Re: [PATCH v2] module/kallsyms: Filter out local mapping symbols during module load

From: Petr Pavlu <petr.pavlu@suse.com>
Date: 2026-08-11 12:20:07
Also in: lkml, loongarch
Subsystem: module support, the rest · Maintainers: Luis Chamberlain, Petr Pavlu, Daniel Gomez, Sami Tolvanen, Linus Torvalds

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().

quoted hunk ↗ jump to hunk

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 <yangtiezhu@loongson.cn>
---
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.

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

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