On Fri, Jul 11, 2025 at 7:23 PM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> On Fri, Jul 11, 2025 at 01:51:31PM +0800, Menglong Dong wrote:
> > On Fri, Jul 11, 2025 at 11:46 AM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
> > >
> > >
> > >
> > > On 7/10/25 8:10 PM, Yonghong Song wrote:
> > > >
> > > >
> > > > On 7/10/25 12:08 AM, Menglong Dong wrote:
> > > >> For now, we lookup the address of the attach target in
> > > >> bpf_check_attach_target() with find_kallsyms_symbol_value or
> > > >> kallsyms_lookup_name, which is not accurate in some cases.
> > > >>
> > > >> For example, we want to attach to the target "t_next", but there are
> > > >> multiple symbols with the name "t_next" exist in the kallsyms, which
> > > >> makes
> > > >> the attach target ambiguous, and the attach should fail.
> > > >>
> > > >> Introduce the function bpf_lookup_attach_addr() to do the address
> > > >> lookup,
> > > >> which will return -EADDRNOTAVAIL when the symbol is not unique.
> > > >>
> > > >> We can do the testing with following shell:
> > > >>
> > > >> for s in $(cat /proc/kallsyms | awk '{print $3}' | sort | uniq -d)
> > > >> do
> > > >> if grep -q "^$s\$"
> > > >> /sys/kernel/debug/tracing/available_filter_functions
> > > >> then
> > > >> bpftrace -e "fentry:$s {printf(\"1\");}" -v
> > > >> fi
> > > >> done
> > > >>
> > > >> The script will find all the duplicated symbols in /proc/kallsyms, which
> > > >> is also in /sys/kernel/debug/tracing/available_filter_functions, and
> > > >> attach them with bpftrace.
> > > >>
> > > >> After this patch, all the attaching fail with the error:
> > > >>
> > > >> The address of function xxx cannot be found
> > > >> or
> > > >> No BTF found for xxx
> > > >>
> > > >> Signed-off-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
> > > >
> > > > Maybe we should prevent vmlinux BTF generation for such symbols
> > > > which are static and have more than one instances? This can
> > > > be done in pahole and downstream libbpf/kernel do not
> > > > need to do anything. This can avoid libbpf/kernel runtime overhead
> > > > since bpf_lookup_attach_addr() could be expensive as it needs
> > > > to go through ALL symbols, even for unique symbols.
> >
> > Hi, yonghong. You are right, the best solution is to solve
> > this problem in the pahole, just like what Jiri said in the V2:
> > https://lore.kernel.org/bpf/aG5hzvaqXi7uI4GL@krava/
> >
> > I wonder will we focus the users to use the latest pahole
> > that supports duplicate symbols filter after we fix this problem
> > in pahole? If so, this patch is useless, and just ignore it. If
> > not, the only usage of this patch is for the users that build
> > the kernel with an old pahole.
> >
> > >
> > > There is a multi-link effort:
> > > https://lore.kernel.org/bpf/20250703121521.1874196-1-dongml2@xxxxxxxxxxxxxxx/
> > > which tries to do similar thing for multi-kprobe. For example, for fentry,
> > > multi-link may pass an array of btf_id's to the kernel. For such cases,
> > > this patch may cause significant performance overhead.
> >
> > For the symbol in the vmlinux, there will be no additional overhead,
> > as the logic is the same as previous. If the symbol is in the
> > modules, it does have additional overhead. Following is the
> > testing that hooks all the symbols with fentry-multi.
> >
> > Without this patch, the time to attach all the symbols:
> > kernel: 0.372660s for 48857 symbols
> > modules: 0.135543s for 8631 symbols
> >
> > And with this patch, the time is:
> > kernel: 0.380087s for 48857 symbols
> > modules: 0.176904s for 8631 symbols
> >
> > One more thing, is there anyone to fix the problem in pahole?
> > I mean, I'm not good at pahole. But if there is nobody, I still can
> > do this job, but I need to learn it first :/
>
> I'm testing change below, I'll send the patch after some more testing
Awesome, thank you :/
>
> jirka
>
>
> ---
> diff --git a/btf_encoder.c b/btf_encoder.c
> index 16739066caae..29ff86bac7de 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -2143,6 +2143,31 @@ int btf_encoder__encode(struct btf_encoder *encoder, struct conf_load *conf)
> return err;
> }
>
> +static void remove_dups(struct elf_functions *functions)
> +{
> + struct elf_function *n = &functions->entries[0];
> + bool matched = false;
> + int i, j;
> +
> + for (i = 0, j = 1; i < functions->cnt && j < functions->cnt; i++, j++) {
> + struct elf_function *a = &functions->entries[i];
> + struct elf_function *b = &functions->entries[j];
> +
> + if (!strcmp(a->name, b->name)) {
> + matched = true;
> + continue;
> + }
> +
> + if (!matched)
> + *n++ = *a;
> + matched = false;
> + }
> +
> + if (!matched)
> + *n++ = functions->entries[functions->cnt - 1];
> + functions->cnt = n - &functions->entries[0];
> +}
> +
> static int elf_functions__collect(struct elf_functions *functions)
> {
> uint32_t nr_symbols = elf_symtab__nr_symbols(functions->symtab);
> @@ -2168,6 +2193,7 @@ static int elf_functions__collect(struct elf_functions *functions)
>
> if (functions->cnt) {
> qsort(functions->entries, functions->cnt, sizeof(*functions->entries), functions_cmp);
> + remove_dups(functions);
> } else {
> err = 0;
> goto out_free;
