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

From: Jessica Yu <[email-masked]>
Date: Tue, 26 Jan 2021 15:25:16 +0100

+++ 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 <[email-masked]>
>---
> 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