[RESEND] x86: fix get_wchan() not support the ORC unwinder

[Qi Zheng]

Currently, the kernel CONFIG_UNWINDER_ORC option is enabled by
default on x86, but the implementation of get_wchan() is still
based on the frame pointer unwinder, so the /proc/<pid>/wchan
always return 0 regardless of whether the task <pid> is running.

We reimplement the get_wchan() by calling stack_trace_save_tsk(),
which is adapted to the ORC and frame pointer unwinders.

Fixes: ee9f8fce9964(x86/unwind: Add the ORC unwinder)
Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
---
 Changlog in resend:
  - rebase on next-20210830

 In addition, this issue has existed for nearly 4 years and no one has
 fixed it, maybe we can return 0 or remove this function directly if
 nobody cares about the return value of the get_wchan() ?

 The following is the previous patch link:

	https://lore.kernel.org/patchwork/patch/1445013/

 arch/x86/kernel/process.c | 51 +++--------------------------------------------
 1 file changed, 3 insertions(+), 48 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 1d9463e3096b..e645925f9f02 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -944,58 +944,13 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
  */
 unsigned long get_wchan(struct task_struct *p)
 {
-	unsigned long start, bottom, top, sp, fp, ip, ret = 0;
-	int count = 0;
+	unsigned long entry = 0;
 
 	if (p == current || task_is_running(p))
 		return 0;
 
-	if (!try_get_task_stack(p))
-		return 0;
-
-	start = (unsigned long)task_stack_page(p);
-	if (!start)
-		goto out;
-
-	/*
-	 * Layout of the stack page:
-	 *
-	 * ----------- topmax = start + THREAD_SIZE - sizeof(unsigned long)
-	 * PADDING
-	 * ----------- top = topmax - TOP_OF_KERNEL_STACK_PADDING
-	 * stack
-	 * ----------- bottom = start
-	 *
-	 * The tasks stack pointer points at the location where the
-	 * framepointer is stored. The data on the stack is:
-	 * ... IP FP ... IP FP
-	 *
-	 * We need to read FP and IP, so we need to adjust the upper
-	 * bound by another unsigned long.
-	 */
-	top = start + THREAD_SIZE - TOP_OF_KERNEL_STACK_PADDING;
-	top -= 2 * sizeof(unsigned long);
-	bottom = start;
-
-	sp = READ_ONCE(p->thread.sp);
-	if (sp < bottom || sp > top)
-		goto out;
-
-	fp = READ_ONCE_NOCHECK(((struct inactive_task_frame *)sp)->bp);
-	do {
-		if (fp < bottom || fp > top)
-			goto out;
-		ip = READ_ONCE_NOCHECK(*(unsigned long *)(fp + sizeof(unsigned long)));
-		if (!in_sched_functions(ip)) {
-			ret = ip;
-			goto out;
-		}
-		fp = READ_ONCE_NOCHECK(*(unsigned long *)fp);
-	} while (count++ < 16 && !task_is_running(p));
-
-out:
-	put_task_stack(p);
-	return ret;
+	stack_trace_save_tsk(p, &entry, 1, 0);
+	return entry;
 }
 
 long do_arch_prctl_common(struct task_struct *task, int option,
-- 
2.11.0