Hi, Petr,
On Tue, Aug 11, 2026 at 8:20 PM Petr Pavlu <[email protected]> wrote:
>
> On 8/11/26 9:15 AM, Tiezhu Yang wrote:
> > The compiler toolchains generate internal local labels on certain
> > architectures (such as LoongArch) for optimizations and relocations.
> >
> > While these local labels are filtered out during runtime lookups in
> > find_kallsyms_symbol(), they still leak into the permanent symbol
> > tables of loaded modules, because layout_symtab() and add_kallsyms()
> > do not check for the mapping symbols during layout generation.
> >
> > Consequently, tracing tools like bpftrace resolve identical addresses
> > into confusing local labels instead of actual clear C function names.
> >
> > Fix this by adding is_mapping_symbol() checks directly into the symbol
> > tracking loops of layout_symtab() and add_kallsyms(). This prevents the
> > mapping symbols from entering the module's memory symbol arrays at load
> > time.
>
> This optimization looks sensible to me. Kallsyms in vmlinux also
> normally doesn't include information about local labels (.L<xyz>) and
> mapping symbols ($a, $d, ...), so it should be ok not to track them in
> modules either.
>
> Note that the patch description mixes up these two symbol categories,
> presumably because the existing is_mapping_symbol() function conflates
> them as well.
>
> >
> > For livepatch modules, core_kallsyms.symtab must remain a complete copy
> > of the original ELF symbol table to prevent delayed relocation failures.
> > Therefore, enforce is_livepatch_module() checking to skip the filtering
> > for livepatch modules.
> >
> > Reproduce steps:
> >
> > 1. Set up a LoongArch VM with "-accel kvm":
> >
> > $ sudo qemu-system-loongarch64 -serial stdio \
> > -machine virt -cpu la464 -smp 4 -m 4G \
> > -bios /usr/share/edk2/loongarch64/QEMU_EFI.fd \
> > -nodefaults -no-reboot -nographic -accel kvm
> >
> > 2. Use bpftrace to capture kstack when vCPU is scheduled out:
> >
> > $ cat trace_sched.bt
> > kprobe:kvm:kvm_sched_out
> > {
> > if (pid == $1) {
> > print(kstack());
> > }
> > }
> >
> > $ sudo bpftrace trace_sched.bt `pgrep -o qemu-system`
> >
> > Test results:
> >
> > 1. Before this patch (Confusing stack with local labels):
> >
> > kvm_sched_out+0
> > __schedule+1584
> > schedule+48
> > .LVL3767+36
> > .LVL3801+8
> > .LVL285+12
> > .LVL425+44
> > .LVL1419+20
> > kvm_exc_entry+260
>
> The description should mention that this change affects what appears in
> /proc/kallsyms, as that is the important part. bpftrace is only one user
> of this data and apparently gets confused because it does not apply any
> filtering of its own, unlike perf which has is_ignored_kernel_symbol().
>
> >
> > 2. After this patch (Accurate stack with function names):
> >
> > kvm_sched_out+0
> > __schedule+1584
> > schedule+48
> > kvm_vcpu_block+112
> > kvm_vcpu_halt+104
> > kvm_emu_idle+284
> > kvm_handle_gspr+1316
> > kvm_handle_exit+456
> > kvm_exc_entry+260
> >
> > Signed-off-by: Tiezhu Yang <[email protected]>
> > ---
> > v2: Fix livepatch support as pointed out by Sashiko bot.
> >
> > kernel/module/kallsyms.c | 9 +++++++++
> > 1 file changed, 9 insertions(+)
> >
> > diff --git a/kernel/module/kallsyms.c b/kernel/module/kallsyms.c
> > index f23126d804b2..aece7aa49dd4 100644
> > --- a/kernel/module/kallsyms.c
> > +++ b/kernel/module/kallsyms.c
> > @@ -130,6 +130,10 @@ void layout_symtab(struct module *mod, struct
> > load_info *info)
> >
> > /* Compute total space required for the core symbols' strtab. */
> > for (ndst = i = 0; i < nsrc; i++) {
> > + if (!is_livepatch_module(mod) &&
> > + is_mapping_symbol(&info->strtab[src[i].st_name]))
> > + continue;
> > +
> > if (i == 0 || is_livepatch_module(mod) ||
> > is_core_symbol(src + i, info->sechdrs, info->hdr->e_shnum,
> > info->index.pcpu)) {
> > @@ -198,6 +202,11 @@ void add_kallsyms(struct module *mod, const struct
> > load_info *info)
> > src = kallsyms->symtab;
> > for (ndst = i = 0; i < kallsyms->num_symtab; i++) {
> > kallsyms->typetab[i] = elf_type(src + i, info);
> > +
> > + if (!is_livepatch_module(mod) &&
> > + is_mapping_symbol(&kallsyms->strtab[src[i].st_name]))
> > + continue;
> > +
> > if (i == 0 || is_livepatch_module(mod) ||
> > is_core_symbol(src + i, info->sechdrs, info->hdr->e_shnum,
> > info->index.pcpu)) {
>
> It is better to avoid duplicating the same logic in layout_symtab() and
> add_kallsyms(). The function is_core_symbol() can be renamed to
> something like is_kept_symbol() and can be solely responsible for
> deciding what to keep.
I'm sorry but I think Tiezhu's patch is simpler and cleaner. :)
Huacai
>
> The following prototype shows my suggestion:
>
> diff --git a/kernel/module/kallsyms.c b/kernel/module/kallsyms.c
> index 0fc11e45df9b..c86879cc5b66 100644
> --- a/kernel/module/kallsyms.c
> +++ b/kernel/module/kallsyms.c
> @@ -75,23 +75,29 @@ static char elf_type(const Elf_Sym *sym, const struct
> load_info *info)
> return '?';
> }
>
> -static bool is_core_symbol(const Elf_Sym *src, const Elf_Shdr *sechdrs,
> - unsigned int shnum, unsigned int pcpundx)
> +static bool is_kept_symbol(struct module *mod, const struct load_info *info,
> + unsigned int symnum, const Elf_Sym *src)
> {
> const Elf_Shdr *sec;
> enum mod_mem_type type;
>
> + if (symnum == 0 || is_livepatch_module(mod))
> + return true;
> +
> if (src->st_shndx == SHN_UNDEF ||
> - src->st_shndx >= shnum ||
> + src->st_shndx >= info->hdr->e_shnum ||
> !src->st_name)
> return false;
>
> + if (is_mapping_symbol(&info->strtab[src->st_name]))
> + return false;
> +
> #ifdef CONFIG_KALLSYMS_ALL
> - if (src->st_shndx == pcpundx)
> + if (src->st_shndx == info->index.pcpu)
> return true;
> #endif
>
> - sec = sechdrs + src->st_shndx;
> + sec = info->sechdrs + src->st_shndx;
> type = sec->sh_entsize >> SH_ENTSIZE_TYPE_SHIFT;
> if (!(sec->sh_flags & SHF_ALLOC)
> #ifndef CONFIG_KALLSYMS_ALL
> @@ -130,12 +136,11 @@ void layout_symtab(struct module *mod, struct load_info
> *info)
>
> /* Compute total space required for the core symbols' strtab. */
> for (ndst = i = 0; i < nsrc; i++) {
> - if (i == 0 || is_livepatch_module(mod) ||
> - is_core_symbol(src + i, info->sechdrs, info->hdr->e_shnum,
> - info->index.pcpu)) {
> - strtab_size += strlen(&info->strtab[src[i].st_name])
> + 1;
> - ndst++;
> - }
> + if (!is_kept_symbol(mod, info, i, src + i))
> + continue;
> +
> + strtab_size += strlen(&info->strtab[src[i].st_name]) + 1;
> + ndst++;
> }
>
> /* Append room for core symbols at end of core part. */
> @@ -197,23 +202,22 @@ void add_kallsyms(struct module *mod, const struct
> load_info *info)
> strtab_size = info->core_typeoffs - info->stroffs;
> src = kallsyms->symtab;
> for (ndst = i = 0; i < kallsyms->num_symtab; i++) {
> + ssize_t ret;
> +
> kallsyms->typetab[i] = elf_type(src + i, info);
> - if (i == 0 || is_livepatch_module(mod) ||
> - is_core_symbol(src + i, info->sechdrs, info->hdr->e_shnum,
> - info->index.pcpu)) {
> - ssize_t ret;
> -
> - mod->core_kallsyms.typetab[ndst] =
> - kallsyms->typetab[i];
> - dst[ndst] = src[i];
> - dst[ndst++].st_name = s - mod->core_kallsyms.strtab;
> - ret = strscpy(s, &kallsyms->strtab[src[i].st_name],
> - strtab_size);
> - if (ret < 0)
> - break;
> - s += ret + 1;
> - strtab_size -= ret + 1;
> - }
> +
> + if (!is_kept_symbol(mod, info, i, src + i))
> + continue;
> +
> + mod->core_kallsyms.typetab[ndst] = kallsyms->typetab[i];
> + dst[ndst] = src[i];
> + dst[ndst++].st_name = s - mod->core_kallsyms.strtab;
> + ret = strscpy(s, &kallsyms->strtab[src[i].st_name],
> + strtab_size);
> + if (ret < 0)
> + break;
> + s += ret + 1;
> + strtab_size -= ret + 1;
> }
>
> /* Set up to point into init section. */
>
> --
> Thanks,
> Petr
>
