. R. Silva]

On Tue, May 17, 2022 at 11:46:01AM -0700, Andrew Morton wrote:
> On Tue, 17 May 2022 23:05:34 +0530 ALOK JHA <alok08jha@gmail.com> 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.
> > 
> > ...
> >
> > --- 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
> > +---------------------------------
> >
> > ...
> >
> 
> Documentation/process/deprecated.rst already has a section "Implicit
> switch case fall-through".  Maybe you're working against an old kernel.
> Please update when resending.

This looks like spam to me.

Let's just ignore this.

--
Gustavo

[PATCH] Makefile: Globally enable fall-through warning

[Omar el Azouny]

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
 
-- 
2.43.0

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

[Miguel Ojeda]

On Mon, May 5, 2025 at 7:17 PM Omar el Azouny <omarlazouny@gmail.com> 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>

I am confused -- hasn't this patch been in mainline since 2019?

    a035d552a93b ("Makefile: Globally enable fall-through warning")

Even if that were not the case, you would need to provide your SoB
etc. Please see:

    https://docs.kernel.org/process/submitting-patches.html

Thanks!

Cheers,
Miguel

[PATCH] Makefile: Globally enable fall-through warning

[MIshraMohit21-LE]

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
 
-- 
2.43.0

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

[Gustavo A. R. Silva]

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
>   

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

[Gustavo A. R. Silva]

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
>>>
>>
>>
> 

[PATCH] Makefile: Globally enable fall-through warning

[Simone Rea]

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
 
-- 
2.52.0

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

[Randy Dunlap]

Hi,

On 1/22/26 7:53 AM, Simone Rea 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>

This is missing your Signed-off-by: line. See
Documentation/process/submitting-patches.rst for info.

Also you should copy the current KBUILD maintainers.
See the MAINTAINERS file.

> ---
>  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.

Given that we have 80 cases of
	/* Fall through */
or	/* fall through */
in the kernel tree and we have over 5000 cases of
	fallthrough;

ITSM that you could go ahead and call for using the latter.


> 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
>  

-- 
~Randy

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

[Andrew Morton]

On Thu, 22 Jan 2026 10:19:56 -0800 Randy Dunlap <rdunlap@infradead.org> wrote:

> On 1/22/26 7:53 AM, Simone Rea 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>
> 
> This is missing your Signed-off-by: line. See
> Documentation/process/submitting-patches.rst for info.
> 
> Also you should copy the current KBUILD maintainers.
> See the MAINTAINERS file.

This went into mainline in 2019 :) I suspect Gustavo had a slight IT
problem.

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

[Randy Dunlap]

On 1/22/26 10:44 AM, Andrew Morton wrote:
> On Thu, 22 Jan 2026 10:19:56 -0800 Randy Dunlap <rdunlap@infradead.org> wrote:
> 
>> On 1/22/26 7:53 AM, Simone Rea 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>
>>
>> This is missing your Signed-off-by: line. See
>> Documentation/process/submitting-patches.rst for info.
>>
>> Also you should copy the current KBUILD maintainers.
>> See the MAINTAINERS file.
> 
> This went into mainline in 2019 :) I suspect Gustavo had a slight IT
> problem.

I think Simone did.

-- 
~Randy

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

[Miguel Ojeda]

On Thu, Jan 22, 2026 at 7:46 PM Randy Dunlap <rdunlap@infradead.org> wrote:
>
> I think Simone did.

Yeah, I don't know what is going on with that particular patch, but
someone sent the same one earlier this month:

  https://lore.kernel.org/all/20260110013412.14426-2-mohit.mishra@lunaredgeit.com/

Maybe an assignment?

Cheers,
Miguel

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

[Gustavo A. R. Silva]

On 1/23/26 03:48, Simone Rea wrote:
> Guys I’m so sorry 🙏🙏🙏 I don’t know what I did, i’m justinf following a
> tutorial…

This is the second time someone submit this same patch by mistake.

I'm curious about that tutorial. Can you please share with us a link to it?

Who's the author?

Are you paying for it?

Thanks
-Gustavo

> 
> Il giorno gio 22 gen 2026 alle 19:46 Randy Dunlap <rdunlap@infradead.org>
> ha scritto:
> 
>>
>>
>> On 1/22/26 10:44 AM, Andrew Morton wrote:
>>> On Thu, 22 Jan 2026 10:19:56 -0800 Randy Dunlap <rdunlap@infradead.org>
>> wrote:
>>>
>>>> On 1/22/26 7:53 AM, Simone Rea 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>
>>>>
>>>> This is missing your Signed-off-by: line. See
>>>> Documentation/process/submitting-patches.rst for info.
>>>>
>>>> Also you should copy the current KBUILD maintainers.
>>>> See the MAINTAINERS file.
>>>
>>> This went into mainline in 2019 :) I suspect Gustavo had a slight IT
>>> problem.
>>
>> I think Simone did.
>>
>> --
>> ~Randy
>>
>>
> 

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

[Gustavo A. R. Silva]

Shuah,

People taking this[1] LF training have been wrongly submitting this
patch since 2022 [2].

Why?

-Gustavo

[1] https://training.linuxfoundation.org/training/a-beginners-guide-to-linux-kernel-development-lfd103/
[2] https://lore.kernel.org/all/20220517195912.GA10952@embeddedor/

On 1/31/26 14:40, david@stennet.com 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
>   

[PATCH] Makefile: Globally enable fall-through warning

[david]

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
 
-- 
2.34.1

[PATCH] Makefile: Globally enable fall-through warning

[Rishabh]

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
 
-- 
2.51.0