powerpc/stacktrace: Fix arch_stack_walk_reliable()

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.

Fixes: eed7c420aac7 ("powerpc: copy_thread differentiate kthreads and user mode threads")
Reported-by: Joe Lawrence <joe.lawrence@redhat.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230921232441.1181843-1-mpe@ellerman.id.au

diff --git a/arch/powerpc/kernel/stacktrace.c b/arch/powerpc/kernel/stacktrace.c
index b15f15dcacb5c42a5b26645460c45e1e5f29340f..e6a958a5da2763deca47037b1246e2d5a83576d5 100644 (file)
--- 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();