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.
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]>
---
Based on the latest modules-next branch of
https://git.kernel.org/pub/scm/linux/kernel/git/modules/linux.git
kernel/module/kallsyms.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/kernel/module/kallsyms.c b/kernel/module/kallsyms.c
index f23126d804b2..61dd5c014a59 100644
--- a/kernel/module/kallsyms.c
+++ b/kernel/module/kallsyms.c
@@ -130,6 +130,9 @@ 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_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 +201,10 @@ 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_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
