bpf: Support kptrs in local storage maps

Enable support for kptrs in local storage maps by wiring up the freeing
of these kptrs from map value. Freeing of bpf_local_storage_map is only
delayed in case there are special fields, therefore bpf_selem_free_*
path can also only dereference smap safely in that case. This is
recorded using a bool utilizing a hole in bpF_local_storage_elem. It
could have been tagged in the pointer value smap using the lowest bit
(since alignment > 1), but since there was already a hole I went with
the simpler option. Only the map structure freeing is delayed using RCU
barriers, as the buckets aren't used when selem is being freed, so they
can be freed once all readers of the bucket lists can no longer access
it.

Cc: Martin KaFai Lau <martin.lau@kernel.org>
Cc: KP Singh <kpsingh@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230225154010.391965-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 44 insertions, 4 deletions

diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
index 58da17ae5124..2bdd722fe293 100644
--- a/kernel/bpf/bpf_local_storage.c
+++ b/kernel/bpf/bpf_local_storage.c
@@ -85,6 +85,7 @@ bpf_selem_alloc(struct bpf_local_storage_map *smap, void *owner,
 	if (selem) {
 		if (value)
 			copy_map_value(&smap->map, SDATA(selem)->data, value);
+		/* No need to call check_and_init_map_value as memory is zero init */
 		return selem;
 	}
 
@@ -113,10 +114,25 @@ static void bpf_selem_free_rcu(struct rcu_head *rcu)
 	struct bpf_local_storage_elem *selem;
 
 	selem = container_of(rcu, struct bpf_local_storage_elem, rcu);
+	/* The can_use_smap bool is set whenever we need to free additional
+	 * fields in selem data before freeing selem. bpf_local_storage_map_free
+	 * only executes rcu_barrier to wait for RCU callbacks when it has
+	 * special fields, hence we can only conditionally dereference smap, as
+	 * by this time the map might have already been freed without waiting
+	 * for our call_rcu callback if it did not have any special fields.
+	 */
+	if (selem->can_use_smap)
+		bpf_obj_free_fields(SDATA(selem)->smap->map.record, SDATA(selem)->data);
+	kfree(selem);
+}
+
+static void bpf_selem_free_tasks_trace_rcu(struct rcu_head *rcu)
+{
+	/* Free directly if Tasks Trace RCU GP also implies RCU GP */
 	if (rcu_trace_implies_rcu_gp())
-		kfree(selem);
+		bpf_selem_free_rcu(rcu);
 	else
-		kfree_rcu(selem, rcu);
+		call_rcu(rcu, bpf_selem_free_rcu);
 }
 
 /* local_storage->lock must be held and selem->local_storage == local_storage.
@@ -170,9 +186,9 @@ static bool bpf_selem_unlink_storage_nolock(struct bpf_local_storage *local_stor
 		RCU_INIT_POINTER(local_storage->cache[smap->cache_idx], NULL);
 
 	if (use_trace_rcu)
-		call_rcu_tasks_trace(&selem->rcu, bpf_selem_free_rcu);
+		call_rcu_tasks_trace(&selem->rcu, bpf_selem_free_tasks_trace_rcu);
 	else
-		kfree_rcu(selem, rcu);
+		call_rcu(&selem->rcu, bpf_selem_free_rcu);
 
 	return free_local_storage;
 }
@@ -240,6 +256,11 @@ void bpf_selem_link_map(struct bpf_local_storage_map *smap,
 	RCU_INIT_POINTER(SDATA(selem)->smap, smap);
 	hlist_add_head_rcu(&selem->map_node, &b->list);
 	raw_spin_unlock_irqrestore(&b->lock, flags);
+
+	/* If our data will have special fields, smap will wait for us to use
+	 * its record in bpf_selem_free_* RCU callbacks before freeing itself.
+	 */
+	selem->can_use_smap = !IS_ERR_OR_NULL(smap->map.record);
 }
 
 void bpf_selem_unlink(struct bpf_local_storage_elem *selem, bool use_trace_rcu)
@@ -723,6 +744,25 @@ void bpf_local_storage_map_free(struct bpf_map *map,
 	 */
 	synchronize_rcu();
 
+	/* Only delay freeing of smap, buckets are not needed anymore */
 	kvfree(smap->buckets);
+
+	/* When local storage has special fields, callbacks for
+	 * bpf_selem_free_rcu and bpf_selem_free_tasks_trace_rcu will keep using
+	 * the map BTF record, we need to execute an RCU barrier to wait for
+	 * them as the record will be freed right after our map_free callback.
+	 */
+	if (!IS_ERR_OR_NULL(smap->map.record)) {
+		rcu_barrier_tasks_trace();
+		/* We cannot skip rcu_barrier() when rcu_trace_implies_rcu_gp()
+		 * is true, because while call_rcu invocation is skipped in that
+		 * case in bpf_selem_free_tasks_trace_rcu (and all local storage
+		 * maps pass use_trace_rcu = true), there can be call_rcu
+		 * callbacks based on use_trace_rcu = false in the earlier while
+		 * ((selem = ...)) loop or from bpf_local_storage_unlink_nolock
+		 * called from owner's free path.
+		 */
+		rcu_barrier();
+	}
 	bpf_map_area_free(smap);
 }