netdev - Re: [PATCH bpf-next] libbpf: fix error when st-prefix

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Message-ID: <CAEf4BzZibyj4cyeMBAhGUWMVJ8P9ZeiwZgXF8HV90L8iwmkjTg@mail.gmail.com>
Date: Fri, 26 Sep 2025 13:12:59 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: "D. Wythe" <alibuda@...ux.alibaba.com>, ast@...nel.org, daniel@...earbox.net, 
	andrii@...nel.org, pabeni@...hat.com, song@...nel.org, sdf@...gle.com, 
	haoluo@...gle.com, yhs@...com, edumazet@...gle.com, john.fastabend@...il.com, 
	kpsingh@...nel.org, jolsa@...nel.org, mjambigi@...ux.ibm.com, 
	wenjia@...ux.ibm.com, wintera@...ux.ibm.com, dust.li@...ux.alibaba.com, 
	tonylu@...ux.alibaba.com, guwen@...ux.alibaba.com, bpf@...r.kernel.org, 
	davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org, 
	sidraya@...ux.ibm.com, jaka@...ux.ibm.com
Subject: Re: [PATCH bpf-next] libbpf: fix error when st-prefix_ops and ops
 from differ btf

On Fri, Sep 26, 2025 at 9:53 AM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>
> On 9/26/25 12:17 AM, D. Wythe wrote:
> > When a module registers a struct_ops, the struct_ops type and its
> > corresponding map_value type ("bpf_struct_ops_") may reside in different
> > btf objects, here are four possible case:
> >
> > +--------+---------------+-------------+---------------------------------+
> > |        |bpf_struct_ops_| xxx_ops     |                                 |
> > +--------+---------------+-------------+---------------------------------+
> > | case 0 | btf_vmlinux   | bft_vmlinux | be used and reg only in vmlinux |
>
> s/bft/btf/
>
> > +--------+---------------+-------------+---------------------------------+
> > | case 1 | btf_vmlinux   | mod_btf     | INVALID                         |
> > +--------+---------------+-------------+---------------------------------+
> > | case 2 | mod_btf       | btf_vmlinux | reg in mod but be used both in  |
> > |        |               |             | vmlinux and mod.                |
> > +--------+---------------+-------------+---------------------------------+
> > | case 3 | mod_btf       | mod_btf     | be used and reg only in mod     |
> > +--------+---------------+-------------+---------------------------------+
> >
> > Currently we figure out the mod_btf by searching with the struct_ops type,
> > which makes it impossible to figure out the mod_btf when the struct_ops
> > type is in btf_vmlinux while it's corresponding map_value type is in
> > mod_btf (case 2).
> >
> > The fix is to use the corresponding map_value type ("bpf_struct_ops_")
> > as the lookup anchor instead of the struct_ops type to figure out the
> > `btf` and `mod_btf` via find_ksym_btf_id(), and then we can locate
> > the kern_type_id via btf__find_by_name_kind() with the `btf` we just
> > obtained from find_ksym_btf_id().
> >
> > With this change the lookup obtains the correct btf and mod_btf for case 2,
> > preserves correct behavior for other valid cases, and still fails as
> > expected for the invalid scenario (case 1).
> >
> > Fixes: 590a00888250 ("bpf: libbpf: Add STRUCT_OPS support")
> > Signed-off-by: D. Wythe <alibuda@...ux.alibaba.com>
> > Acked-by: Andrii Nakryiko <andrii@...nel.org>
> > ---
> >   tools/lib/bpf/libbpf.c | 37 ++++++++++++++++++-------------------
> >   1 file changed, 18 insertions(+), 19 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 5161c2b39875..a93eed660404 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -1018,35 +1018,34 @@ find_struct_ops_kern_types(struct bpf_object *obj, const char *tname_raw,
> >       const struct btf_member *kern_data_member;
> >       struct btf *btf = NULL;
> >       __s32 kern_vtype_id, kern_type_id;
> > -     char tname[256];
> > +     char tname[256], stname[256];

I adjusted tname size down to 192 to avoid compiler complains that
snprintf below can truncate output (useless warning, but oh well)

> >       __u32 i;
> >
> >       snprintf(tname, sizeof(tname), "%.*s",
> >                (int)bpf_core_essential_name_len(tname_raw), tname_raw);
> >
> > -     kern_type_id = find_ksym_btf_id(obj, tname, BTF_KIND_STRUCT,
> > -                                     &btf, mod_btf);
> > -     if (kern_type_id < 0) {
> > -             pr_warn("struct_ops init_kern: struct %s is not found in kernel BTF\n",
> > -                     tname);
> > -             return kern_type_id;
> > -     }
> > -     kern_type = btf__type_by_id(btf, kern_type_id);
> > +     snprintf(stname, sizeof(stname), "%s%.*s", STRUCT_OPS_VALUE_PREFIX,
> > +              (int)strlen(tname), tname);
>
> nit. strlen(tname) should not be needed. Others lgtm.

yeah, dropped that while applying, thanks. this was a copy/paste from
tname's snprint, but there we do actually need a prefix of a type, not
here, so strlen() makes no sense


applied to bpf-next

>
> Acked-by: Martin KaFai Lau <martin.lau@...nel.org>
>
>
>