Re: [PATCH bpf-next v1 8/9] bpf: Introduce cgroup iter

From: Yonghong Song
Date: Wed Mar 02 2022 - 21:03:48 EST



On 3/2/22 2:45 PM, Kumar Kartikeya Dwivedi wrote:
On Sat, Feb 26, 2022 at 05:13:38AM IST, Hao Luo wrote:
Introduce a new type of iter prog: cgroup. Unlike other bpf_iter, this
iter doesn't iterate a set of kernel objects. Instead, it is supposed to
be parameterized by a cgroup id and prints only that cgroup. So one
needs to specify a target cgroup id when attaching this iter.

The target cgroup's state can be read out via a link of this iter.
Typically, we can monitor cgroup creation and deletion using sleepable
tracing and use it to create corresponding directories in bpffs and pin
a cgroup id parameterized link in the directory. Then we can read the
auto-pinned iter link to get cgroup's state. The output of the iter link
is determined by the program. See the selftest test_cgroup_stats.c for
an example.

Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
---
include/linux/bpf.h | 1 +
include/uapi/linux/bpf.h | 6 ++
kernel/bpf/Makefile | 2 +-
kernel/bpf/cgroup_iter.c | 141 +++++++++++++++++++++++++++++++++
tools/include/uapi/linux/bpf.h | 6 ++
5 files changed, 155 insertions(+), 1 deletion(-)
create mode 100644 kernel/bpf/cgroup_iter.c

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 759ade7b24b3..3ce9b0b7ed89 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1595,6 +1595,7 @@ int bpf_obj_get_path(bpfptr_t pathname, int flags);

struct bpf_iter_aux_info {
struct bpf_map *map;
+ u64 cgroup_id;
};

typedef int (*bpf_iter_attach_target_t)(struct bpf_prog *prog,
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index a5dbc794403d..855ad80d9983 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -91,6 +91,9 @@ union bpf_iter_link_info {
struct {
__u32 map_fd;
} map;
+ struct {
+ __u64 cgroup_id;
+ } cgroup;
};

/* BPF syscall commands, see bpf(2) man-page for more details. */
@@ -5887,6 +5890,9 @@ struct bpf_link_info {
struct {
__u32 map_id;
} map;
+ struct {
+ __u64 cgroup_id;
+ } cgroup;
};
} iter;
struct {
diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
index c1a9be6a4b9f..52a0e4c6e96e 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_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/cgroup_iter.c b/kernel/bpf/cgroup_iter.c
new file mode 100644
index 000000000000..011d9dcd1d51
--- /dev/null
+++ b/kernel/bpf/cgroup_iter.c
@@ -0,0 +1,141 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2022 Google */
+#include <linux/bpf.h>
+#include <linux/btf_ids.h>
+#include <linux/cgroup.h>
+#include <linux/kernel.h>
+#include <linux/seq_file.h>
+
+struct bpf_iter__cgroup {
+ __bpf_md_ptr(struct bpf_iter_meta *, meta);
+ __bpf_md_ptr(struct cgroup *, cgroup);
+};
+
+static void *cgroup_iter_seq_start(struct seq_file *seq, loff_t *pos)
+{
+ struct cgroup *cgroup;
+ u64 cgroup_id;
+
+ /* Only one session is supported. */
+ if (*pos > 0)
+ return NULL;
+
+ cgroup_id = *(u64 *)seq->private;
+ cgroup = cgroup_get_from_id(cgroup_id);
+ if (!cgroup)
+ return NULL;
+
+ if (*pos == 0)
+ ++*pos;
+
+ return cgroup;
+}
+
+static void *cgroup_iter_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+{
+ ++*pos;
+ return NULL;
+}
+
+static int cgroup_iter_seq_show(struct seq_file *seq, void *v)
+{
+ struct bpf_iter__cgroup 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_iter_seq_stop(struct seq_file *seq, void *v)
+{
+ if (v)
+ cgroup_put(v);
+}

I think in existing iterators, we make a final call to seq_show, with v as NULL,
is there a specific reason to do it differently for this? There is logic in
bpf_iter.c to trigger ->stop() callback again when ->start() or ->next() returns
NULL, to execute BPF program with NULL p, see the comment above stop label.

If you do add the seq_show call with NULL, you'd also need to change the
ctx_arg_info PTR_TO_BTF_ID to PTR_TO_BTF_ID_OR_NULL.

Kumar, PTR_TO_BTF_ID should be okay since the show() never takes a non-NULL cgroup. But we do have issues for cgroup_iter_seq_stop() which I missed earlier.

For cgroup_iter, the following is the current workflow:
start -> not NULL -> show -> next -> NULL -> stop
or
start -> NULL -> stop

So for cgroup_iter_seq_stop, the input parameter 'v' will be NULL, so
the cgroup_put() is not actually called, i.e., corresponding cgroup is
not freed.

There are two ways to fix the issue:
. call cgroup_put() in next() before return NULL. This way,
stop() will be a noop.
. put cgroup_get_from_id() and cgroup_put() in
bpf_iter_attach_cgroup() and bpf_iter_detach_cgroup().

I prefer the second approach as it is cleaner.


+
+static const struct seq_operations cgroup_iter_seq_ops = {
+ .start = cgroup_iter_seq_start,
+ .next = cgroup_iter_seq_next,
+ .stop = cgroup_iter_seq_stop,
+ .show = cgroup_iter_seq_show,
+};
+
+BTF_ID_LIST_SINGLE(bpf_cgroup_btf_id, struct, cgroup)
+
+static int cgroup_iter_seq_init(void *priv_data, struct bpf_iter_aux_info *aux)
+{
+ *(u64 *)priv_data = aux->cgroup_id;
+ return 0;
+}
+
+static void cgroup_iter_seq_fini(void *priv_data)
+{
+}
+
+static const struct bpf_iter_seq_info cgroup_iter_seq_info = {
+ .seq_ops = &cgroup_iter_seq_ops,
+ .init_seq_private = cgroup_iter_seq_init,
+ .fini_seq_private = cgroup_iter_seq_fini,
+ .seq_priv_size = sizeof(u64),
+};
+
+static int bpf_iter_attach_cgroup(struct bpf_prog *prog,
+ union bpf_iter_link_info *linfo,
+ struct bpf_iter_aux_info *aux)
+{
+ aux->cgroup_id = linfo->cgroup.cgroup_id;
+ return 0;
+}
+
+static void bpf_iter_detach_cgroup(struct bpf_iter_aux_info *aux)
+{
+}
+
+void bpf_iter_cgroup_show_fdinfo(const struct bpf_iter_aux_info *aux,
+ struct seq_file *seq)
+{
+ char buf[64] = {0};
+
+ cgroup_path_from_kernfs_id(aux->cgroup_id, buf, sizeof(buf));
+ seq_printf(seq, "cgroup_id:\t%lu\n", aux->cgroup_id);
+ seq_printf(seq, "cgroup_path:\t%s\n", buf);
+}
+
+int bpf_iter_cgroup_fill_link_info(const struct bpf_iter_aux_info *aux,
+ struct bpf_link_info *info)
+{
+ info->iter.cgroup.cgroup_id = aux->cgroup_id;
+ return 0;
+}
+
+DEFINE_BPF_ITER_FUNC(cgroup, struct bpf_iter_meta *meta,
+ struct cgroup *cgroup)
+
+static struct bpf_iter_reg bpf_cgroup_reg_info = {
+ .target = "cgroup",
+ .attach_target = bpf_iter_attach_cgroup,
+ .detach_target = bpf_iter_detach_cgroup,
+ .show_fdinfo = bpf_iter_cgroup_show_fdinfo,
+ .fill_link_info = bpf_iter_cgroup_fill_link_info,
+ .ctx_arg_info_size = 1,
+ .ctx_arg_info = {
+ { offsetof(struct bpf_iter__cgroup, cgroup),
+ PTR_TO_BTF_ID },
+ },
+ .seq_info = &cgroup_iter_seq_info,
+};
+
+static int __init bpf_cgroup_iter_init(void)
+{
+ bpf_cgroup_reg_info.ctx_arg_info[0].btf_id = bpf_cgroup_btf_id[0];
+ return bpf_iter_reg_target(&bpf_cgroup_reg_info);
+}
+
+late_initcall(bpf_cgroup_iter_init);
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index a5dbc794403d..855ad80d9983 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -91,6 +91,9 @@ union bpf_iter_link_info {
struct {
__u32 map_fd;
} map;
+ struct {
+ __u64 cgroup_id;
+ } cgroup;
};

/* BPF syscall commands, see bpf(2) man-page for more details. */
@@ -5887,6 +5890,9 @@ struct bpf_link_info {
struct {
__u32 map_id;
} map;
+ struct {
+ __u64 cgroup_id;
+ } cgroup;
};
} iter;
struct {
--
2.35.1.574.g5d30c73bfb-goog


--
Kartikeya