[PATCH v5] docs: Explain the desired position of function attributes

[PATCH v5] docs: Explain the desired position of function attributes

[Kees Cook]

While discussing how to format the addition of various function
attributes, some "unwritten rules" of ordering surfaced[1]. Capture as
close as possible to Linus's preferences for future reference.

(Though I note the dissent voiced by Joe Perches, Alexey Dobriyan, and
others that would prefer all attributes live on a separate leading line.)

[1] https://lore.kernel.org/mm-commits/CAHk-=wiOCLRny5aifWNhr621kYrJwhfURsa0vFPeUEm8mF0ufg@mail.gmail.com/

Signed-off-by: Kees Cook <keescook@chromium.org>
---
v5:
- drop extern (joe)
- fix __malloc position (miguel)
v4: https://lore.kernel.org/lkml/20210930235754.2635912-1-keescook@chromium.org
---
 Documentation/process/coding-style.rst | 37 +++++++++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst
index 42969ab37b34..5756ff775233 100644
--- a/Documentation/process/coding-style.rst
+++ b/Documentation/process/coding-style.rst
@@ -480,13 +480,48 @@ closing function brace line.  E.g.:
 	}
 	EXPORT_SYMBOL(system_is_up);
 
+6.1) Function prototypes
+************************
+
 In function prototypes, include parameter names with their data types.
 Although this is not required by the C language, it is preferred in Linux
 because it is a simple way to add valuable information for the reader.
 
-Do not use the ``extern`` keyword with function prototypes as this makes
+Do not use the ``extern`` keyword with function declarations as this makes
 lines longer and isn't strictly necessary.
 
+When writing function prototypes, please keep the `order of elements regular
+<https://lore.kernel.org/mm-commits/CAHk-=wiOCLRny5aifWNhr621kYrJwhfURsa0vFPeUEm8mF0ufg@mail.gmail.com/>`_.
+For example, using this function declaration example::
+
+ __init void * __must_check action(enum magic value, size_t size, u8 count,
+				   char *fmt, ...) __printf(4, 5) __malloc;
+
+The preferred order of elements for a function prototype is:
+
+- storage class (below, ``static __always_inline``, noting that ``__always_inline``
+  is technically an attribute but is treated like ``inline``)
+- storage class attributes (here, ``__init`` -- i.e. section declarations, but also
+  things like ``__cold``)
+- return type (here, ``void *``)
+- return type attributes (here, ``__must_check``)
+- function name (here, ``action``)
+- function parameters (here, ``(enum magic value, size_t size, u8 count, char *fmt, ...)``,
+  noting that parameter names should always be included)
+- function parameter attributes (here, ``__printf(4, 5)``)
+- function behavior attributes (here, ``__malloc``)
+
+Note that for a function **definition** (i.e. the actual function body),
+the compiler does not allow function parameter attributes after the
+function parameters. In these cases, they should go after the storage
+class attributes (e.g. note the changed position of ``__printf(4, 5)``
+below, compared to the **declaration** example above)::
+
+ static __always_inline __init __printf(4, 5) void * __must_check action(enum magic value,
+		size_t size, u8 count, char *fmt, ...) __malloc
+ {
+	...
+ }
 
 7) Centralized exiting of functions
 -----------------------------------
-- 
2.30.2

Re: [PATCH v5] docs: Explain the desired position of function attributes

[Joe Perches]

On Tue, 2021-10-05 at 08:26 -0700, Kees Cook wrote:
> While discussing how to format the addition of various function
> attributes, some "unwritten rules" of ordering surfaced[1]. Capture as
> close as possible to Linus's preferences for future reference.
> +For example, using this function declaration example::
> +
> + __init void * __must_check action(enum magic value, size_t size, u8 count,
> +				   char *fmt, ...) __printf(4, 5) __malloc;

trivia: almost all fmt declarations should be const char *

> +Note that for a function **definition** (i.e. the actual function body),
> +the compiler does not allow function parameter attributes after the
> +function parameters. In these cases, they should go after the storage
> +class attributes (e.g. note the changed position of ``__printf(4, 5)``
> +below, compared to the **declaration** example above)::
> +
> + static __always_inline __init __printf(4, 5) void * __must_check action(enum magic value,
> +		size_t size, u8 count, char *fmt, ...) __malloc

here too, and 80 columns?

> + {
> +	...
> + }

Or just put all the attributes before the storage class... <grumble/chuckle>

cheers, Joe

Re: [PATCH v5] docs: Explain the desired position of function attributes

[Kees Cook]

On Tue, Oct 05, 2021 at 08:39:14AM -0700, Joe Perches wrote:
> On Tue, 2021-10-05 at 08:26 -0700, Kees Cook wrote:
> > While discussing how to format the addition of various function
> > attributes, some "unwritten rules" of ordering surfaced[1]. Capture as
> > close as possible to Linus's preferences for future reference.
> > +For example, using this function declaration example::
> > +
> > + __init void * __must_check action(enum magic value, size_t size, u8 count,
> > +				   char *fmt, ...) __printf(4, 5) __malloc;
> 
> trivia: almost all fmt declarations should be const char *

Heh, good point!

> > +Note that for a function **definition** (i.e. the actual function body),
> > +the compiler does not allow function parameter attributes after the
> > +function parameters. In these cases, they should go after the storage
> > +class attributes (e.g. note the changed position of ``__printf(4, 5)``
> > +below, compared to the **declaration** example above)::
> > +
> > + static __always_inline __init __printf(4, 5) void * __must_check action(enum magic value,
> > +		size_t size, u8 count, char *fmt, ...) __malloc
> 
> here too, and 80 columns?

Kernel standard is now 100. *shrug*

> > + {
> > +	...
> > + }
> 
> Or just put all the attributes before the storage class... <grumble/chuckle>

I hear ya...

-- 
Kees Cook

Re: [PATCH v5] docs: Explain the desired position of function attributes

[Randy Dunlap]

On 10/5/21 10:04 AM, Kees Cook wrote:
> On Tue, Oct 05, 2021 at 08:39:14AM -0700, Joe Perches wrote:
>> On Tue, 2021-10-05 at 08:26 -0700, Kees Cook wrote:
>>> While discussing how to format the addition of various function
>>> attributes, some "unwritten rules" of ordering surfaced[1]. Capture as
>>> close as possible to Linus's preferences for future reference.
>>> +For example, using this function declaration example::
>>> +
>>> + __init void * __must_check action(enum magic value, size_t size, u8 count,
>>> +				   char *fmt, ...) __printf(4, 5) __malloc;
>>
>> trivia: almost all fmt declarations should be const char *
> 
> Heh, good point!
> 
>>> +Note that for a function **definition** (i.e. the actual function body),
>>> +the compiler does not allow function parameter attributes after the
>>> +function parameters. In these cases, they should go after the storage
>>> +class attributes (e.g. note the changed position of ``__printf(4, 5)``
>>> +below, compared to the **declaration** example above)::
>>> +
>>> + static __always_inline __init __printf(4, 5) void * __must_check action(enum magic value,
>>> +		size_t size, u8 count, char *fmt, ...) __malloc
>>
>> here too, and 80 columns?
> 
> Kernel standard is now 100. *shrug*

That's more for exceptions, not the common rule.
AFAIUI.

> 
>>> + {
>>> +	...
>>> + }
>>
>> Or just put all the attributes before the storage class... <grumble/chuckle>
> 
> I hear ya...
> 


-- 
~Randy

Re: [PATCH v5] docs: Explain the desired position of function attributes

[Joe Perches]

On Tue, 2021-10-05 at 12:15 -0700, Randy Dunlap wrote:
> On 10/5/21 10:04 AM, Kees Cook wrote:
> > On Tue, Oct 05, 2021 at 08:39:14AM -0700, Joe Perches wrote:
> > > On Tue, 2021-10-05 at 08:26 -0700, Kees Cook wrote:
> > > > While discussing how to format the addition of various function
> > > > attributes, some "unwritten rules" of ordering surfaced[1]. Capture as
> > > > close as possible to Linus's preferences for future reference.
> > > > +For example, using this function declaration example::
> > > > +
> > > > + __init void * __must_check action(enum magic value, size_t size, u8 count,
> > > > +				   char *fmt, ...) __printf(4, 5) __malloc;
> > > 
> > > trivia: almost all fmt declarations should be const char *
> > 
> > Heh, good point!
> > 
> > > > +Note that for a function **definition** (i.e. the actual function body),
> > > > +the compiler does not allow function parameter attributes after the
> > > > +function parameters. In these cases, they should go after the storage
> > > > +class attributes (e.g. note the changed position of ``__printf(4, 5)``
> > > > +below, compared to the **declaration** example above)::
> > > > +
> > > > + static __always_inline __init __printf(4, 5) void * __must_check action(enum magic value,
> > > > +		size_t size, u8 count, char *fmt, ...) __malloc
> > > 
> > > here too, and 80 columns?
> > 
> > Kernel standard is now 100. *shrug*
> 
> That's more for exceptions, not the common rule.
> AFAIUI.

And for function definitions that are not static inline, when
separate function declarations exist, the function definition
does not need any attribute marking at all.

Re: [PATCH v5] docs: Explain the desired position of function attributes

[Jonathan Corbet]

Kees Cook <keescook@chromium.org> writes:

> While discussing how to format the addition of various function
> attributes, some "unwritten rules" of ordering surfaced[1]. Capture as
> close as possible to Linus's preferences for future reference.
>
> (Though I note the dissent voiced by Joe Perches, Alexey Dobriyan, and
> others that would prefer all attributes live on a separate leading line.)
>
> [1] https://lore.kernel.org/mm-commits/CAHk-=wiOCLRny5aifWNhr621kYrJwhfURsa0vFPeUEm8mF0ufg@mail.gmail.com/
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> v5:
> - drop extern (joe)
> - fix __malloc position (miguel)
> v4: https://lore.kernel.org/lkml/20210930235754.2635912-1-keescook@chromium.org
> ---
>  Documentation/process/coding-style.rst | 37 +++++++++++++++++++++++++-
>  1 file changed, 36 insertions(+), 1 deletion(-)

I've applied this, thanks.

jon