Subject: Re: [PATCH] powerpc/stacktrace: Fix arch_stack_walk_reliable()

From: Joe Lawrence <[email-masked]>
Date: Mon, 25 Sep 2023 15:02:50 -0400

On Fri, Sep 22, 2023 at 09:24:41AM +1000, Michael Ellerman wrote:
> 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 <[email-masked]>
> Signed-off-by: Michael Ellerman <[email-masked]>
> 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
> 
> 

Reviewed-by: Joe Lawrence <[email-masked]>

Thanks for posting, Michael.

Livepatching kselftests are happy now.  Minimal kpatch testing good, too
(we have not rebased our full integration tests to latest upstreams just
yet).

--
Joe