Re: [PATCH 04/13] livepatch: move klp_find_object_module to module.c

[Jessica Yu]

+++ Christoph Hellwig [21/01/21 08:49 +0100]:
> To uncouple the livepatch code from module loader internals move a
> slightly refactored version of klp_find_object_module to module.c
> This allows to mark find_module static and removes one of the last
> users of module_mutex outside of module.c.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> include/linux/module.h  |  3 +--
> kernel/livepatch/core.c | 39 +++++++++++++--------------------------
> kernel/module.c         | 17 ++++++++++++++++-
> 3 files changed, 30 insertions(+), 29 deletions(-)
>
> diff --git a/include/linux/module.h b/include/linux/module.h
> index b4654f8a408134..8588482bde4116 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -586,8 +586,7 @@ static inline bool within_module(unsigned long addr, const struct module *mod)
> 	return within_module_init(addr, mod) || within_module_core(addr, mod);
> }
>
> -/* Search for module by name: must hold module_mutex. */
> -struct module *find_module(const char *name);
> +struct module *find_klp_module(const char *name);
>
> /* Check if a module is loaded. */
> bool module_loaded(const char *name);
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index a7f625dc24add3..878759baadd81c 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -49,30 +49,6 @@ static bool klp_is_module(struct klp_object *obj)
> 	return obj->name;
> }
>
> -/* sets obj->mod if object is not vmlinux and module is found */
> -static void klp_find_object_module(struct klp_object *obj)
> -{
> -	struct module *mod;
> -
> -	mutex_lock(&module_mutex);
> -	/*
> -	 * We do not want to block removal of patched modules and therefore
> -	 * we do not take a reference here. The patches are removed by
> -	 * klp_module_going() instead.
> -	 */
> -	mod = find_module(obj->name);
> -	/*
> -	 * Do not mess work of klp_module_coming() and klp_module_going().
> -	 * Note that the patch might still be needed before klp_module_going()
> -	 * is called. Module functions can be called even in the GOING state
> -	 * until mod->exit() finishes. This is especially important for
> -	 * patches that modify semantic of the functions.
> -	 */
> -	if (mod && mod->klp_alive)
> -		obj->mod = mod;
> -	mutex_unlock(&module_mutex);
> -}

Hmm, I am not a huge fan of moving more livepatch code into module.c, I
wonder if we can keep them separate.

Why not have module_is_loaded() kill two birds with one stone? That
is, just have it return a module pointer to signify that the module is
loaded, NULL if not. Then we don't need an extra find_klp_module()
function just to call find_module() and return a pointer, as
module_is_loaded() can just do that for us.

As for the mod->klp_alive check, I believe this function
(klp_find_object_module()) is called with klp_mutex held, and
mod->klp_alive is only modified under klp_mutex. Also, if klp_alive is
true, the module is at least COMING and cannot be GOING until it
acquires the klp_mutex again in klp_module_going(). So does that hunk
really need to be under module_mutex? It has been a long time since
I've looked at livepatch code so it would be great if someone could
double check.

Jessica