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

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Date:   Tue,  5 Oct 2021 08:26:11 -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,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Subject: [PATCH v5] 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>
---
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