[PATCH v1 2/2] LoongArch: Return 0 for user tasks in arch_stack_walk_reliable()

[Tiezhu Yang]

When testing the kernel live patching with "modprobe livepatch-sample",
there is a timeout over 15 seconds from "starting patching transition"
to "patching complete", dmesg shows "unreliable stack" for user tasks
in debug mode. When executing "rmmod livepatch-sample", there exists
the similar issue.

Like x86, arch_stack_walk_reliable() should return 0 for user tasks.
It is necessary to set regs->csr_prmd as task->thread.csr_prmd first,
then use user_mode() to check whether the task is in userspace.

Here are the call chains:

  klp_enable_patch()
    klp_try_complete_transition()
      klp_try_switch_task()
        klp_check_and_switch_task()
          klp_check_stack()
            stack_trace_save_tsk_reliable()
              arch_stack_walk_reliable()

With this patch, it takes a short time for patching and unpatching.

Before:

  # modprobe livepatch-sample
  # dmesg -T | tail -3
  [Sat Sep  6 11:00:20 2025] livepatch: 'livepatch_sample': starting patching 
transition
  [Sat Sep  6 11:00:35 2025] livepatch: signaling remaining tasks
  [Sat Sep  6 11:00:36 2025] livepatch: 'livepatch_sample': patching complete

  # echo 0 > /sys/kernel/livepatch/livepatch_sample/enabled
  # rmmod livepatch_sample
  rmmod: ERROR: Module livepatch_sample is in use
  # rmmod livepatch_sample
  # dmesg -T | tail -3
  [Sat Sep  6 11:06:05 2025] livepatch: 'livepatch_sample': starting unpatching 
transition
  [Sat Sep  6 11:06:20 2025] livepatch: signaling remaining tasks
  [Sat Sep  6 11:06:21 2025] livepatch: 'livepatch_sample': unpatching complete

After:

  # modprobe livepatch-sample
  # dmesg -T | tail -2
  [Sat Sep  6 11:19:00 2025] livepatch: 'livepatch_sample': starting patching 
transition
  [Sat Sep  6 11:19:01 2025] livepatch: 'livepatch_sample': patching complete

  # echo 0 > /sys/kernel/livepatch/livepatch_sample/enabled
  # rmmod livepatch_sample
  # dmesg -T | tail -2
  [Sat Sep  6 11:21:10 2025] livepatch: 'livepatch_sample': starting unpatching 
transition
  [Sat Sep  6 11:21:11 2025] livepatch: 'livepatch_sample': unpatching complete

While at it, do the similar thing for arch_stack_walk() to avoid
potential issues.

Cc: sta...@vger.kernel.org # v6.9+
Fixes: 199cc14cb4f1 ("LoongArch: Add kernel livepatching support")
Reported-by: Xi Zhang <zhan...@kylinos.cn>
Signed-off-by: Tiezhu Yang <yangtie...@loongson.cn>
---
 arch/loongarch/kernel/stacktrace.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/loongarch/kernel/stacktrace.c 
b/arch/loongarch/kernel/stacktrace.c
index 9a038d1070d7..0454cce3b667 100644
--- a/arch/loongarch/kernel/stacktrace.c
+++ b/arch/loongarch/kernel/stacktrace.c
@@ -30,10 +30,15 @@ void arch_stack_walk(stack_trace_consume_fn consume_entry, 
void *cookie,
                }
                regs->regs[1] = 0;
                regs->regs[22] = 0;
+               regs->csr_prmd = task->thread.csr_prmd;
        }
 
        for (unwind_start(&state, task, regs);
             !unwind_done(&state); unwind_next_frame(&state)) {
+               /* Success path for user tasks */
+               if (user_mode(regs))
+                       return;
+
                addr = unwind_get_return_address(&state);
                if (!addr || !consume_entry(cookie, addr))
                        break;
@@ -57,9 +62,14 @@ int arch_stack_walk_reliable(stack_trace_consume_fn 
consume_entry,
        }
        regs->regs[1] = 0;
        regs->regs[22] = 0;
+       regs->csr_prmd = task->thread.csr_prmd;
 
        for (unwind_start(&state, task, regs);
             !unwind_done(&state) && !unwind_error(&state); 
unwind_next_frame(&state)) {
+               /* Success path for user tasks */
+               if (user_mode(regs))
+                       return 0;
+
                addr = unwind_get_return_address(&state);
 
                /*
-- 
2.42.0