[PATCH] powerpc/stacktrace: Fix arch_stack_walk_reliable()

[Michael Ellerman]

The changes to copy_thread() made in commit eed7c420aac7 ("powerpc:
copy_thread differentiate kthreads and user mode threads") inadvertently
broke arch_stack_walk_reliable() because it has knowledge of the stack
layout.

Fix it by changing the condition to match the new logic in
copy_thread(). The changes make the comments about the stack layout
incorrect, rather than rephrasing them just refer the reader to
copy_thread().

Also the comment about the stack backchain is no longer true, since
commit edbd0387f324 ("powerpc: copy_thread add a back chain to the
switch stack frame"), so remove that as well.

Reported-by: Joe Lawrence <joe.lawrence at redhat.com>
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
Fixes: eed7c420aac7 ("powerpc: copy_thread differentiate kthreads and user mode threads")
---
 arch/powerpc/kernel/stacktrace.c | 27 +++++----------------------
 1 file changed, 5 insertions(+), 22 deletions(-)

diff --git a/arch/powerpc/kernel/stacktrace.c b/arch/powerpc/kernel/stacktrace.c
index b15f15dcacb5..e6a958a5da27 100644
--- a/arch/powerpc/kernel/stacktrace.c
+++ b/arch/powerpc/kernel/stacktrace.c
@@ -73,29 +73,12 @@ int __no_sanitize_address arch_stack_walk_reliable(stack_trace_consume_fn consum
 	bool firstframe;
 
 	stack_end = stack_page + THREAD_SIZE;
-	if (!is_idle_task(task)) {
-		/*
-		 * For user tasks, this is the SP value loaded on
-		 * kernel entry, see "PACAKSAVE(r13)" in _switch() and
-		 * system_call_common().
-		 *
-		 * Likewise for non-swapper kernel threads,
-		 * this also happens to be the top of the stack
-		 * as setup by copy_thread().
-		 *
-		 * Note that stack backlinks are not properly setup by
-		 * copy_thread() and thus, a forked task() will have
-		 * an unreliable stack trace until it's been
-		 * _switch()'ed to for the first time.
-		 */
-		stack_end -= STACK_USER_INT_FRAME_SIZE;
-	} else {
-		/*
-		 * idle tasks have a custom stack layout,
-		 * c.f. cpu_idle_thread_init().
-		 */
+
+	// See copy_thread() for details.
+	if (task->flags & PF_KTHREAD)
 		stack_end -= STACK_FRAME_MIN_SIZE;
-	}
+	else
+		stack_end -= STACK_USER_INT_FRAME_SIZE;
 
 	if (task == current)
 		sp = current_stack_frame();
-- 
2.41.0