5 files changed, 104 insertions, 47 deletions

diff --git a/Documentation/livepatch/shadow-vars.txt b/Documentation/livepatch/shadow-vars.txt
index 89c66634d600..9c7ae191641c 100644
--- a/Documentation/livepatch/shadow-vars.txt
+++ b/Documentation/livepatch/shadow-vars.txt
@@ -34,9 +34,13 @@ meta-data and shadow-data:
   - data[] - storage for shadow data
 
 It is important to note that the klp_shadow_alloc() and
-klp_shadow_get_or_alloc() calls, described below, store a *copy* of the
-data that the functions are provided.  Callers should provide whatever
-mutual exclusion is required of the shadow data.
+klp_shadow_get_or_alloc() are zeroing the variable by default.
+They also allow to call a custom constructor function when a non-zero
+value is needed. Callers should provide whatever mutual exclusion
+is required.
+
+Note that the constructor is called under klp_shadow_lock spinlock. It allows
+to do actions that can be done only once when a new variable is allocated.
 
 * klp_shadow_get() - retrieve a shadow variable data pointer
   - search hashtable for <obj, id> pair
@@ -47,7 +51,7 @@ mutual exclusion is required of the shadow data.
     - WARN and return NULL
   - if <obj, id> doesn't already exist
     - allocate a new shadow variable
-    - copy data into the new shadow variable
+    - initialize the variable using a custom constructor and data when provided
     - add <obj, id> to the global hashtable
 
 * klp_shadow_get_or_alloc() - get existing or alloc a new shadow variable
@@ -56,7 +60,7 @@ mutual exclusion is required of the shadow data.
     - return existing shadow variable
   - if <obj, id> doesn't already exist
     - allocate a new shadow variable
-    - copy data into the new shadow variable
+    - initialize the variable using a custom constructor and data when provided
     - add <obj, id> pair to the global hashtable
 
 * klp_shadow_free() - detach and free a <obj, id> shadow variable
@@ -107,7 +111,8 @@ struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata,
         sta = kzalloc(sizeof(*sta) + hw->sta_data_size, gfp);
 
         /* Attach a corresponding shadow variable, then initialize it */
-        ps_lock = klp_shadow_alloc(sta, PS_LOCK, NULL, sizeof(*ps_lock), gfp);
+        ps_lock = klp_shadow_alloc(sta, PS_LOCK, sizeof(*ps_lock), gfp,
+                                   NULL, NULL);
         if (!ps_lock)
                 goto shadow_fail;
         spin_lock_init(ps_lock);
@@ -148,16 +153,24 @@ shadow variables to parents already in-flight.
 For commit 1d147bfa6429, a good spot to allocate a shadow spinlock is
 inside ieee80211_sta_ps_deliver_wakeup():
 
+int ps_lock_shadow_ctor(void *obj, void *shadow_data, void *ctor_data)
+{
+        spinlock_t *lock = shadow_data;
+
+        spin_lock_init(lock);
+        return 0;
+}
+
 #define PS_LOCK 1
 void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta)
 {
-        DEFINE_SPINLOCK(ps_lock_fallback);
         spinlock_t *ps_lock;
 
         /* sync with ieee80211_tx_h_unicast_ps_buf */
         ps_lock = klp_shadow_get_or_alloc(sta, PS_LOCK,
-                        &ps_lock_fallback, sizeof(ps_lock_fallback),
-                        GFP_ATOMIC);
+                        sizeof(*ps_lock), GFP_ATOMIC,
+                        ps_lock_shadow_ctor, NULL);
+
         if (ps_lock)
                 spin_lock(ps_lock);
         ...
diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index 4754f01c1abb..7e084321b146 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -186,11 +186,17 @@ static inline bool klp_have_reliable_stack(void)
                IS_ENABLED(CONFIG_HAVE_RELIABLE_STACKTRACE);
 }
 
+typedef int (*klp_shadow_ctor_t)(void *obj,
+                                 void *shadow_data,
+                                 void *ctor_data);
+
 void *klp_shadow_get(void *obj, unsigned long id);
-void *klp_shadow_alloc(void *obj, unsigned long id, void *data,
-                       size_t size, gfp_t gfp_flags);
-void *klp_shadow_get_or_alloc(void *obj, unsigned long id, void *data,
-                              size_t size, gfp_t gfp_flags);
+void *klp_shadow_alloc(void *obj, unsigned long id,
+                       size_t size, gfp_t gfp_flags,
+                       klp_shadow_ctor_t ctor, void *ctor_data);
+void *klp_shadow_get_or_alloc(void *obj, unsigned long id,
+                              size_t size, gfp_t gfp_flags,
+                              klp_shadow_ctor_t ctor, void *ctor_data);
 void klp_shadow_free(void *obj, unsigned long id);
 void klp_shadow_free_all(unsigned long id);
 
diff --git a/kernel/livepatch/shadow.c b/kernel/livepatch/shadow.c
index fdac27588d60..b10a0bbb7f84 100644
--- a/kernel/livepatch/shadow.c
+++ b/kernel/livepatch/shadow.c
@@ -113,8 +113,10 @@ void *klp_shadow_get(void *obj, unsigned long id)
 }
 EXPORT_SYMBOL_GPL(klp_shadow_get);
 
-static void *__klp_shadow_get_or_alloc(void *obj, unsigned long id, void *data,
-                       size_t size, gfp_t gfp_flags, bool warn_on_exist)
+static void *__klp_shadow_get_or_alloc(void *obj, unsigned long id,
+                                       size_t size, gfp_t gfp_flags,
+                                       klp_shadow_ctor_t ctor, void *ctor_data,
+                                       bool warn_on_exist)
 {
         struct klp_shadow *new_shadow;
         void *shadow_data;
@@ -125,18 +127,15 @@ static void *__klp_shadow_get_or_alloc(void *obj, unsigned long id, void *data,
         if (shadow_data)
                 goto exists;
 
-        /* Allocate a new shadow variable for use inside the lock below */
+        /*
+         * Allocate a new shadow variable.  Fill it with zeroes by default.
+         * More complex setting can be done by @ctor function.  But it is
+         * called only when the buffer is really used (under klp_shadow_lock).
+         */
         new_shadow = kzalloc(size + sizeof(*new_shadow), gfp_flags);
         if (!new_shadow)
                 return NULL;
 
-        new_shadow->obj = obj;
-        new_shadow->id = id;
-
-        /* Initialize the shadow variable if data provided */
-        if (data)
-                memcpy(new_shadow->data, data, size);
-
         /* Look for <obj, id> again under the lock */
         spin_lock_irqsave(&klp_shadow_lock, flags);
         shadow_data = klp_shadow_get(obj, id);
@@ -150,6 +149,22 @@ static void *__klp_shadow_get_or_alloc(void *obj, unsigned long id, void *data,
                 goto exists;
         }
 
+        new_shadow->obj = obj;
+        new_shadow->id = id;
+
+        if (ctor) {
+                int err;
+
+                err = ctor(obj, new_shadow->data, ctor_data);
+                if (err) {
+                        spin_unlock_irqrestore(&klp_shadow_lock, flags);
+                        kfree(new_shadow);
+                        pr_err("Failed to construct shadow variable <%p, %lx> (%d)\n",
+                               obj, id, err);
+                        return NULL;
+                }
+        }
+
         /* No <obj, id> found, so attach the newly allocated one */
         hash_add_rcu(klp_shadow_hash, &new_shadow->node,
                      (unsigned long)new_shadow->obj);
@@ -170,26 +185,32 @@ exists:
  * klp_shadow_alloc() - allocate and add a new shadow variable
  * @obj:        pointer to parent object
  * @id:         data identifier
- * @data:       pointer to data to attach to parent
  * @size:       size of attached data
  * @gfp_flags:  GFP mask for allocation
+ * @ctor:       custom constructor to initialize the shadow data (optional)
+ * @ctor_data:  pointer to any data needed by @ctor (optional)
+ *
+ * Allocates @size bytes for new shadow variable data using @gfp_flags.
+ * The data are zeroed by default.  They are further initialized by @ctor
+ * function if it is not NULL.  The new shadow variable is then added
+ * to the global hashtable.
  *
- * Allocates @size bytes for new shadow variable data using @gfp_flags
- * and copies @size bytes from @data into the new shadow variable's own
- * data space.  If @data is NULL, @size bytes are still allocated, but
- * no copy is performed.  The new shadow variable is then added to the
- * global hashtable.
+ * If an existing <obj, id> shadow variable can be found, this routine will
+ * issue a WARN, exit early and return NULL.
  *
- * If an existing <obj, id> shadow variable can be found, this routine
- * will issue a WARN, exit early and return NULL.
+ * This function guarantees that the constructor function is called only when
+ * the variable did not exist before.  The cost is that @ctor is called
+ * in atomic context under a spin lock.
  *
  * Return: the shadow variable data element, NULL on duplicate or
  * failure.
  */
-void *klp_shadow_alloc(void *obj, unsigned long id, void *data,
-                       size_t size, gfp_t gfp_flags)
+void *klp_shadow_alloc(void *obj, unsigned long id,
+                       size_t size, gfp_t gfp_flags,
+                       klp_shadow_ctor_t ctor, void *ctor_data)
 {
-        return __klp_shadow_get_or_alloc(obj, id, data, size, gfp_flags, true);
+        return __klp_shadow_get_or_alloc(obj, id, size, gfp_flags,
+                                         ctor, ctor_data, true);
 }
 EXPORT_SYMBOL_GPL(klp_shadow_alloc);
 
@@ -197,25 +218,28 @@ EXPORT_SYMBOL_GPL(klp_shadow_alloc);
  * klp_shadow_get_or_alloc() - get existing or allocate a new shadow variable
  * @obj:        pointer to parent object
  * @id:         data identifier
- * @data:       pointer to data to attach to parent
  * @size:       size of attached data
  * @gfp_flags:  GFP mask for allocation
+ * @ctor:       custom constructor to initialize the shadow data (optional)
+ * @ctor_data:  pointer to any data needed by @ctor (optional)
  *
  * Returns a pointer to existing shadow data if an <obj, id> shadow
  * variable is already present.  Otherwise, it creates a new shadow
  * variable like klp_shadow_alloc().
  *
- * This function guarantees that only one shadow variable exists with
- * the given @id for the given @obj.  It also guarantees that the shadow
- * variable will be initialized by the given @data only when it did not
- * exist before.
+ * This function guarantees that only one shadow variable exists with the given
+ * @id for the given @obj.  It also guarantees that the constructor function
+ * will be called only when the variable did not exist before.  The cost is
+ * that @ctor is called in atomic context under a spin lock.
  *
  * Return: the shadow variable data element, NULL on failure.
  */
-void *klp_shadow_get_or_alloc(void *obj, unsigned long id, void *data,
-                               size_t size, gfp_t gfp_flags)
+void *klp_shadow_get_or_alloc(void *obj, unsigned long id,
+                              size_t size, gfp_t gfp_flags,
+                              klp_shadow_ctor_t ctor, void *ctor_data)
 {
-        return __klp_shadow_get_or_alloc(obj, id, data, size, gfp_flags, false);
+        return __klp_shadow_get_or_alloc(obj, id, size, gfp_flags,
+                                         ctor, ctor_data, false);
 }
 EXPORT_SYMBOL_GPL(klp_shadow_get_or_alloc);
 
diff --git a/samples/livepatch/livepatch-shadow-fix1.c b/samples/livepatch/livepatch-shadow-fix1.c
index 830c55514f9f..04151c7f2631 100644
--- a/samples/livepatch/livepatch-shadow-fix1.c
+++ b/samples/livepatch/livepatch-shadow-fix1.c
@@ -56,6 +56,21 @@ struct dummy {
         unsigned long jiffies_expire;
 };
 
+/*
+ * The constructor makes more sense together with klp_shadow_get_or_alloc().
+ * In this example, it would be safe to assign the pointer also to the shadow
+ * variable returned by klp_shadow_alloc().  But we wanted to show the more
+ * complicated use of the API.
+ */
+static int shadow_leak_ctor(void *obj, void *shadow_data, void *ctor_data)
+{
+        void **shadow_leak = shadow_data;
+        void *leak = ctor_data;
+
+        *shadow_leak = leak;
+        return 0;
+}
+
 struct dummy *livepatch_fix1_dummy_alloc(void)
 {
         struct dummy *d;
@@ -74,7 +89,8 @@ struct dummy *livepatch_fix1_dummy_alloc(void)
          * pointer to handle resource release.
          */
         leak = kzalloc(sizeof(int), GFP_KERNEL);
-        klp_shadow_alloc(d, SV_LEAK, &leak, sizeof(leak), GFP_KERNEL);
+        klp_shadow_alloc(d, SV_LEAK, sizeof(leak), GFP_KERNEL,
+                         shadow_leak_ctor, leak);
 
         pr_info("%s: dummy @ %p, expires @ %lx\n",
                 __func__, d, d->jiffies_expire);
diff --git a/samples/livepatch/livepatch-shadow-fix2.c b/samples/livepatch/livepatch-shadow-fix2.c
index ff9948f0ec00..d6c62844dc15 100644
--- a/samples/livepatch/livepatch-shadow-fix2.c
+++ b/samples/livepatch/livepatch-shadow-fix2.c
@@ -53,17 +53,15 @@ struct dummy {
 bool livepatch_fix2_dummy_check(struct dummy *d, unsigned long jiffies)
 {
         int *shadow_count;
-        int count;
 
         /*
          * Patch: handle in-flight dummy structures, if they do not
          * already have a SV_COUNTER shadow variable, then attach a
          * new one.
          */
-        count = 0;
         shadow_count = klp_shadow_get_or_alloc(d, SV_COUNTER,
-                                               &count, sizeof(count),
-                                               GFP_NOWAIT);
+                                sizeof(*shadow_count), GFP_NOWAIT,
+                                NULL, NULL);
         if (shadow_count)
                 *shadow_count += 1;