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

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.

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

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)) {
-- 
2.42.0