bpf: Disable migration in bpf_selem_free_rcu

bpf_selem_free_rcu() calls bpf_obj_free_fields() to free the special
fields in map value (e.g., kptr). Since kptrs may be allocated from bpf
memory allocator, migrate_{disable|enable} pairs are necessary for the
freeing of these kptrs.

To simplify reasoning about when migrate_disable() is needed for the
freeing of these dynamically-allocated kptrs, let the caller to
guarantee migration is disabled before invoking bpf_obj_free_fields().

Therefore, the patch adds migrate_{disable|enable} pair in
bpf_selem_free_rcu(). The migrate_{disable|enable} pairs in the
underlying implementation of bpf_obj_free_fields() will be removed by
the following patch.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20250108010728.207536-10-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Hou Tao

kernel/bpf/bpf_local_storage.c

@@ -217,7 +217,10 @@ static void bpf_selem_free_rcu(struct rcu_head *rcu)
 	selem = container_of(rcu, struct bpf_local_storage_elem, rcu);
 	/* The bpf_local_storage_map_free will wait for rcu_barrier */
 	smap = rcu_dereference_check(SDATA(selem)->smap, 1);
+
+	migrate_disable();
 	bpf_obj_free_fields(smap->map.record, SDATA(selem)->data);
+	migrate_enable();
 	bpf_mem_cache_raw_free(selem);
 }