[PATCH 2/4] LoongArch: Get stack without NMI when providing regs parameter

[Enze Li <lienze@kylinos.cn>]

Currently, executing arch_stack_walk can only get the full stack
information including NMI.  This is because the implementation
of arch_stack_walk is forced to ignore the information passed by the
regs parameter and use the current stack information instead.

For some detection systems like KFENCE, only partial stack information
is needed.  In particular, the stack frame where the interrupt occurred.

To support KFENCE, this patch modifies the implementation of the
arch_stack_walk function so that if this function is called with the
regs argument passed, it retains all the stack information in regs and
uses it to provide accurate information.

Before the patch applied, I get,
[    1.531195 ] ==================================================================
[    1.531442 ] BUG: KFENCE: out-of-bounds read in stack_trace_save_regs+0x48/0x6c
[    1.531442 ]
[    1.531900 ] Out-of-bounds read at 0xffff800012267fff (1B left of kfence-#12):
[    1.532046 ]  stack_trace_save_regs+0x48/0x6c
[    1.532169 ]  kfence_report_error+0xa4/0x528
[    1.532276 ]  kfence_handle_page_fault+0x124/0x270
[    1.532388 ]  no_context+0x50/0x94
[    1.532453 ]  do_page_fault+0x1a8/0x36c
[    1.532524 ]  tlb_do_page_fault_0+0x118/0x1b4
[    1.532623 ]  test_out_of_bounds_read+0xa0/0x1d8
[    1.532745 ]  kunit_generic_run_threadfn_adapter+0x1c/0x28
[    1.532854 ]  kthread+0x124/0x130
[    1.532922 ]  ret_from_kernel_thread+0xc/0xa4
<snip>

With this patch applied, I get the correct stack information.
[    1.320220 ] ==================================================================
[    1.320401 ] BUG: KFENCE: out-of-bounds read in test_out_of_bounds_read+0xa8/0x1d8
[    1.320401 ]
[    1.320898 ] Out-of-bounds read at 0xffff800012257fff (1B left of kfence-#10):
[    1.321134 ]  test_out_of_bounds_read+0xa8/0x1d8
[    1.321264 ]  kunit_generic_run_threadfn_adapter+0x1c/0x28
[    1.321392 ]  kthread+0x124/0x130
[    1.321459 ]  ret_from_kernel_thread+0xc/0xa4
<snip>

Signed-off-by: Enze Li <lienze@kylinos.cn>
---
 arch/loongarch/kernel/stacktrace.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/loongarch/kernel/stacktrace.c b/arch/loongarch/kernel/stacktrace.c
index 2463d2fea21f..21f60811e26f 100644
--- a/arch/loongarch/kernel/stacktrace.c
+++ b/arch/loongarch/kernel/stacktrace.c
@@ -18,16 +18,20 @@ void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
 	struct pt_regs dummyregs;
 	struct unwind_state state;
 
-	regs = &dummyregs;
-
 	if (task == current) {
-		regs->regs[3] = (unsigned long)__builtin_frame_address(0);
-		regs->csr_era = (unsigned long)__builtin_return_address(0);
+		if (regs)
+			memcpy(&dummyregs, regs, sizeof(*regs));
+		else {
+			dummyregs.regs[3] = (unsigned long)__builtin_frame_address(0);
+			dummyregs.csr_era = (unsigned long)__builtin_return_address(0);
+		}
 	} else {
-		regs->regs[3] = thread_saved_fp(task);
-		regs->csr_era = thread_saved_ra(task);
+		dummyregs.regs[3] = thread_saved_fp(task);
+		dummyregs.csr_era = thread_saved_ra(task);
 	}
 
+	regs = &dummyregs;
+
 	regs->regs[1] = 0;
 	for (unwind_start(&state, task, regs);
 	     !unwind_done(&state) && !unwind_error(&state); unwind_next_frame(&state)) {
-- 
2.34.1