[RFC PATCH 4/5] bpf: Add a new bpf helper to get cgroup ino

[Chuyi Zhou]

This patch adds a new bpf helper bpf_get_ino_from_cgroup_id, so that
we can get the inode number once we know the cgroup id.

Cgroup_id is used to identify a cgroup in BPF prog. However we can't
get the cgroup id directly in userspace applications. In userspace,
we are used to identifying cgroups by their paths or their inodes.
However, cgroup id is not always equal to the inode number, depending
on the sizeof ino_t.

For example, given some cgroup paths, we only care about the events
related to those cgroups. We can only do this by updating these paths
in a map and doing string comparison in BPF prog, which is not very
convenient. However with this new helper, we just need to record
the inode in a map and lookup a inode number in BPF prog.

Signed-off-by: Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx>
---
 include/uapi/linux/bpf.h       |  7 +++++++
 kernel/bpf/core.c              |  1 +
 kernel/bpf/helpers.c           | 17 +++++++++++++++++
 tools/include/uapi/linux/bpf.h |  7 +++++++
 4 files changed, 32 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 9da0d61cf703..01efb289fa14 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -5575,6 +5575,12 @@ union bpf_attr {
  *		0 on success.
  *
  *		**-ENOENT** if the bpf_local_storage cannot be found.
+ *
+ * u64 bpf_get_ino_from_cgroup_id(u64 id)
+ *	Description
+ *		Get inode number from a *cgroup id*.
+ *	Return
+ *		Inode number.
  */
 #define ___BPF_FUNC_MAPPER(FN, ctx...)			\
 	FN(unspec, 0, ##ctx)				\
@@ -5789,6 +5795,7 @@ union bpf_attr {
 	FN(user_ringbuf_drain, 209, ##ctx)		\
 	FN(cgrp_storage_get, 210, ##ctx)		\
 	FN(cgrp_storage_delete, 211, ##ctx)		\
+	FN(get_ino_from_cgroup_id, 212, ##ctx)		\
 	/* */
 
 /* backwards-compatibility macros for users of __BPF_FUNC_MAPPER that don't
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index dc85240a0134..49dfdb2dd336 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -2666,6 +2666,7 @@ const struct bpf_func_proto bpf_snprintf_btf_proto __weak;
 const struct bpf_func_proto bpf_seq_printf_btf_proto __weak;
 const struct bpf_func_proto bpf_set_retval_proto __weak;
 const struct bpf_func_proto bpf_get_retval_proto __weak;
+const struct bpf_func_proto bpf_get_ino_from_cgroup_id_proto __weak;
 
 const struct bpf_func_proto * __weak bpf_get_trace_printk_proto(void)
 {
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 9e80efa59a5d..e87328b008d3 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -433,6 +433,21 @@ const struct bpf_func_proto bpf_get_current_ancestor_cgroup_id_proto = {
 	.ret_type	= RET_INTEGER,
 	.arg1_type	= ARG_ANYTHING,
 };
+
+BPF_CALL_1(bpf_get_ino_from_cgroup_id, u64, id)
+{
+	u64 ino = kernfs_id_ino(id);
+
+	return ino;
+}
+
+const struct bpf_func_proto bpf_get_ino_from_cgroup_id_proto = {
+	.func		= bpf_get_ino_from_cgroup_id,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_ANYTHING,
+};
+
 #endif /* CONFIG_CGROUPS */
 
 #define BPF_STRTOX_BASE_MASK 0x1F
@@ -1767,6 +1782,8 @@ bpf_base_func_proto(enum bpf_func_id func_id)
 		return &bpf_get_current_cgroup_id_proto;
 	case BPF_FUNC_get_current_ancestor_cgroup_id:
 		return &bpf_get_current_ancestor_cgroup_id_proto;
+	case BPF_FUNC_get_ino_from_cgroup_id:
+		return &bpf_get_ino_from_cgroup_id_proto;
 #endif
 	default:
 		break;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 9da0d61cf703..661d97aacb85 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -5575,6 +5575,12 @@ union bpf_attr {
  *		0 on success.
  *
  *		**-ENOENT** if the bpf_local_storage cannot be found.
+ *
+ * u64 bpf_get_ino_from_cgroup_id(u64 id)
+ *	Description
+ *		Get inode number from a *cgroup id*.
+ *	Return
+ *		Inode number.
  */
 #define ___BPF_FUNC_MAPPER(FN, ctx...)			\
 	FN(unspec, 0, ##ctx)				\
@@ -5789,6 +5795,7 @@ union bpf_attr {
 	FN(user_ringbuf_drain, 209, ##ctx)		\
 	FN(cgrp_storage_get, 210, ##ctx)		\
 	FN(cgrp_storage_delete, 211, ##ctx)		\
+	FN(get_ino_from_cgroup_id, 212, ##ctx)  \
 	/* */
 
 /* backwards-compatibility macros for users of __BPF_FUNC_MAPPER that don't
-- 
2.20.1