linux-kernel - Re: [PATCH v5] printk: Userspace format enumeration support

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Date:   Thu, 18 Mar 2021 12:31:44 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Chris Down <chris@...isdown.name>, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        John Ogness <john.ogness@...utronix.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>, kernel-team@...com
Subject: Re: [PATCH v5] printk: Userspace format enumeration support

On 18/03/2021 11.46, Petr Mladek wrote:

> BTW: Is the trick with int (printk)(const char *s, ...) documented
> somewhere? Is it portable?

It is completely standard and portable C, explicitly spelled out in the
C standard itself. C99:

===
6.10.3 Macro replacement

10 [...] Each subsequent instance of the
function-like macro name followed by a ( as the next preprocessing token
introduces the
sequence of preprocessing tokens that is replaced by the replacement
list in the definition
(an invocation of the macro). [...]
===

and later

===
7.1.4 Use of library functions

1 [...] one
of the techniques shown below can be used to ensure the declaration is
not affected by
such a macro. Any macro definition of a function can be suppressed
locally by enclosing
the name of the function in parentheses, because the name is then not
followed by the left
parenthesis that indicates expansion of a macro function name. For the
same syntactic
reason, it is permitted to take the address of a library function even
if it is also defined as
a macro.
===

Also, the use of printk() inside the definition of a printk()
function-like macro does not lead to infinite recursion, by

===
6.10.3.4 Rescanning and further replacement

2 If the name of the macro being replaced is found during this scan of
the replacement list
(not including the rest of the source file’s preprocessing tokens), it
is not replaced.
===

Rasmus