[PATCH RFC bpf-next v2 3/5] bpf: cgroup_view iter

[Hao Luo <haoluo@google.com>]

Introduce a new type of iter prog: 'cgroup_view'. It prints out cgroup's
state.

Cgroup_view is supposed to be used together with directory tagging. When
cgroup_view is pinned in a directory, it tags that directory as
KERNFS_REP, i.e. a replicate of the cgroup hierarchy. Whenever a
subdirectory is created, if there is a child cgroup of the same name
exists, the subdirectory inherits the pinned cgroup_view object from
its parent and holds a reference of the corresponding kernfs node.

The cgroup_view prog takes a pointer to the cgroup and can use family
of seq_print helpers to print out cgroup state. A typical use case of
cgroup_view is to extend the cgroupfs interface.

Signed-off-by: Hao Luo <haoluo@google.com>
---
 include/linux/bpf.h           |   2 +
 kernel/bpf/Makefile           |   2 +-
 kernel/bpf/bpf_iter.c         |  11 ++++
 kernel/bpf/cgroup_view_iter.c | 114 ++++++++++++++++++++++++++++++++++
 4 files changed, 128 insertions(+), 1 deletion(-)
 create mode 100644 kernel/bpf/cgroup_view_iter.c

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 6eb0b180d33b..494927b2b3c2 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1610,6 +1610,7 @@ typedef const struct bpf_func_proto *
 
 enum bpf_iter_feature {
 	BPF_ITER_RESCHED	= BIT(0),
+	BPF_ITER_INHERIT	= BIT(1),
 };
 
 #define BPF_ITER_CTX_ARG_MAX 2
@@ -1647,6 +1648,7 @@ bpf_iter_get_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog);
 int bpf_iter_link_attach(const union bpf_attr *attr, bpfptr_t uattr, struct bpf_prog *prog);
 int bpf_iter_new_fd(struct bpf_link *link);
 bool bpf_link_is_iter(struct bpf_link *link);
+bool bpf_link_support_inherit(struct bpf_link *link);
 struct bpf_prog *bpf_iter_get_info(struct bpf_iter_meta *meta, bool in_stop);
 int bpf_iter_run_prog(struct bpf_prog *prog, void *ctx);
 void bpf_iter_map_show_fdinfo(const struct bpf_iter_aux_info *aux,
diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
index c1a9be6a4b9f..d9d2b8541ba7 100644
--- a/kernel/bpf/Makefile
+++ b/kernel/bpf/Makefile
@@ -8,7 +8,7 @@ CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-nogcse-yy)
 
 obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
 obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o bloom_filter.o
-obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o
+obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o cgroup_view_iter.o
 obj-$(CONFIG_BPF_SYSCALL) += bpf_local_storage.o bpf_task_storage.o
 obj-${CONFIG_BPF_LSM}	  += bpf_inode_storage.o
 obj-$(CONFIG_BPF_SYSCALL) += disasm.o
diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
index 110029ede71e..ff5577a5f73a 100644
--- a/kernel/bpf/bpf_iter.c
+++ b/kernel/bpf/bpf_iter.c
@@ -496,6 +496,17 @@ bool bpf_link_is_iter(struct bpf_link *link)
 	return link->ops == &bpf_iter_link_lops;
 }
 
+bool bpf_link_support_inherit(struct bpf_link *link)
+{
+	struct bpf_iter_link *iter_link;
+
+	if (!bpf_link_is_iter(link))
+		return false;
+
+	iter_link = container_of(link, struct bpf_iter_link, link);
+	return iter_link->tinfo->reg_info->feature & BPF_ITER_INHERIT;
+}
+
 int bpf_iter_link_attach(const union bpf_attr *attr, bpfptr_t uattr,
 			 struct bpf_prog *prog)
 {
diff --git a/kernel/bpf/cgroup_view_iter.c b/kernel/bpf/cgroup_view_iter.c
new file mode 100644
index 000000000000..a44d115235c4
--- /dev/null
+++ b/kernel/bpf/cgroup_view_iter.c
@@ -0,0 +1,114 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2022 Google */
+#include <linux/bpf.h>
+#include <linux/fs.h>
+#include <linux/filter.h>
+#include <linux/kernel.h>
+#include <linux/btf_ids.h>
+#include <linux/cgroup.h>
+#include <linux/kernfs.h>
+#include "inode.h"
+
+static void *cgroup_view_seq_start(struct seq_file *seq, loff_t *pos)
+{
+	struct bpf_dir_tag *tag;
+	struct kernfs_node *kn;
+	struct cgroup *cgroup;
+	struct inode *dir;
+
+	/* Only one session is supported. */
+	if (*pos > 0)
+		return NULL;
+
+	dir = d_inode(seq->file->f_path.dentry->d_parent);
+	tag = dir->i_private;
+	if (!tag)
+		return NULL;
+
+	kn = tag->private;
+
+	rcu_read_lock();
+	cgroup = rcu_dereference(*(void __rcu __force **)&kn->priv);
+	if (!cgroup || !cgroup_tryget(cgroup))
+		cgroup = NULL;
+	rcu_read_unlock();
+
+	if (!cgroup)
+		return NULL;
+
+	if (*pos == 0)
+		++*pos;
+	return cgroup;
+}
+
+static void *cgroup_view_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+{
+	++*pos;
+	return NULL;
+}
+
+struct bpf_iter__cgroup_view {
+	__bpf_md_ptr(struct bpf_iter_meta *, meta);
+	__bpf_md_ptr(struct cgroup *, cgroup);
+};
+
+DEFINE_BPF_ITER_FUNC(cgroup_view, struct bpf_iter_meta *meta, struct cgroup *cgroup)
+
+static int cgroup_view_seq_show(struct seq_file *seq, void *v)
+{
+	struct bpf_iter__cgroup_view ctx;
+	struct bpf_iter_meta meta;
+	struct bpf_prog *prog;
+	int ret = 0;
+
+	ctx.meta = &meta;
+	ctx.cgroup = v;
+	meta.seq = seq;
+	prog = bpf_iter_get_info(&meta, false);
+	if (prog)
+		ret = bpf_iter_run_prog(prog, &ctx);
+
+	return ret;
+}
+
+static void cgroup_view_seq_stop(struct seq_file *seq, void *v)
+{
+	if (v)
+		cgroup_put(v);
+}
+
+static const struct seq_operations cgroup_view_seq_ops = {
+	.start	= cgroup_view_seq_start,
+	.next	= cgroup_view_seq_next,
+	.stop	= cgroup_view_seq_stop,
+	.show	= cgroup_view_seq_show,
+};
+
+BTF_ID_LIST(btf_cgroup_id)
+BTF_ID(struct, cgroup)
+
+static const struct bpf_iter_seq_info cgroup_view_seq_info = {
+	.seq_ops		= &cgroup_view_seq_ops,
+	.init_seq_private	= NULL,
+	.fini_seq_private	= NULL,
+	.seq_priv_size		= 0,
+};
+
+static struct bpf_iter_reg cgroup_view_reg_info = {
+	.target			= "cgroup_view",
+	.feature		= BPF_ITER_INHERIT,
+	.ctx_arg_info_size	= 1,
+	.ctx_arg_info		= {
+		{ offsetof(struct bpf_iter__cgroup_view, cgroup),
+		  PTR_TO_BTF_ID },
+	},
+	.seq_info		= &cgroup_view_seq_info,
+};
+
+static int __init cgroup_view_init(void)
+{
+	cgroup_view_reg_info.ctx_arg_info[0].btf_id = *btf_cgroup_id;
+	return bpf_iter_reg_target(&cgroup_view_reg_info);
+}
+
+late_initcall(cgroup_view_init);
-- 
2.35.0.rc2.247.g8bbb082509-goog