linux-kernel - Re: Early crash (was: Re: module: show version information for built-in modules in sysfs)

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Date:	Wed, 02 Feb 2011 23:52:04 +0100
From:	Andreas Schwab <schwab@...ux-m68k.org>
To:	Dmitry Torokhov <dtor@...are.com>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux/m68k <linux-m68k@...r.kernel.org>,
	Linux-Arch <linux-arch@...r.kernel.org>
Subject: Re: Early crash (was: Re: module: show version information for built-in modules in sysfs)

Dmitry Torokhov <dtor@...are.com> writes:

> But why is it aligned on 2-byte boundary and why m64k is not happy with
> module_version_attribute but is happy with kernel_param which is also
> aligned similarly?

struct kernel_parm doesn't contain internal padding on 32 bit
architectures (it does on 64bit architectures though).

> If we unroll module_version_attribute it woud look like this:
>
> struct module_version_attribute {
>
> 	struct module_attribute {
>
> 		struct attribute {
> 			const char *name;
> 			mode_t mode;
> 		} attr;
> 		...
>
> 	} mattr;
>
> 	const char *module_name;
> 	const char *version;
> };
>
> So I would expect it be aligned on (char *) boundary which should be the
> same as (void *).

mode_t is a 16 bit type, thus any following member becomes aligned on an
odd 2 byte boundary.  On 32bit architectures with 4 byte alignment and
16 bit mode_t struct attribute contains 2 bytes of internal padding.
(64bit architectures typically have a 32bit mode_t, and there are 4
bytes of padding.)

> Will it help if we rearrange module_version_attribute definition to
> explicitly have first field being a pointer so it is more like
> kernel_param, like this:
>
> struct module_version_attribute {
> 	const char *module_name;
> 	const char *version;
> 	struct module_attribute mattr;
> };

That won't change the total size of the structure.

Andreas.

-- 
Andreas Schwab, schwab@...ux-m68k.org
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