[Devel] Re: [PATCH] Fix some kallsyms_lookup() vs rmmod races

[Alexey Dobriyan]

On Thu, Mar 15, 2007 at 04:53:59PM +0000, Paulo Marques wrote:
> Alexey Dobriyan wrote:
> >[cc'ing folks whose proc files are affected]
> >
> >kallsyms_lookup() can call module_address_lookup() which iterates over
> >modules list without module_mutex taken. Comment at the top of
> >module_address_lookup() says it's for oops resolution so races are
> >irrelevant, but in some cases it's reachable from regular code:
>
> So maybe we should just add a new parameter to "kallsyms_lookup" to
> inform it if it is safe to take a mutex or not.

You have to drag "mod->name" out of kallsyms_lookup(), so if you drop
module_mutex in it, you still have a bug.

We can agree on kallsyms_lookup() or whatever other low-level function
to copy everything into caller-supplied structure and not spreading
module_mutex. module's name is 64 minus a little, so stack usage should
be fine.

> Spreading module_mutex everywhere doesn't seem like the right interface
> for several reasons:
>
>  - new users of "kallsyms_lookup" might not be aware that they should
> take module_mutex if it is safe

Well, yes.

>  - many times we will be taking module_mutex even when we are fetching
> a kernel symbol that shouldn't require the mutex at all
>
>  - it just creates new dependencies (hint: this patch shouldn't even
> compile with current git since module_mutex is not declared in module.h,

Yeah, I remembered about it only in subway :).

This patch is techically dependent on http://lkml.org/lkml/2007/3/14/128 aka
[PATCH v2] Race between cat /proc/kallsyms and rmmod

> not to mention compile when CONFIG_MODULES not set)

OK, I'll fix it.

> IMHO we should not expose module_mutex outside of module.c. That is just
> wrong from an encapsulation point of view.