On Mon, Jun 23, 2025 at 01:59:18PM -0700, Andrii Nakryiko wrote:

On Mon, Jun 23, 2025 at 10:56 AM Alexei Starovoitov
[off-list ref] wrote:

quoted

On Mon, Jun 23, 2025 at 6:44 AM Tao Chen [off-list ref] wrote:

quoted

Alexei suggested, 'link_type' can be more precise and differentiate
for human in fdinfo. In fact BPF_LINK_TYPE_KPROBE_MULTI includes
kretprobe_multi type, the same as BPF_LINK_TYPE_UPROBE_MULTI, so we
can show it more concretely.

link_type:      kprobe_multi
link_id:        1
prog_tag:       d2b307e915f0dd37
...
link_type:      kretprobe_multi
link_id:        2
prog_tag:       ab9ea0545870781d
...
link_type:      uprobe_multi
link_id:        9
prog_tag:       e729f789e34a8eca
...
link_type:      uretprobe_multi
link_id:        10
prog_tag:       7db356c03e61a4d4

Signed-off-by: Tao Chen <redacted>
---
 include/linux/trace_events.h | 10 ++++++++++
 kernel/bpf/syscall.c         |  9 ++++++++-
 kernel/trace/bpf_trace.c     | 28 ++++++++++++++++++++++++++++
 3 files changed, 46 insertions(+), 1 deletion(-)

Change list:
  v4 -> v5:
    - Add patch1 to show precise link_type for
      {uprobe,kprobe}_multi.(Alexei)
    - patch2,3 just remove type field, which will be showed in
      link_type
  v4:
  https://lore.kernel.org/bpf/20250619034257.70520-1-chen.dylane@linux.dev (local)

  v3 -> v4:
    - use %pS to print func info.(Alexei)
  v3:
  https://lore.kernel.org/bpf/20250616130233.451439-1-chen.dylane@linux.dev (local)

  v2 -> v3:
    - show info in one line for multi events.(Jiri)
  v2:
  https://lore.kernel.org/bpf/20250615150514.418581-1-chen.dylane@linux.dev (local)

  v1 -> v2:
    - replace 'func_cnt' with 'uprobe_cnt'.(Andrii)
    - print func name is more readable and security for kprobe_multi.(Alexei)
  v1:
  https://lore.kernel.org/bpf/20250612115556.295103-1-chen.dylane@linux.dev (local)

diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
index fa9cf4292df..951c91babbc 100644
--- a/include/linux/trace_events.h
+++ b/include/linux/trace_events.h

@@ -780,6 +780,8 @@ int bpf_get_perf_event_info(const struct perf_event *event, u32 *prog_id,
                            unsigned long *missed);
 int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog);
 int bpf_uprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog);
+void bpf_kprobe_multi_link_type_show(const struct bpf_link *link, char *link_type, int len);
+void bpf_uprobe_multi_link_type_show(const struct bpf_link *link, char *link_type, int len);
 #else
 static inline unsigned int trace_call_bpf(struct trace_event_call *call, void *ctx)
 {

@@ -832,6 +834,14 @@ bpf_uprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 {
        return -EOPNOTSUPP;
 }
+static inline void
+bpf_kprobe_multi_link_type_show(const struct bpf_link *link, char *link_type, int len)
+{
+}
+static inline void
+bpf_uprobe_multi_link_type_show(const struct bpf_link *link, char *link_type, int len)
+{
+}
 #endif

 enum {

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 51ba1a7aa43..43b821b37bc 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c

@@ -3226,9 +3226,16 @@ static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
        const struct bpf_prog *prog = link->prog;
        enum bpf_link_type type = link->type;
        char prog_tag[sizeof(prog->tag) * 2 + 1] = { };
+       char link_type[64] = {};

        if (type < ARRAY_SIZE(bpf_link_type_strs) && bpf_link_type_strs[type]) {
-               seq_printf(m, "link_type:\t%s\n", bpf_link_type_strs[type]);
+               if (link->type == BPF_LINK_TYPE_KPROBE_MULTI)
+                       bpf_kprobe_multi_link_type_show(link, link_type, sizeof(link_type));
+               else if (link->type == BPF_LINK_TYPE_UPROBE_MULTI)
+                       bpf_uprobe_multi_link_type_show(link, link_type, sizeof(link_type));
+               else
+                       strscpy(link_type, bpf_link_type_strs[type], sizeof(link_type));
+               seq_printf(m, "link_type:\t%s\n", link_type);

New callbacks just to print a string?
Let's find a different way.

How about moving 'flags' from bpf_[ku]probe_multi_link into bpf_link ?
(There is a 7 byte hole there anyway)
and checking flags inline.

Jiri, Andrii,

better ideas?

We can just remember original attr->link_create.attach_type in
bpf_link itself, and then have a small helper that will accept link
type and attach type, and fill out link type representation based on
those two. Internally we can do the special-casing of  uprobe vs
uretprobe and kprobe vs kretprobe transparently to all the other code.
And use that here in show_fdinfo

but you'd still need the flags, no? to find out if it's return probe

I tried what Alexei suggested and it seems ok and simple enough

jirka


---

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 5dd556e89cce..287c956cdbd2 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -1702,6 +1702,7 @@ struct bpf_link {
 	 * link's semantics is determined by target attach hook
 	 */
 	bool sleepable;
+	u32 flags;
 	/* rcu is used before freeing, work can be used to schedule that
 	 * RCU-based freeing before that, so they never overlap
 	 */

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 56500381c28a..f1d9ee9717a1 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c

@@ -3228,7 +3228,14 @@ static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
 	char prog_tag[sizeof(prog->tag) * 2 + 1] = { };
 
 	if (type < ARRAY_SIZE(bpf_link_type_strs) && bpf_link_type_strs[type]) {
-		seq_printf(m, "link_type:\t%s\n", bpf_link_type_strs[type]);
+		if (link->type == BPF_LINK_TYPE_KPROBE_MULTI)
+			seq_printf(m, "link_type:\t%s\n", link->flags == BPF_F_KPROBE_MULTI_RETURN ?
+				   "kretprobe_multi" : "kprobe_multi");
+		else if (link->type == BPF_LINK_TYPE_UPROBE_MULTI)
+			seq_printf(m, "link_type:\t%s\n", link->flags == BPF_F_UPROBE_MULTI_RETURN ?
+				   "uretprobe_multi" : "uprobe_multi");
+		else
+			seq_printf(m, "link_type:\t%s\n", bpf_link_type_strs[type]);
 	} else {
 		WARN_ONCE(1, "missing BPF_LINK_TYPE(...) for link type %u\n", type);
 		seq_printf(m, "link_type:\t<%u>\n", type);

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 0a06ea6638fe..81d7a4e5ae15 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c

@@ -2466,7 +2466,6 @@ struct bpf_kprobe_multi_link {
 	u32 cnt;
 	u32 mods_cnt;
 	struct module **mods;
-	u32 flags;
 };
 
 struct bpf_kprobe_multi_run_ctx {

@@ -2586,7 +2585,7 @@ static int bpf_kprobe_multi_link_fill_link_info(const struct bpf_link *link,
 
 	kmulti_link = container_of(link, struct bpf_kprobe_multi_link, link);
 	info->kprobe_multi.count = kmulti_link->cnt;
-	info->kprobe_multi.flags = kmulti_link->flags;
+	info->kprobe_multi.flags = kmulti_link->link.flags;
 	info->kprobe_multi.missed = kmulti_link->fp.nmissed;
 
 	if (!uaddrs)

@@ -2976,7 +2975,7 @@ int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
 	link->addrs = addrs;
 	link->cookies = cookies;
 	link->cnt = cnt;
-	link->flags = flags;
+	link->link.flags = flags;
 
 	if (cookies) {
 		/*

@@ -3045,7 +3044,6 @@ struct bpf_uprobe_multi_link {
 	struct path path;
 	struct bpf_link link;
 	u32 cnt;
-	u32 flags;
 	struct bpf_uprobe *uprobes;
 	struct task_struct *task;
 };

@@ -3109,7 +3107,7 @@ static int bpf_uprobe_multi_link_fill_link_info(const struct bpf_link *link,
 
 	umulti_link = container_of(link, struct bpf_uprobe_multi_link, link);
 	info->uprobe_multi.count = umulti_link->cnt;
-	info->uprobe_multi.flags = umulti_link->flags;
+	info->uprobe_multi.flags = umulti_link->link.flags;
 	info->uprobe_multi.pid = umulti_link->task ?
 				 task_pid_nr_ns(umulti_link->task, task_active_pid_ns(current)) : 0;
 

@@ -3369,7 +3367,7 @@ int bpf_uprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
 	link->uprobes = uprobes;
 	link->path = path;
 	link->task = task;
-	link->flags = flags;
+	link->link.flags = flags;
 
 	bpf_link_init(&link->link, BPF_LINK_TYPE_UPROBE_MULTI,
 		      &bpf_uprobe_multi_link_lops, prog);