[PATCH v3] bpftool: Add 'prepend' option for tcx attach to insert at chain start

[gyutae.opensource@navercorp.com]

From: Gyutae Bae <gyutae.bae@navercorp.com>

Add support for the 'prepend' option when attaching tcx_ingress and
tcx_egress programs. This option allows inserting a BPF program at
the beginning of the TCX chain instead of appending it at the end.

The implementation uses BPF_F_BEFORE flag which automatically inserts
the program at the beginning of the chain when no relative reference
is specified.

This change includes:
- Modify do_attach_tcx() to support prepend insertion using BPF_F_BEFORE
- Update documentation to describe the new 'prepend' option
- Add bash completion support for the 'prepend' option on tcx attach types
- Add example usage in the documentation

The 'prepend' option is only valid for tcx_ingress and tcx_egress attach
types. For XDP attach types, the existing 'overwrite' option remains
available.

Example usage:
  # bpftool net attach tcx_ingress name tc_prog dev lo prepend

This feature is useful when the order of program execution in the TCX
chain matters and users need to ensure certain programs run first.

Co-developed-by: Siwan Kim <siwan.kim@navercorp.com>
Signed-off-by: Siwan Kim <siwan.kim@navercorp.com>
Signed-off-by: Gyutae Bae <gyutae.bae@navercorp.com>
---
Hi Daniel.

Thank you for the detailed feedback. Thanks to your explanation,
I now understand that BPF_F_BEFORE and BPF_F_AFTER work as standalone flags.
This has made the implementation much simpler and cleaner.

Thanks,
Gyutae.

Changes in v3:
- Simplified implementation by using BPF_F_BEFORE alone (Daniel)
- Removed get_first_tcx_prog_id() helper function (Daniel)

Changes in v2:
- Renamed 'head' to 'prepend' for consistency with 'overwrite' (Quentin)
- Moved relative_id variable to relevant scope inside if block (Quentin)
- Changed condition style from '== 0' to '!' (Quentin)
- Updated documentation to clarify 'overwrite' is XDP-only (Quentin)
- Removed outdated "only XDP-related modes are supported" note (Quentin)
- Removed extra help text from do_help() for consistency (Quentin)

 .../bpf/bpftool/Documentation/bpftool-net.rst | 30 ++++++++++++++-----
 tools/bpf/bpftool/bash-completion/bpftool     |  9 +++++-
 tools/bpf/bpftool/net.c                       | 23 +++++++++++---
 3 files changed, 50 insertions(+), 12 deletions(-)

diff --git a/tools/bpf/bpftool/Documentation/bpftool-net.rst b/tools/bpf/bpftool/Documentation/bpftool-net.rst
index a9ed8992800f..22da07087e42 100644
--- a/tools/bpf/bpftool/Documentation/bpftool-net.rst
+++ b/tools/bpf/bpftool/Documentation/bpftool-net.rst
@@ -24,7 +24,7 @@ NET COMMANDS
 ============

 | **bpftool** **net** { **show** | **list** } [ **dev** *NAME* ]
-| **bpftool** **net attach** *ATTACH_TYPE* *PROG* **dev** *NAME* [ **overwrite** ]
+| **bpftool** **net attach** *ATTACH_TYPE* *PROG* **dev** *NAME* [ **overwrite** | **prepend** ]
 | **bpftool** **net detach** *ATTACH_TYPE* **dev** *NAME*
 | **bpftool** **net help**
 |
@@ -58,11 +58,9 @@ bpftool net { show | list } [ dev *NAME* ]
     then all bpf programs attached to non clsact qdiscs, and finally all bpf
     programs attached to root and clsact qdisc.

-bpftool net attach *ATTACH_TYPE* *PROG* dev *NAME* [ overwrite ]
+bpftool net attach *ATTACH_TYPE* *PROG* dev *NAME* [ overwrite | prepend ]
     Attach bpf program *PROG* to network interface *NAME* with type specified
-    by *ATTACH_TYPE*. Previously attached bpf program can be replaced by the
-    command used with **overwrite** option. Currently, only XDP-related modes
-    are supported for *ATTACH_TYPE*.
+    by *ATTACH_TYPE*.

     *ATTACH_TYPE* can be of:
     **xdp** - try native XDP and fallback to generic XDP if NIC driver does not support it;
@@ -72,11 +70,18 @@ bpftool net attach *ATTACH_TYPE* *PROG* dev *NAME* [ overwrite ]
     **tcx_ingress** - Ingress TCX. runs on ingress net traffic;
     **tcx_egress** - Egress TCX. runs on egress net traffic;

+    For XDP-related attach types (**xdp**, **xdpgeneric**, **xdpdrv**,
+    **xdpoffload**), the **overwrite** option can be used to replace a
+    previously attached bpf program.
+
+    For **tcx_ingress** and **tcx_egress** attach types, the **prepend** option
+    can be used to attach the program at the beginning of the chain instead of
+    at the end.
+
 bpftool net detach *ATTACH_TYPE* dev *NAME*
     Detach bpf program attached to network interface *NAME* with type specified
     by *ATTACH_TYPE*. To detach bpf program, same *ATTACH_TYPE* previously used
-    for attach must be specified. Currently, only XDP-related modes are
-    supported for *ATTACH_TYPE*.
+    for attach must be specified.

 bpftool net help
     Print short help message.
@@ -191,6 +196,17 @@ EXAMPLES
       tc:
       lo(1) tcx/ingress tc_prog prog_id 29

+|
+| **# bpftool net attach tcx_ingress name tc_prog2 dev lo prepend**
+| **# bpftool net**
+|
+
+::
+
+      tc:
+      lo(1) tcx/ingress tc_prog2 prog_id 30
+      lo(1) tcx/ingress tc_prog prog_id 29
+
 |
 | **# bpftool net attach tcx_ingress name tc_prog dev lo**
 | **# bpftool net detach tcx_ingress dev lo**
diff --git a/tools/bpf/bpftool/bash-completion/bpftool b/tools/bpf/bpftool/bash-completion/bpftool
index 53bcfeb1a76e..a28f0cc522e4 100644
--- a/tools/bpf/bpftool/bash-completion/bpftool
+++ b/tools/bpf/bpftool/bash-completion/bpftool
@@ -1142,7 +1142,14 @@ _bpftool()
                             return 0
                             ;;
                         8)
-                            _bpftool_once_attr 'overwrite'
+                            case ${words[3]} in
+                                tcx_ingress|tcx_egress)
+                                    _bpftool_once_attr 'prepend'
+                                    ;;
+                                *)
+                                    _bpftool_once_attr 'overwrite'
+                                    ;;
+                            esac
                             return 0
                             ;;
                     esac
diff --git a/tools/bpf/bpftool/net.c b/tools/bpf/bpftool/net.c
index cfc6f944f7c3..1a2ba3312a82 100644
--- a/tools/bpf/bpftool/net.c
+++ b/tools/bpf/bpftool/net.c
@@ -666,10 +666,16 @@ static int get_tcx_type(enum net_attach_type attach_type)
 	}
 }

-static int do_attach_tcx(int progfd, enum net_attach_type attach_type, int ifindex)
+static int do_attach_tcx(int progfd, enum net_attach_type attach_type, int ifindex, bool prepend)
 {
 	int type = get_tcx_type(attach_type);

+	if (prepend) {
+		LIBBPF_OPTS(bpf_prog_attach_opts, opts,
+			.flags = BPF_F_BEFORE
+		);
+		return bpf_prog_attach_opts(progfd, ifindex, type, &opts);
+	}
 	return bpf_prog_attach(progfd, ifindex, type, 0);
 }

@@ -685,6 +691,7 @@ static int do_attach(int argc, char **argv)
 	enum net_attach_type attach_type;
 	int progfd, ifindex, err = 0;
 	bool overwrite = false;
+	bool prepend = false;

 	/* parse attach args */
 	if (!REQ_ARGS(5))
@@ -710,8 +717,16 @@ static int do_attach(int argc, char **argv)
 	if (argc) {
 		if (is_prefix(*argv, "overwrite")) {
 			overwrite = true;
+		} else if (is_prefix(*argv, "prepend")) {
+			if (attach_type != NET_ATTACH_TYPE_TCX_INGRESS &&
+			    attach_type != NET_ATTACH_TYPE_TCX_EGRESS) {
+				p_err("'prepend' is only supported for tcx_ingress/tcx_egress");
+				err = -EINVAL;
+				goto cleanup;
+			}
+			prepend = true;
 		} else {
-			p_err("expected 'overwrite', got: '%s'?", *argv);
+			p_err("expected 'overwrite' or 'prepend', got: '%s'?", *argv);
 			err = -EINVAL;
 			goto cleanup;
 		}
@@ -728,7 +743,7 @@ static int do_attach(int argc, char **argv)
 	/* attach tcx prog */
 	case NET_ATTACH_TYPE_TCX_INGRESS:
 	case NET_ATTACH_TYPE_TCX_EGRESS:
-		err = do_attach_tcx(progfd, attach_type, ifindex);
+		err = do_attach_tcx(progfd, attach_type, ifindex, prepend);
 		break;
 	default:
 		break;
@@ -985,7 +1000,7 @@ static int do_help(int argc, char **argv)

 	fprintf(stderr,
 		"Usage: %1$s %2$s { show | list } [dev <devname>]\n"
-		"       %1$s %2$s attach ATTACH_TYPE PROG dev <devname> [ overwrite ]\n"
+		"       %1$s %2$s attach ATTACH_TYPE PROG dev <devname> [ overwrite | prepend ]\n"
 		"       %1$s %2$s detach ATTACH_TYPE dev <devname>\n"
 		"       %1$s %2$s help\n"
 		"\n"
--
2.39.5 (Apple Git-154)