Re: [PATCH v3] livepatch/module: Correctly handle coming and going modules

[Josh Poimboeuf <jpoimboe@redhat.com>]

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