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SubjectRe: Early crash (was: Re: module: show version information for built-in modules in sysfs)
Date
Dmitry Torokhov <dtor@vmware.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@linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756 01D3 44D5 214B 8276 4ED5
"And now for something completely different."


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