linux-kernel - Re: livepatch: reuse module loader code to write relocations

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Message-ID: <alpine.LNX.2.00.1603212213340.3656@cbobk.fhfr.pm>
Date:	Mon, 21 Mar 2016 22:16:17 +0100 (CET)
From:	Jiri Kosina <jikos@...nel.org>
To:	Jessica Yu <jeyu@...hat.com>
cc:	Miroslav Benes <mbenes@...e.cz>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Petr Mladek <pmladek@...e.com>,
	Jonathan Corbet <corbet@....net>, linux-api@...r.kernel.org,
	live-patching@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-doc@...r.kernel.org
Subject: Re: livepatch: reuse module loader code to write relocations

On Mon, 21 Mar 2016, Jessica Yu wrote:

> Yes, this is a concern and I'm not sure what the best way to fix it
> is. If both MODULE_NAME_LEN and KSYM_NAME_LEN were straight up
> constants, then I think Josh's stringify approach would have worked
> perfectly. However since MODULE_NAME_LEN translates to an expression
> (64 - sizeof(unsigned long)), which the preprocessor cannot evaluate,
> we will need another approach. Building the format strings at run time
> might be messier than we'd like. Alternatively we could just go the
> simple route and simply be a bit more aggressive on the upper bound
> for the format width; though the size of long varies on different
> architectures, afaik the max size it could ever be on any arch is 8
> bytes, so perhaps 64 - 8 = 56 (then - 1 to make room for \0) might be
> an appropriate field width. This would deserve a comment as well.

So how about actually modifying MAX_PARAM_PREFIX_LEN so 
that it's actually properly evaluable at preprocessing time, 
i.e. something along the lines of

diff --git a/include/linux/moduleparam.h b/include/linux/moduleparam.h
index 52666d9..954dae9 100644
--- a/include/linux/moduleparam.h
+++ b/include/linux/moduleparam.h
@@ -14,7 +14,7 @@
 #endif
 
 /* Chosen so that structs with an unsigned long line up. */
-#define MAX_PARAM_PREFIX_LEN (64 - sizeof(unsigned long))
+#define MAX_PARAM_PREFIX_LEN (64 - __SIZEOF_LONG__)
 
 #ifdef MODULE
 #define __MODULE_INFO(tag, name, info)					  \

-- 
Jiri Kosina
SUSE Labs