Re: [PATCH 1/3] module: deal with alignment issues in built-in module versions

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Mon, 2011-02-21 at 14:30 +1030, Rusty Russell wrote:
> > Except that .long is 32-bit on ppc64 :-( You need .llong for 64-bit.
> 
> OK, all options suck.  Do we want the workaround or not?

The only sane thing I can see is make sure that such structures that
we put into sections "arrays" like that are naturally aligned with
padding.

Cheers,
Ben.

> Cheers,
> Rusty.
> 
> Subject: module: deal with alignment issues in built-in module versions
> Date: Mon, 7 Feb 2011 16:02:25 -0800
> From: Dmitry Torokhov <dtor@vmware.com>
> 
> On m68k natural alignment is 2-byte boundary but we are trying to
> align structures in __modver section on sizeof(void *) boundary.
> This causes trouble when we try to access elements in this section
> in array-like fashion when create "version" attributes for built-in
> modules.
> 
> Moreover, as DaveM said, we can't reliably put structures into
> independent objects, put them into a special section, and then expect
> array access over them (via the section boundaries) after linking the
> objects together to just "work" due to variable alignment choices in
> different situations. The only solution that seems to work reliably
> is to make an array of plain pointers to the objects in question and
> put those pointers in the special section.
> 
> Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
> Signed-off-by: Dmitry Torokhov <dtor@vmware.com>
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> ---
>  include/linux/module.h |   10 +++++-----
>  kernel/params.c        |    9 ++++++---
>  2 files changed, 11 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/module.h b/include/linux/module.h
> index 9bdf27c..9b7081a 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -174,10 +174,7 @@ extern struct module __this_module;
>  #define MODULE_VERSION(_version)					\
>  	extern ssize_t __modver_version_show(struct module_attribute *,	\
>  					     struct module *, char *);	\
> -	static struct module_version_attribute __modver_version_attr	\
> -	__used								\
> -    __attribute__ ((__section__ ("__modver"),aligned(sizeof(void *)))) \
> -	= {								\
> +	static struct module_version_attribute ___modver_attr = {	\
>  		.mattr	= {						\
>  			.attr	= {					\
>  				.name	= "version",			\
> @@ -187,7 +184,10 @@ extern struct module __this_module;
>  		},							\
>  		.module_name	= KBUILD_MODNAME,			\
>  		.version	= _version,				\
> -	}
> +	};								\
> +	static const struct module_version_attribute			\
> +	__used __attribute__ ((__section__ ("__modver")))		\
> +	* __moduleparam_const __modver_attr = &___modver_attr
>  #endif
>  
>  /* Optional firmware file (or files) needed by the module
> diff --git a/kernel/params.c b/kernel/params.c
> index 0da1411..09a08cb 100644
> --- a/kernel/params.c
> +++ b/kernel/params.c
> @@ -821,15 +821,18 @@ ssize_t __modver_version_show(struct module_attribute *mattr,
>  	return sprintf(buf, "%s\n", vattr->version);
>  }
>  
> -extern struct module_version_attribute __start___modver[], __stop___modver[];
> +extern const struct module_version_attribute *__start___modver[];
> +extern const struct module_version_attribute *__stop___modver[];
>  
>  static void __init version_sysfs_builtin(void)
>  {
> -	const struct module_version_attribute *vattr;
> +	const struct module_version_attribute **p;
>  	struct module_kobject *mk;
>  	int err;
>  
> -	for (vattr = __start___modver; vattr < __stop___modver; vattr++) {
> +	for (p = __start___modver; p < __stop___modver; p++) {
> +		const struct module_version_attribute *vattr = *p;
> +
>  		mk = locate_module_kobject(vattr->module_name);
>  		if (mk) {
>  			err = sysfs_create_file(&mk->kobj, &vattr->mattr.attr);
> --
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