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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4aa7f3229dbsm91077035e9.1.2026.10.29.15.33.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 29 Oct 2026 15:33:46 -0700 (PDT) Date: Thu, 29 Oct 2026 22:33:44 +0000 From: David Laight To: Samuel Wu Cc: Jiri Olsa , Jonathan Corbet , Shuah Khan , Randy Dunlap , Alexander Viro , Christian Brauner , Jan Kara , Andrii Nakryiko , Eduard Zingerman , Ihor Solodrai , Alexei Starovoitov , Daniel Borkmann , Kumar Kartikeya Dwivedi , Martin KaFai Lau , Song Liu , Yonghong Song , Emil Tsalapatis , Nathan Chancellor , Nick Desaulniers , Bill Wendling , Justin Stitt , kernel-team@android.com, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-doc@vger.kernel.org, bpf@vger.kernel.org, linux-kselftest@vger.kernel.org, llvm@lists.linux.dev Subject: Re: [PATCH bpf-next v2 4/6] libbpf: Resolve typeless ksyms via KALLSYMS_QUERY ioctl Message-ID: <20261029223344.5dbe60d5@pumpkin> In-Reply-To: References: <20261023001603.560210-8-wusamuel@google.com> <20261023001603.560210-12-wusamuel@google.com> <20261028140127.7bfeab46@pumpkin> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) X-Mailing-List: llvm@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Thu, 29 Oct 2026 14:35:36 -0700 Samuel Wu wrote: > On Wed, Oct 28, 2026 at 7:01=E2=80=AFAM David Laight > wrote: > > > > On Tue, 27 Oct 2026 15:07:44 -0700 > > Samuel Wu wrote: > > =20 > > > On Mon, Oct 26, 2026 at 2:28=E2=80=AFAM Jiri Olsa wrote: =20 > > > > > > > > On Thu, Oct 22, 2026 at 05:16:05PM -0700, Samuel Wu wrote: > > > > > > > > SNIP > > > > =20 > > > > > +static int bpf_object__query_ksyms(struct bpf_object *obj) > > > > > +{ > > > > > + struct kallsyms_query q; > > > > > + struct extern_desc *ext; > > > > > + int err =3D 0; > > > > > + int fd; > > > > > + int i; > > > > > + > > > > > + fd =3D open("/proc/kallsyms", O_RDONLY | O_CLOEXEC); > > > > > + if (fd < 0) > > > > > + return -errno; > > > > > + > > > > > + for (i =3D 0; i < obj->nr_extern; i++) { > > > > > + ext =3D &obj->externs[i]; > > > > > + if (ext->type !=3D EXT_KSYM || ext->ksym.type_id) > > > > > + continue; > > > > > + > > > > > + q =3D (struct kallsyms_query){ > > > > > + .size =3D sizeof(q), > > > > > + .query_flags =3D KALLSYMS_QUERY_BY_SYMBOL, > > > > > + .sym_name_addr =3D ptr_to_u64(ext->name), > > > > > + }; > > > > > + > > > > > + err =3D -errno; > > > > > + if (err =3D=3D -ENOENT) { > > > > > + err =3D 0; > > > > > + continue; > > > > > + } > > > > > + if (err =3D=3D -EADDRNOTAVAIL) > > > > > + pr_warn("extern (ksym) '%s': resolu= tion is ambiguous\n", > > > > > + ext->name); > > > > > + break; > > > > > + } > > > > > + ext->is_set =3D true; > > > > > + ext->ksym.addr =3D q.sym_addr; > > > > > + pr_debug("extern (ksym) '%s': resolved in kernel to= 0x%llx\n", > > > > > + ext->name, ext->ksym.addr); > > > > > + } > > > > > + close(fd); > > > > > + return err; > > > > > +} > > > > > + > > > > > +static int bpf_object__resolve_typeless_ksyms(struct bpf_object = *obj) > > > > > { > > > > > + int err; > > > > > + > > > > > + err =3D bpf_object__query_ksyms(obj); =20 > > > > > > > > hi, > > > > we could also add similar support for libbpf_available_kallsyms_par= se and > > > > probably in some other places like bpf_get_ksyms.. which makes me t= hink it > > > > could be useful to have KALLSYMS_QUERY support for multiple symbols > > > > > > > > thanks, > > > > jirka > > > > =20 > > > > > > Thanks for lookiing over this Jiri! > > > > > > Using KALLSYMS_QUERY for libbpf_available_kallsyms_parse() would > > > theoretically by O(m*log(n)) instead of O(n*log(m)), where m is the > > > number of tracefs symbols, and n is lines in kallsyms, with n >> m. > > > But AFAICT, libbpf_available_kallsyms_parse() would only be called in > > > pre-6.5 kernels, for kernels without commit 83f74441 (ftrace: Show all > > > functions with addresses in available_filter_functions_addrs); so this > > > ioctl could never be called. =20 > > > > Isn't that code also O(n*log(m)) ? =20 >=20 > Yes, sorry maybe I wasn't clear before, but the current code is > O(n*log(m)), and the ioctl approach would be better since it's m > lookups of O(log(n)) for vmlinux symbols, where n >> m. However, this > is a red herring because that path only runs on pre-6.5 kernels, which > won't have this ioctl. I was thinking of the ftrace_lookup_symbols() code (called from bbf_kprobe_multi_link_attach) which O(n*log(m)) (the wrong way around). David >=20 > > I've got a patch (I might send it after the merge window) that changes > > one of the ftrace functions to look up every symbol by name, not just > > when only asked to look up one. > > It can also use the 'callback' function to detect duplicates. > > That should be massively faster - especially with my ~18 fold improveme= nt > > (also waiting for the merge window to end before posting a new > > (not much changed) version). =20 >=20 > Excellent, I'll keep an eye out for it! >=20 > > Does look the the code could do with getting the 'symbol name table ind= ex' > > for the symbol so that it can read the entries sequentially from that > > point onwards. > > > > David > > =20 > > > > > > And regarding bpf_get_ksyms(), it loads all traceable functions and > > > needs the full syms table, so a /proc/kallsyms should fits better. > > > > > > I did profile a slightly different form of querying multiple symbols > > > at once [1], but didn't see much benefit. struct kallsyms_query is > > > extensible via size and query_flags, so we can update in the future as > > > needed. > > > > > > [1]: https://lore.kernel.org/all/CAG2KctozPSev+EXSpHitM3JWfjufQaEjsPJ= HS_1HJ1wSm7sJ1w@mail.gmail.com/ > > > > > > [...] > > > =20 > > =20