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, 16 insertions, 11 deletions

diff --git a/samples/livepatch/livepatch-shadow-fix2.c b/samples/livepatch/livepatch-shadow-fix2.c
index d6c62844dc15..b34c7bf83356 100644
--- a/samples/livepatch/livepatch-shadow-fix2.c
+++ b/samples/livepatch/livepatch-shadow-fix2.c
@@ -68,22 +68,27 @@ bool livepatch_fix2_dummy_check(struct dummy *d, unsigned long jiffies)
         return time_after(jiffies, d->jiffies_expire);
 }
 
+static void livepatch_fix2_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_fix2_dummy_free(struct dummy *d)
 {
-        void **shadow_leak, *leak;
+        void **shadow_leak;
         int *shadow_count;
 
         /* Patch: copy the memory leak patch from the fix1 module. */
         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_fix2_dummy_leak_dtor);
+        else
                 pr_info("%s: dummy @ %p leaked!\n", __func__, d);
-        }
 
         /*
          * Patch: fetch the SV_COUNTER shadow variable and display
@@ -93,7 +98,7 @@ void livepatch_fix2_dummy_free(struct dummy *d)
         if (shadow_count) {
                 pr_info("%s: dummy @ %p, check counter = %d\n",
                         __func__, d, *shadow_count);
-                klp_shadow_free(d, SV_COUNTER);
+                klp_shadow_free(d, SV_COUNTER, NULL);
         }
 
         kfree(d);
@@ -140,7 +145,7 @@ static int livepatch_shadow_fix2_init(void)
 static void livepatch_shadow_fix2_exit(void)
 {
         /* Cleanup any existing SV_COUNTER shadow variables */
-        klp_shadow_free_all(SV_COUNTER);
+        klp_shadow_free_all(SV_COUNTER, NULL);
 
         WARN_ON(klp_unregister_patch(&patch));
 }