linux-kernel - Re: [PATCH v1 2/2] LoongArch: Return 0 for user tasks in arch_stack_walk

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Message-ID: <c3431ce4-0026-3a05-fa50-281cd34aba4e@loongson.cn>
Date: Thu, 11 Sep 2025 19:49:50 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Jinyang He <hejinyang@...ngson.cn>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, Huacai Chen
 <chenhuacai@...nel.org>, Xi Zhang <zhangxi@...inos.cn>,
 live-patching@...r.kernel.org, loongarch@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] LoongArch: Return 0 for user tasks in
 arch_stack_walk_reliable()

On 2025/9/10 上午9:11, Jinyang He wrote:
> On 2025-09-09 19:31, Tiezhu Yang wrote:
> 
>> 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.

...

>> @@ -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);
>>           /*
> Hi, Tiezhu,
> 
> We update stack info by get_stack_info when meet ORC_TYPE_REGS in
> unwind_next_frame. And in arch_stack_walk(_reliable), we always
> do unwind_done before unwind_next_frame. So is there anything
> error in get_stack_info which causing regs is user_mode while
> stack is not STACK_TYPE_UNKNOWN?

When testing the kernel live patching, the error code path in
unwind_next_frame() is:

   switch (orc->fp_reg) {
           case ORC_REG_PREV_SP:
                   p = (unsigned long *)(state->sp + orc->fp_offset);
                   if (!stack_access_ok(state, (unsigned long)p, 
sizeof(unsigned long)))
                           goto err;

for this case, get_stack_info() does not return 0 due to in_task_stack()
is not true, then goto error, state->stack_info.type = STACK_TYPE_UNKNOWN
and state->error = true. In arch_stack_walk_reliable(), the loop will be
break and it returns -EINVAL, thus causing unreliable stack.

Maybe it can check whether the task is in userspace and set
state->stack_info.type = STACK_TYPE_UNKNOWN in get_stack_info(),
but I think no need to do that because it has similar effect with
this patch.

Thanks,
Tiezhu