[RFC PATCH bpf-next 3/4] bpf: Introduce css_descendant open-coded iterator kfuncs

[Chuyi Zhou]

This Patch adds kfuncs bpf_iter_css_{new,next,destroy} which allow creation
and manipulation of struct bpf_iter_css in open-coded iterator style. These
kfuncs actually wrapps css_next_descendant_{pre, post}. BPF programs can
use these kfuncs through bpf_for_each macro for iteration of all descendant
css under a root css.

Normally, css_next_descendant_{pre, post} should be called with rcu
locking. Although we have bpf_rcu_read_lock(), here we still calls
rcu_read_lock in bpf_iter_css_new and unlock in bpf_iter_css_destroy
for convenience use.

Signed-off-by: Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx>
---
 include/uapi/linux/bpf.h       |  5 +++++
 kernel/bpf/helpers.c           |  3 +++
 kernel/bpf/task_iter.c         | 39 ++++++++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h |  5 +++++
 tools/lib/bpf/bpf_helpers.h    |  6 ++++++
 5 files changed, 58 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index cfbd527e3733..19f1f1bf9301 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -7203,4 +7203,9 @@ struct bpf_iter_process {
 	__u64 __opaque[1];
 } __attribute__((aligned(8)));
 
+struct bpf_iter_css {
+	__u64 __opaque[2];
+	char __opaque_c[1];
+} __attribute__((aligned(8)));
+
 #endif /* _UAPI__LINUX_BPF_H__ */
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 81a2005edc26..47d46a51855f 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2461,6 +2461,9 @@ BTF_ID_FLAGS(func, bpf_iter_css_task_destroy, KF_ITER_DESTROY)
 BTF_ID_FLAGS(func, bpf_iter_process_new, KF_ITER_NEW)
 BTF_ID_FLAGS(func, bpf_iter_process_next, KF_ITER_NEXT | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_iter_process_destroy, KF_ITER_DESTROY)
+BTF_ID_FLAGS(func, bpf_iter_css_new, KF_ITER_NEW)
+BTF_ID_FLAGS(func, bpf_iter_css_next, KF_ITER_NEXT | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_iter_css_destroy, KF_ITER_DESTROY)
 BTF_ID_FLAGS(func, bpf_dynptr_adjust)
 BTF_ID_FLAGS(func, bpf_dynptr_is_null)
 BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index a6717a76c1e0..ef9aef62f1ac 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -893,6 +893,45 @@ __bpf_kfunc void bpf_iter_process_destroy(struct bpf_iter_process *it)
 	rcu_read_unlock();
 }
 
+struct bpf_iter_css_kern {
+	struct cgroup_subsys_state *root;
+	struct cgroup_subsys_state *pos;
+	char flag;
+} __attribute__((aligned(8)));
+
+__bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css *it,
+		struct cgroup_subsys_state *root, char flag)
+{
+	struct bpf_iter_css_kern *kit = (void *)it;
+
+	BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != sizeof(struct bpf_iter_css));
+	BUILD_BUG_ON(__alignof__(struct bpf_iter_css_kern) != __alignof__(struct bpf_iter_css));
+	kit->root = root;
+	kit->pos = NULL;
+	kit->flag = flag;
+	rcu_read_lock();
+	return 0;
+}
+
+__bpf_kfunc struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it)
+{
+	struct bpf_iter_css_kern *kit = (void *)it;
+
+	if (!kit->pos)
+		kit->pos = kit->flag ? css_next_descendant_post(NULL, kit->root)
+					: css_next_descendant_pre(NULL, kit->root);
+	else
+		kit->pos = kit->flag ? css_next_descendant_post(kit->pos, kit->root)
+					: css_next_descendant_pre(kit->pos, kit->root);
+
+	return kit->pos;
+}
+
+__bpf_kfunc void bpf_iter_css_destroy(struct bpf_iter_css *it)
+{
+	rcu_read_unlock();
+}
+
 DEFINE_PER_CPU(struct mmap_unlock_irq_work, mmap_unlock_work);
 
 static void do_mmap_read_unlock(struct irq_work *entry)
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index cfbd527e3733..19f1f1bf9301 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -7203,4 +7203,9 @@ struct bpf_iter_process {
 	__u64 __opaque[1];
 } __attribute__((aligned(8)));
 
+struct bpf_iter_css {
+	__u64 __opaque[2];
+	char __opaque_c[1];
+} __attribute__((aligned(8)));
+
 #endif /* _UAPI__LINUX_BPF_H__ */
diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index 7d6a828d98b5..bb56295b1442 100644
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h
@@ -314,6 +314,12 @@ extern int bpf_iter_process_new(struct bpf_iter_process *it) __weak __ksym;
 extern struct task_struct *bpf_iter_process_next(struct bpf_iter_process *it) __weak __ksym;
 extern void bpf_iter_process_destroy(struct bpf_iter_process *it) __weak __ksym;
 
+struct bpf_iter_css;
+extern int bpf_iter_css_new(struct bpf_iter_css *it,
+		struct cgroup_subsys_state *root, char flag) __weak __ksym;
+extern struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it) __weak __ksym;
+extern void bpf_iter_css_destroy(struct bpf_iter_css *it) __weak __ksym;
+
 #ifndef bpf_for_each
 /* bpf_for_each(iter_type, cur_elem, args...) provides generic construct for
  * using BPF open-coded iterators without having to write mundane explicit
-- 
2.20.1