bpf: Introduce cgroup iter

Cgroup_iter is a type of bpf_iter. It walks over cgroups in four modes:

 - walking a cgroup's descendants in pre-order.
 - walking a cgroup's descendants in post-order.
 - walking a cgroup's ancestors.
 - process only the given cgroup.

When attaching cgroup_iter, one can set a cgroup to the iter_link
created from attaching. This cgroup is passed as a file descriptor
or cgroup id and serves as the starting point of the walk. If no
cgroup is specified, the starting point will be the root cgroup v2.

For walking descendants, one can specify the order: either pre-order or
post-order. For walking ancestors, the walk starts at the specified
cgroup and ends at the root.

One can also terminate the walk early by returning 1 from the iter
program.

Note that because walking cgroup hierarchy holds cgroup_mutex, the iter
program is called with cgroup_mutex held.

Currently only one session is supported, which means, depending on the
volume of data bpf program intends to send to user space, the number
of cgroups that can be walked is limited. For example, given the current
buffer size is 8 * PAGE_SIZE, if the program sends 64B data for each
cgroup, assuming PAGE_SIZE is 4kb, the total number of cgroups that can
be walked is 512. This is a limitation of cgroup_iter. If the output
data is larger than the kernel buffer size, after all data in the
kernel buffer is consumed by user space, the subsequent read() syscall
will signal EOPNOTSUPP. In order to work around, the user may have to
update their program to reduce the volume of data sent to output. For
example, skip some uninteresting cgroups. In future, we may extend
bpf_iter flags to allow customizing buffer size.

Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Hao Luo <haoluo@google.com>
Link: https://lore.kernel.org/r/20220824233117.1312810-2-haoluo@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 30 insertions, 0 deletions

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 4fb685591035..5056cef2112f 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -87,10 +87,29 @@ struct bpf_cgroup_storage_key {
 	__u32	attach_type;		/* program attach type (enum bpf_attach_type) */
 };
 
+enum bpf_cgroup_iter_order {
+	BPF_ITER_ORDER_UNSPEC = 0,
+	BPF_ITER_SELF_ONLY,		/* process only a single object. */
+	BPF_ITER_DESCENDANTS_PRE,	/* walk descendants in pre-order. */
+	BPF_ITER_DESCENDANTS_POST,	/* walk descendants in post-order. */
+	BPF_ITER_ANCESTORS_UP,		/* walk ancestors upward. */
+};
+
 union bpf_iter_link_info {
 	struct {
 		__u32	map_fd;
 	} map;
+	struct {
+		enum bpf_cgroup_iter_order order;
+
+		/* At most one of cgroup_fd and cgroup_id can be non-zero. If
+		 * both are zero, the walk starts from the default cgroup v2
+		 * root. For walking v1 hierarchy, one should always explicitly
+		 * specify cgroup_fd.
+		 */
+		__u32	cgroup_fd;
+		__u64	cgroup_id;
+	} cgroup;
 };
 
 /* BPF syscall commands, see bpf(2) man-page for more details. */
@@ -6176,11 +6195,22 @@ struct bpf_link_info {
 		struct {
 			__aligned_u64 target_name; /* in/out: target_name buffer ptr */
 			__u32 target_name_len;	   /* in/out: target_name buffer len */
+
+			/* If the iter specific field is 32 bits, it can be put
+			 * in the first or second union. Otherwise it should be
+			 * put in the second union.
+			 */
 			union {
 				struct {
 					__u32 map_id;
 				} map;
 			};
+			union {
+				struct {
+					__u64 cgroup_id;
+					__u32 order;
+				} cgroup;
+			};
 		} iter;
 		struct  {
 			__u32 netns_ino;