livepatch: Allow to call a custom callback when freeing shadow variables

We might need to do some actions before the shadow variable is freed.
For example, we might need to remove it from a list or free some data
that it points to.

This is already possible now. The user can get the shadow variable
by klp_shadow_get(), do the necessary actions, and then call
klp_shadow_free().

This patch allows to do it a more elegant way. The user could implement
the needed actions in a callback that is passed to klp_shadow_free()
as a parameter. The callback usually does reverse operations to
the constructor callback that can be called by klp_shadow_*alloc().

It is especially useful for klp_shadow_free_all(). There we need to do
these extra actions for each found shadow variable with the given ID.

Note that the memory used by the shadow variable itself is still released
later by rcu callback. It is needed to protect internal structures that
keep all shadow variables. But the destructor is called immediately.
The shadow variable must not be access anyway after klp_shadow_free()
is called. The user is responsible to protect this any suitable way.

Be aware that the destructor is called under klp_shadow_lock. It is
the same as for the contructor in klp_shadow_alloc().

Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

1 files changed, 15 insertions, 10 deletions

diff --git a/samples/livepatch/livepatch-shadow-fix1.c b/samples/livepatch/livepatch-shadow-fix1.c
index 04151c7f2631..49b13553eaae 100644
--- a/samples/livepatch/livepatch-shadow-fix1.c
+++ b/samples/livepatch/livepatch-shadow-fix1.c
@@ -98,9 +98,19 @@ struct dummy *livepatch_fix1_dummy_alloc(void)
 	return d;
 }
 
+static void livepatch_fix1_dummy_leak_dtor(void *obj, void *shadow_data)
+{
+	void *d = obj;
+	void **shadow_leak = shadow_data;
+
+	kfree(*shadow_leak);
+	pr_info("%s: dummy @ %p, prevented leak @ %p\n",
+			 __func__, d, *shadow_leak);
+}
+
 void livepatch_fix1_dummy_free(struct dummy *d)
 {
-	void **shadow_leak, *leak;
+	void **shadow_leak;
 
 	/*
 	 * Patch: fetch the saved SV_LEAK shadow variable, detach and
@@ -109,15 +119,10 @@ void livepatch_fix1_dummy_free(struct dummy *d)
 	 * was loaded.)
 	 */
 	shadow_leak = klp_shadow_get(d, SV_LEAK);
-	if (shadow_leak) {
-		leak = *shadow_leak;
-		klp_shadow_free(d, SV_LEAK);
-		kfree(leak);
-		pr_info("%s: dummy @ %p, prevented leak @ %p\n",
-			 __func__, d, leak);
-	} else {
+	if (shadow_leak)
+		klp_shadow_free(d, SV_LEAK, livepatch_fix1_dummy_leak_dtor);
+	else
 		pr_info("%s: dummy @ %p leaked!\n", __func__, d);
-	}
 
 	kfree(d);
 }
@@ -163,7 +168,7 @@ static int livepatch_shadow_fix1_init(void)
 static void livepatch_shadow_fix1_exit(void)
 {
 	/* Cleanup any existing SV_LEAK shadow variables */
-	klp_shadow_free_all(SV_LEAK);
+	klp_shadow_free_all(SV_LEAK, livepatch_fix1_dummy_leak_dtor);
 
 	WARN_ON(klp_unregister_patch(&patch));
 }