Re: [PATCH bpf-next RESEND v2] libbpf: Fix expected_attach_type set when kernel not support

From: Tao Chen
Date: Sat Sep 14 2024 - 10:29:40 EST


在 2024/9/14 04:46, Andrii Nakryiko 写道:
On Fri, Sep 13, 2024 at 9:44 AM Tao Chen <chen.dylane@xxxxxxxxx> wrote:

The commit "5902da6d8a52" set expected_attach_type again with
filed of bpf_program after libpf_prepare_prog_load, which makes
expected_attach_type = 0 no sense when kenrel not support the
attach_type feature, so fix it.

Fixes: 5902da6d8a52 ("libbpf: Add uprobe multi link support to bpf_program__attach_usdt")
Suggested-by: Jiri Olsa <jolsa@xxxxxxxxxx>
Signed-off-by: Tao Chen <chen.dylane@xxxxxxxxx>
---
tools/lib/bpf/libbpf.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)

Change list:
- v1 -> v2:
- restore the original initialization way suggested by Jiri

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 219facd0e66e..df2244397ba1 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -7353,7 +7353,7 @@ static int libbpf_prepare_prog_load(struct bpf_program *prog,

/* special check for usdt to use uprobe_multi link */
if ((def & SEC_USDT) && kernel_supports(prog->obj, FEAT_UPROBE_MULTI_LINK))
- prog->expected_attach_type = BPF_TRACE_UPROBE_MULTI;
+ opts->expected_attach_type = BPF_TRACE_UPROBE_MULTI;


Ok, took me a bit to understand what the issue is. But the above is
not quite correct, for the above case of setting
BPF_TRACE_UPROBE_MULTI we do want to record BPF_TRACE_UPROBE_MULTI in
prog->expected_attach_type, because user might want to query that
later.

So I agree with the part of the fix below, but here I think we need
*both* update opts' and prog's expected_attach_type, so we will have:

prog->expected_attach_type = BPF_TRACE_UPROBE_MULTI;
opts->expected_attach_type = BPF_TRACE_UPROBE_MULTI;

pw-bot: cr

if ((def & SEC_ATTACH_BTF) && !prog->attach_btf_id) {
int btf_obj_fd = 0, btf_type_id = 0, err;
@@ -7443,6 +7443,7 @@ static int bpf_object_load_prog(struct bpf_object *obj, struct bpf_program *prog
load_attr.attach_btf_id = prog->attach_btf_id;
load_attr.kern_version = kern_version;
load_attr.prog_ifindex = prog->prog_ifindex;
+ load_attr.expected_attach_type = prog->expected_attach_type;

/* specify func_info/line_info only if kernel supports them */
if (obj->btf && btf__fd(obj->btf) >= 0 && kernel_supports(obj, FEAT_BTF_FUNC)) {
@@ -7474,9 +7475,6 @@ static int bpf_object_load_prog(struct bpf_object *obj, struct bpf_program *prog
insns_cnt = prog->insns_cnt;
}

- /* allow prog_prepare_load_fn to change expected_attach_type */
- load_attr.expected_attach_type = prog->expected_attach_type;
-
if (obj->gen_loader) {
bpf_gen__prog_load(obj->gen_loader, prog->type, prog->name,
license, insns, insns_cnt, &load_attr,
--
2.25.1


Hi, Andrii, thank you for your reply. I will send v3 as you suggested.

--
Best Regards
Dylane Chen