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

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Message-ID: <CAAhV-H5AO38DAqxUyawwKW_f=ZuCaROj+s--vE8ham0sTWMxog@mail.gmail.com>
Date: Tue, 25 Nov 2025 16:08:44 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
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()

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?

Huacai

> +
>         state->pc = bt_address(pc);
>         if (!state->pc) {
>                 pr_err("cannot find unwind pc at %p\n", (void *)pc);
> --
> 2.42.0
>
>