bpf: Add alloc/xchg/direct_access support for local percpu kptr

Add two new kfunc's, bpf_percpu_obj_new_impl() and
bpf_percpu_obj_drop_impl(), to allocate a percpu obj.
Two functions are very similar to bpf_obj_new_impl()
and bpf_obj_drop_impl(). The major difference is related
to percpu handling.

    bpf_rcu_read_lock()
    struct val_t __percpu_kptr *v = map_val->percpu_data;
    ...
    bpf_rcu_read_unlock()

For a percpu data map_val like above 'v', the reg->type
is set as
	PTR_TO_BTF_ID | MEM_PERCPU | MEM_RCU
if inside rcu critical section.

MEM_RCU marking here is similar to NON_OWN_REF as 'v'
is not a owning reference. But NON_OWN_REF is
trusted and typically inside the spinlock while
MEM_RCU is under rcu read lock. RCU is preferred here
since percpu data structures mean potential concurrent
access into its contents.

Also, bpf_percpu_obj_new_impl() is restricted such that
no pointers or special fields are allowed. Therefore,
the bpf_list_head and bpf_rb_root will not be supported
in this patch set to avoid potential memory leak issue
due to racing between bpf_obj_free_fields() and another
bpf_kptr_xchg() moving an allocated object to
bpf_list_head and bpf_rb_root.
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230827152744.1996739-1-yonghong.song@linux.devSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/helpers.c

@@ -1902,6 +1902,14 @@ __bpf_kfunc void *bpf_obj_new_impl(u64 local_type_id__k, void *meta__ign)
 	return p;
 }
 
+__bpf_kfunc void *bpf_percpu_obj_new_impl(u64 local_type_id__k, void *meta__ign)
+{
+	u64 size = local_type_id__k;
+
+	/* The verifier has ensured that meta__ign must be NULL */
+	return bpf_mem_alloc(&bpf_global_percpu_ma, size);
+}
+
 /* Must be called under migrate_disable(), as required by bpf_mem_free */
 void __bpf_obj_drop_impl(void *p, const struct btf_record *rec)
 {
@@ -1930,6 +1938,12 @@ __bpf_kfunc void bpf_obj_drop_impl(void *p__alloc, void *meta__ign)
 	__bpf_obj_drop_impl(p, meta ? meta->record : NULL);
 }
 
+__bpf_kfunc void bpf_percpu_obj_drop_impl(void *p__alloc, void *meta__ign)
+{
+	/* The verifier has ensured that meta__ign must be NULL */
+	bpf_mem_free_rcu(&bpf_global_percpu_ma, p__alloc);
+}
+
 __bpf_kfunc void *bpf_refcount_acquire_impl(void *p__refcounted_kptr, void *meta__ign)
 {
 	struct btf_struct_meta *meta = meta__ign;
@@ -2442,7 +2456,9 @@ BTF_SET8_START(generic_btf_ids)
 BTF_ID_FLAGS(func, crash_kexec, KF_DESTRUCTIVE)
 #endif
 BTF_ID_FLAGS(func, bpf_obj_new_impl, KF_ACQUIRE | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_percpu_obj_new_impl, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_obj_drop_impl, KF_RELEASE)
+BTF_ID_FLAGS(func, bpf_percpu_obj_drop_impl, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_refcount_acquire_impl, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_list_push_front_impl)
 BTF_ID_FLAGS(func, bpf_list_push_back_impl)