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

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Message-ID: <20251125063327.31586-1-yangtiezhu@loongson.cn>
Date: Tue, 25 Nov 2025 14:33:27 +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: [PATCH v1] LoongArch: Handle special PC in unwind_next_frame()

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