linux-kernel - [PATCH v4] docs: Explain the desired position of function attributes

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Message-Id: <20210930235754.2635912-1-keescook@chromium.org>
Date:   Thu, 30 Sep 2021 16:57:54 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jonathan Corbet <corbet@....net>
Cc:     Kees Cook <keescook@...omium.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Joe Perches <joe@...ches.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: [PATCH v4] docs: Explain the desired position of function attributes

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@...omium.org>
---
v4:
- fix another stray "void"! This is why code needs a compiler... (thx randy)
---
 Documentation/process/coding-style.rst | 30 ++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst
index 42969ab37b34..45b48510f5ec 100644
--- a/Documentation/process/coding-style.rst
+++ b/Documentation/process/coding-style.rst
@@ -487,6 +487,36 @@ 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
 lines longer and isn't strictly necessary.
 
+When writing a function declarations, please keep the `order of elements regular
+<https://lore.kernel.org/mm-commits/CAHk-=wiOCLRny5aifWNhr621kYrJwhfURsa0vFPeUEm8mF0ufg@mail.gmail.com/>`_.
+For example::
+
+ extern __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 (here, ``extern``, and things like ``static __always_inline`` even though
+  ``__always_inline`` is technically an attribute, it 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 (e.g. ``static inline``), 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)``)::
+
+ 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