On Tue, Mar 10, 2015 at 05:58:02PM +0100, Petr Mladek wrote:
> On Tue 2015-03-10 09:47:01, Josh Poimboeuf wrote:
> > On Tue, Mar 10, 2015 at 03:36:17PM +0100, Petr Mladek wrote:
> > > On Tue 2015-03-10 09:22:04, Josh Poimboeuf wrote:
> > > > On Tue, Mar 10, 2015 at 01:01:07PM +0100, Petr Mladek wrote:
> > > > > On Mon 2015-03-09 09:40:55, Josh Poimboeuf wrote:
> > > > > > On Mon, Mar 09, 2015 at 02:25:28PM +0100, Petr Mladek wrote:
> > > > > > > diff --git a/kernel/module.c b/kernel/module.c
> > > > > > > index d856e96a3cce..b3ffc231ce0d 100644
> > > > > > > --- a/kernel/module.c
> > > > > > > +++ b/kernel/module.c
> > > > > > > @@ -3271,6 +3271,10 @@ static int load_module(struct load_info *info, const char __user *uargs,
> > > > > > > }
> > > > > > > #endif
> > > > > > >
> > > > > > > +#ifdef CONFIG_LIVEPATCH
> > > > > > > + mod->klp_alive = false;
> > > > > > > +#endif
> > > > > > > +
> > > > > >
> > > > > > I don't think you need this initialization. It looks like the module
> > > > > > struct is embedded in the mod->module_core region which is initialized
> > > > > > to zero in move_module().
> > > > >
> > > > > I have looked at this before but I was not able to find a code
> > > > > zeroing struct module. If I get it correctly, mod->module_core
> > > > > is a location where symbol table sections are copied or so.
> > > >
> > > > Yeah, it's far from obvious. AFAICT, it's cleared by the
> > > > "memset(ptr, 0, mod->core_size)" line.
> > >
> > > What I wanted to say is that module_core is not struct module. It
> > > seems that it points to the module code. See within_module_core() and
> > > how it is used().
> > >
> > > By other words, I think that struct module is not zeroed and we need
> > > to initialize the bool.
> > >
> > > Or did I miss anything?
> >
> > My understanding is that module_core is not only code. It also contains the
> > struct module itself. Verified in crash:
> >
> > crash> mod |head -n2
> > MODULE NAME SIZE OBJECT FILE
> > ffffffffa0003180 video 19905 (not loaded) [CONFIG_KALLSYMS]
> > crash> module.module_core,core_size 0xffffffffa0003180
> > module_core = 0xffffffffa0000000
> > core_size = 0x4dc1
>
> OK, you are right that struct module is inside mod->module_core. But I
> am still not convinced that the structure is zeroed.
>
> There are the following commands in move_module()
>
> ptr = module_alloc_update_bounds(mod->core_size);
> [...]
> memset(ptr, 0, mod->core_size);
> mod->module_core = ptr;
>
> if (mod->init_size) {
> [...]
> } else
> mod->module_init = NULL;
>
>
> The needed memory is allocated and zeroed but the pointer
> is written to the temporary place.
>
> I do not see any code that would copy parts of struct module from the
> temporary place to the newly allocated one. It seems that the whole
> structure is copied.
>
> Huh, the code is really twisted but I think that the space for the
> temporary structure is not zeroed. One week proof is that the code
> does mod->module_init = NULL; It would not make sense if the
> temporary location was zeroed.
>
>
> Of course, I will be happy if anyone convince me that I am wrong and
> we could omit the initialization.
Ok, I didn't realize the struct module was copied as part of the elf
section copy logic. I think we're getting close now :-)
>From a generated mod.C file:
__visible struct module __this_module
__attribute__((section(".gnu.linkonce.this_module"))) = {
.name = KBUILD_MODNAME,
.init = init_module,
#ifdef CONFIG_MODULE_UNLOAD
.exit = cleanup_module,
#endif
.arch = MODULE_ARCH_INIT,
};
The struct is statically initialized as a global variable in the module.
So the elements not specified in the static initializer should all be
cleared to zero by the compiler.
--
Josh
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