linux-kernel - Re: [PATCH v1] LoongArch: Handle special PC in unwind_next

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Message-ID: <332975a8-c7f1-76d5-0628-cd7260ad1180@loongson.cn>
Date: Tue, 25 Nov 2025 16:54:35 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: Xi Zhang <zhangxi@...inos.cn>, Xianglai Li <lixianglai@...ngson.cn>,
 loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] LoongArch: Handle special PC in unwind_next_frame()

On 2025/11/25 下午4:08, Huacai Chen wrote:
> Hi, Tiezhu,
> 
> On Tue, Nov 25, 2025 at 2:34 PM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>>
>> When running virtual machine before testing the kernel live patching with
>> "modprobe livepatch-sample", there is a timeout over 15 seconds, the dmesg
>> command shows "unreliable stack" for user tasks in debug mode.
>>
>> The "unreliable stack" is because it can not unwind from kvm_handle_exit()
>> to its previous frame kvm_exc_entry() due to the PC is not a valid kernel
>> address, the root cause is that the code of kvm_exc_entry() was copied to
>> the DMW area in kvm_loongarch_env_init(), so it should check the PC range
>> and then finish unwinding for this special case.
>>
>> How to test:
>>
>> (1) Update kernel with LIVEPATCH
>>
>>    git clone https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
>>    cd linux && make mrproper defconfig
>>    scripts/config -e FTRACE -e FUNCTION_TRACER \
>>    -e LIVEPATCH -e SAMPLES -m SAMPLE_LIVEPATCH
>>    make olddefconfig all -j"$(nproc)"
>>    sudo make modules_install
>>    sudo make install
>>    sudo reboot
>>
>> (2) Set up a VM with "-accel kvm", no need to specify kernel and initrd
>>
>>    sudo yum -y install edk2-loongarch64 qemu
>>    qemu-system-loongarch64 -serial stdio \
>>    -machine virt -cpu la464 -smp 8 -m 4G \
>>    -bios /usr/share/edk2/loongarch64/QEMU_EFI.fd \
>>    -nodefaults -no-reboot -nographic -accel kvm
>>
>> (3) Test the kernel live patching
>>
>>    cat /proc/cmdline
>>    sudo modprobe livepatch-sample
>>    cat /proc/cmdline
>>    sudo sh -c "echo 0 > /sys/kernel/livepatch/livepatch_sample/enabled"
>>    sudo rmmod livepatch_sample
>>    cat /proc/cmdline
>>    dmesg -T
>>
>> Cc: stable@...r.kernel.org # v6.9+
>> Fixes: cb8a2ef0848c ("LoongArch: Add ORC stack unwinder support")
>> Reported-by: Xi Zhang <zhangxi@...inos.cn>
>> Reported-by: Xianglai Li <lixianglai@...ngson.cn>
>> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
>> ---
>>   arch/loongarch/kernel/unwind_orc.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/arch/loongarch/kernel/unwind_orc.c b/arch/loongarch/kernel/unwind_orc.c
>> index 0d5fa64a2225..1d60a593479a 100644
>> --- a/arch/loongarch/kernel/unwind_orc.c
>> +++ b/arch/loongarch/kernel/unwind_orc.c
>> @@ -506,6 +506,9 @@ bool unwind_next_frame(struct unwind_state *state)
>>                  goto err;
>>          }
>>
>> +       if (pc >= (unsigned long)_end && pc < (unsigned long)XKVRANGE)
>> +               goto end;
> The __kernel_text_address() judgement after bt_address() do the same
> thing exactly, so why do we need this?

They are different code paths, the current path is "goto err", it will
set state->error as true, then arch_stack_walk_reliable() will return
-EINVAL.

With this patch, the code path is "goto end" for this case, it will
not set state->error as true, then arch_stack_walk_reliable() will
return 0.

Thanks,
Tiezhu