[PATCH bpf-next v5 0/2] bpf, xdp: Add tracepoint to xdp attaching failure

[PATCH bpf-next v5 0/2] bpf, xdp: Add tracepoint to xdp attaching failure

[Leon Hwang]

This series introduces a new tracepoint in bpf_xdp_link_attach(). By
this tracepoint, error message will be captured when error happens in
dev_xdp_attach(), e.g. invalid attaching flags.

v4 -> v5:
* Initialise the extack variable.
* Fix code style issue of variable declaration lines.

v3 -> v4:
* Fix selftest-crashed issue.

Leon Hwang (2):
  bpf, xdp: add tracepoint to xdp attaching failure
  selftests/bpf: Add testcase for xdp attaching failure tracepoint

 include/trace/events/xdp.h                    | 17 +++++
 net/core/dev.c                                |  5 +-
 .../selftests/bpf/prog_tests/xdp_attach.c     | 65 +++++++++++++++++++
 .../bpf/progs/test_xdp_attach_fail.c          | 54 +++++++++++++++
 4 files changed, 140 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/progs/test_xdp_attach_fail.c


base-commit: a33d978500acd8fb67efac9773ba0a8502c1ff06
-- 
2.41.0

[PATCH bpf-next v5 1/2] bpf, xdp: Add tracepoint to xdp attaching failure

[Leon Hwang]

When error happens in dev_xdp_attach(), it should have a way to tell
users the error message like the netlink approach.

To avoid breaking uapi, adding a tracepoint in bpf_xdp_link_attach() is
an appropriate way to notify users the error message.

Hence, bpf libraries are able to retrieve the error message by this
tracepoint, and then report the error message to users.

Signed-off-by: Leon Hwang <hffilwlqm@gmail.com>
---
 include/trace/events/xdp.h | 17 +++++++++++++++++
 net/core/dev.c             |  5 ++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/include/trace/events/xdp.h b/include/trace/events/xdp.h
index c40fc97f94171..cd89f1d5ce7b8 100644
--- a/include/trace/events/xdp.h
+++ b/include/trace/events/xdp.h
@@ -404,6 +404,23 @@ TRACE_EVENT(mem_return_failed,
 	)
 );
 
+TRACE_EVENT(bpf_xdp_link_attach_failed,
+
+	TP_PROTO(const char *msg),
+
+	TP_ARGS(msg),
+
+	TP_STRUCT__entry(
+		__string(msg, msg)
+	),
+
+	TP_fast_assign(
+		__assign_str(msg, msg);
+	),
+
+	TP_printk("errmsg=%s", __get_str(msg))
+);
+
 #endif /* _TRACE_XDP_H */
 
 #include <trace/define_trace.h>
diff --git a/net/core/dev.c b/net/core/dev.c
index 8e7d0cb540cdb..002fec07de739 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -133,6 +133,7 @@
 #include <trace/events/net.h>
 #include <trace/events/skb.h>
 #include <trace/events/qdisc.h>
+#include <trace/events/xdp.h>
 #include <linux/inetdevice.h>
 #include <linux/cpu_rmap.h>
 #include <linux/static_key.h>
@@ -9470,6 +9471,7 @@ int bpf_xdp_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 {
 	struct net *net = current->nsproxy->net_ns;
 	struct bpf_link_primer link_primer;
+	struct netlink_ext_ack extack = {};
 	struct bpf_xdp_link *link;
 	struct net_device *dev;
 	int err, fd;
@@ -9497,12 +9499,13 @@ int bpf_xdp_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 		goto unlock;
 	}
 
-	err = dev_xdp_attach_link(dev, NULL, link);
+	err = dev_xdp_attach_link(dev, &extack, link);
 	rtnl_unlock();
 
 	if (err) {
 		link->dev = NULL;
 		bpf_link_cleanup(&link_primer);
+		trace_bpf_xdp_link_attach_failed(extack._msg);
 		goto out_put_dev;
 	}
 
-- 
2.41.0

Re: [PATCH bpf-next v5 1/2] bpf, xdp: Add tracepoint to xdp attaching failure

[Jakub Kicinski]

On Tue,  1 Aug 2023 22:26:20 +0800 Leon Hwang wrote:
> When error happens in dev_xdp_attach(), it should have a way to tell
> users the error message like the netlink approach.
> 
> To avoid breaking uapi, adding a tracepoint in bpf_xdp_link_attach() is
> an appropriate way to notify users the error message.
> 
> Hence, bpf libraries are able to retrieve the error message by this
> tracepoint, and then report the error message to users.

Whatevered-by: Jakub Kicinski <kuba@kernel.org> ?

Re: [PATCH bpf-next v5 1/2] bpf, xdp: Add tracepoint to xdp attaching failure

[Leon Hwang]

On 2/8/23 06:08, Jakub Kicinski wrote:
> On Tue,  1 Aug 2023 22:26:20 +0800 Leon Hwang wrote:
>> When error happens in dev_xdp_attach(), it should have a way to tell
>> users the error message like the netlink approach.
>>
>> To avoid breaking uapi, adding a tracepoint in bpf_xdp_link_attach() is
>> an appropriate way to notify users the error message.
>>
>> Hence, bpf libraries are able to retrieve the error message by this
>> tracepoint, and then report the error message to users.
> 
> Whatevered-by: Jakub Kicinski <kuba@kernel.org> ?

Oh, it's not responsible for libraries to retrieve the error message. It
should be users' responsibility.

Sorry for the misguiding text.

In the previous patch[0], which changes uapi, it's able to back-propagate
the error message from dev_xdp_attach() to userspace through BPF syscall.
Then, tracepoint idea is suggested to avoid changing uapi. I do agree to
implement a tracepoint to avoid changing uapi.

[0] bpf: Introduce user log
https://lore.kernel.org/bpf/20230708040750.72570-1-hffilwlqm@gmail.com/

Thanks,
Leon

[PATCH bpf-next v5 2/2] selftests/bpf: Add testcase for xdp attaching failure tracepoint

[Leon Hwang]

Add a test case for the tracepoint of xdp attaching failure by bpf
tracepoint when attach XDP to a device with invalid flags option.

The bpf tracepoint retrieves error message from the tracepoint, and
then put the error message to a perf buffer. The testing code receives
error message from perf buffer, and then ASSERT "Invalid XDP flags for
BPF link attachment".

Signed-off-by: Leon Hwang <hffilwlqm@gmail.com>
---
 .../selftests/bpf/prog_tests/xdp_attach.c     | 65 +++++++++++++++++++
 .../bpf/progs/test_xdp_attach_fail.c          | 54 +++++++++++++++
 2 files changed, 119 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/test_xdp_attach_fail.c

diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_attach.c b/tools/testing/selftests/bpf/prog_tests/xdp_attach.c
index fa3cac5488f5d..e6bcb6051402d 100644
--- a/tools/testing/selftests/bpf/prog_tests/xdp_attach.c
+++ b/tools/testing/selftests/bpf/prog_tests/xdp_attach.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <test_progs.h>
+#include "test_xdp_attach_fail.skel.h"
 
 #define IFINDEX_LO 1
 #define XDP_FLAGS_REPLACE		(1U << 4)
@@ -85,10 +86,74 @@ static void test_xdp_attach(const char *file)
 	bpf_object__close(obj1);
 }
 
+#define ERRMSG_LEN 64
+
+struct xdp_errmsg {
+	char msg[ERRMSG_LEN];
+};
+
+static void on_xdp_errmsg(void *ctx, int cpu, void *data, __u32 size)
+{
+	struct xdp_errmsg *ctx_errmg = ctx, *tp_errmsg = data;
+
+	memcpy(&ctx_errmg->msg, &tp_errmsg->msg, ERRMSG_LEN);
+}
+
+static const char tgt_errmsg[] = "Invalid XDP flags for BPF link attachment";
+
+static void test_xdp_attach_fail(const char *file)
+{
+	struct test_xdp_attach_fail *skel = NULL;
+	struct xdp_errmsg errmsg = {};
+	struct perf_buffer *pb = NULL;
+	struct bpf_object *obj = NULL;
+	int err, fd_xdp;
+
+	LIBBPF_OPTS(bpf_link_create_opts, opts);
+
+	skel = test_xdp_attach_fail__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "test_xdp_attach_fail__open_and_load"))
+		goto out_close;
+
+	err = test_xdp_attach_fail__attach(skel);
+	if (!ASSERT_EQ(err, 0, "test_xdp_attach_fail__attach"))
+		goto out_close;
+
+	/* set up perf buffer */
+	pb = perf_buffer__new(bpf_map__fd(skel->maps.xdp_errmsg_pb), 1,
+			      on_xdp_errmsg, NULL, &errmsg, NULL);
+	if (!ASSERT_OK_PTR(pb, "perf_buffer__new"))
+		goto out_close;
+
+	err = bpf_prog_test_load(file, BPF_PROG_TYPE_XDP, &obj, &fd_xdp);
+	if (!ASSERT_EQ(err, 0, "bpf_prog_test_load"))
+		goto out_close;
+
+	opts.flags = 0xFF; // invalid flags to fail to attach XDP prog
+	err = bpf_link_create(fd_xdp, IFINDEX_LO, BPF_XDP, &opts);
+	if (!ASSERT_EQ(err, -EINVAL, "bpf_link_create"))
+		goto out_close;
+
+	/* read perf buffer */
+	err = perf_buffer__poll(pb, 100);
+	if (!ASSERT_GT(err, -1, "perf_buffer__poll"))
+		goto out_close;
+
+	ASSERT_STRNEQ((const char *) errmsg.msg, tgt_errmsg,
+		      42 /* strlen(tgt_errmsg) */, "check error message");
+
+out_close:
+	perf_buffer__free(pb);
+	bpf_object__close(obj);
+	test_xdp_attach_fail__destroy(skel);
+}
+
 void serial_test_xdp_attach(void)
 {
 	if (test__start_subtest("xdp_attach"))
 		test_xdp_attach("./test_xdp.bpf.o");
 	if (test__start_subtest("xdp_attach_dynptr"))
 		test_xdp_attach("./test_xdp_dynptr.bpf.o");
+	if (test__start_subtest("xdp_attach_failed"))
+		test_xdp_attach_fail("./xdp_dummy.bpf.o");
 }
diff --git a/tools/testing/selftests/bpf/progs/test_xdp_attach_fail.c b/tools/testing/selftests/bpf/progs/test_xdp_attach_fail.c
new file mode 100644
index 0000000000000..2ff1b596e87ea
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_xdp_attach_fail.c
@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright Leon Hwang */
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+#define ERRMSG_LEN 64
+
+struct xdp_errmsg {
+	char msg[ERRMSG_LEN];
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
+	__type(key, int);
+	__type(value, int);
+} xdp_errmsg_pb SEC(".maps");
+
+struct xdp_attach_error_ctx {
+	unsigned long unused;
+
+	/*
+	 * bpf does not support tracepoint __data_loc directly.
+	 *
+	 * Actually, this field is a 32 bit integer whose value encodes
+	 * information on where to find the actual data. The first 2 bytes is
+	 * the size of the data. The last 2 bytes is the offset from the start
+	 * of the tracepoint struct where the data begins.
+	 * -- https://github.com/iovisor/bpftrace/pull/1542
+	 */
+	__u32 msg; // __data_loc char[] msg;
+};
+
+/*
+ * Catch the error message at the tracepoint.
+ */
+
+SEC("tp/xdp/bpf_xdp_link_attach_failed")
+int tp__xdp__bpf_xdp_link_attach_failed(struct xdp_attach_error_ctx *ctx)
+{
+	char *msg = (void *)(__u64) ((void *) ctx + (__u16) ctx->msg);
+	struct xdp_errmsg errmsg = {};
+
+	bpf_probe_read_kernel_str(&errmsg.msg, ERRMSG_LEN, msg);
+	bpf_perf_event_output(ctx, &xdp_errmsg_pb, BPF_F_CURRENT_CPU, &errmsg,
+			      ERRMSG_LEN);
+	return 0;
+}
+
+/*
+ * Reuse the XDP program in xdp_dummy.c.
+ */
+
+char LICENSE[] SEC("license") = "GPL";
-- 
2.41.0

Re: [PATCH bpf-next v5 0/2] bpf, xdp: Add tracepoint to xdp attaching failure

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Tue,  1 Aug 2023 22:26:19 +0800 you wrote:
> This series introduces a new tracepoint in bpf_xdp_link_attach(). By
> this tracepoint, error message will be captured when error happens in
> dev_xdp_attach(), e.g. invalid attaching flags.
> 
> v4 -> v5:
> * Initialise the extack variable.
> * Fix code style issue of variable declaration lines.
> 
> [...]

Here is the summary with links:
  - [bpf-next,v5,1/2] bpf, xdp: Add tracepoint to xdp attaching failure
    https://git.kernel.org/bpf/bpf-next/c/bf4ea1d0b2cb
  - [bpf-next,v5,2/2] selftests/bpf: Add testcase for xdp attaching failure tracepoint
    https://git.kernel.org/bpf/bpf-next/c/7fedbf32fcc7

You are awesome, thank you!
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