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([2a01:4b00:bd1f:f500:f867:fc8a:5174:5755]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48e530c1e84sm43857065e9.10.2026.05.07.09.14.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 May 2026 09:14:38 -0700 (PDT) Message-ID: <232803d2-cdbb-4fdf-b782-6d6f8f77d89f@gmail.com> Date: Thu, 7 May 2026 17:14:38 +0100 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] tools/lib/bpf/libbpf: Prioritize module kfuncs over vmlinux kfuncs To: Song Chen , andrii@kernel.org, eddyz87@gmail.com, ast@kernel.org, daniel@iogearbox.net, martin.lau@linux.dev, song@kernel.org, yonghong.song@linux.dev, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com, jolsa@kernel.org, alexei.starovoitov@gmail.com Cc: bpf@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260430054707.38965-1-chensong_2000@126.com> Content-Language: en-US From: Mykyta Yatsenko In-Reply-To: <20260430054707.38965-1-chensong_2000@126.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/30/26 6:47 AM, Song Chen wrote: > Change the kfunc resolution order in find_ksym_btf_id() to search > module BTFs before vmlinux BTF. This allows kernel modules to override > vmlinux kfuncs with the same name, enabling a form of live-patching > for kfuncs. > > Previously, vmlinux kfuncs were always preferred, making it impossible > for modules to provide enhanced or fixed versions of existing kfuncs. > With this change, modules can now override kernel kfuncs, while > programs that don't use module BTFs remain unaffected. > > Suggested-by: Alexei Starovoitov > Signed-off-by: Song Chen > --- > tools/lib/bpf/libbpf.c | 34 ++++++++++++++++++---------------- > 1 file changed, 18 insertions(+), 16 deletions(-) > > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c > index 0be7017800fe..9c308930bddb 100644 > --- a/tools/lib/bpf/libbpf.c > +++ b/tools/lib/bpf/libbpf.c > @@ -8538,29 +8538,31 @@ static int find_ksym_btf_id(struct bpf_object *obj, const char *ksym_name, > { > struct module_btf *mod_btf; > struct btf *btf; > - int i, id, err; > + int i, id = 0, err; nit: I think you can keep id uninitialized, it's set below unconditionally. > > - btf = obj->btf_vmlinux; > mod_btf = NULL; nit: It looks like you don't need setting mod_btf to NULL here, both code paths set it anyway. > - id = btf__find_by_name_kind(btf, ksym_name, kind); > > - if (id == -ENOENT) { > - err = load_module_btfs(obj); > - if (err) > - return err; > + err = load_module_btfs(obj); > + if (err) > + goto search_vmlinux; > > - for (i = 0; i < obj->btf_module_cnt; i++) { > - /* we assume module_btf's BTF FD is always >0 */ > - mod_btf = &obj->btf_modules[i]; > - btf = mod_btf->btf; > - id = btf__find_by_name_kind_own(btf, ksym_name, kind); > - if (id != -ENOENT) > - break; > - } > + for (i = 0; i < obj->btf_module_cnt; i++) { > + /* we assume module_btf's BTF FD is always >0 */ > + mod_btf = &obj->btf_modules[i]; > + btf = mod_btf->btf; > + id = btf__find_by_name_kind_own(btf, ksym_name, kind); > + if (id != -ENOENT) > + goto found; very nit: Perhaps setting *res_btf and *res_mod_btf here inline and return will make code a bit easier to follow and allow removing found label. Overall it looks like the refactoring looks correct, module btf is processed before the vmlinux btf. Acked-by: Mykyta Yatsenko > } > - if (id <= 0) > + > +search_vmlinux: > + btf = obj->btf_vmlinux; > + mod_btf = NULL; > + id = btf__find_by_name_kind(btf, ksym_name, kind); > + if (id == -ENOENT) > return -ESRCH; > > +found: > *res_btf = btf; > *res_mod_btf = mod_btf; > return id;