Re: User stacktrace garbage when USER_STACKTRACE_SUPPORT is not enabled

[Vasily Gorbik <gor@linux.ibm.com>]

On Wed, Mar 31, 2021 at 10:37:49AM -0400, Steven Rostedt wrote:
> But after writing all of the above, I think I found a bug! It's this:
> 
> 	size = nr_entries * sizeof(unsigned long);
> 	event = __trace_buffer_lock_reserve(buffer, TRACE_STACK,
> 					    sizeof(*entry) + size, trace_ctx);
> 
> 
> I said the above commit did not play a role in output, but it does play a
> role in creating the struct stack_trace entry. And by making it a fixed
> array (even though it's not used) it added 8 more entries to the stack!
> 
> This should fix the problem:
> 
> -- Steve
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 3c605957bb5c..507a30bf26e4 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2985,7 +2985,8 @@ static void __ftrace_trace_stack(struct trace_buffer *buffer,
>  
>  	size = nr_entries * sizeof(unsigned long);
>  	event = __trace_buffer_lock_reserve(buffer, TRACE_STACK,
> -					    sizeof(*entry) + size, trace_ctx);
> +				    (sizeof(*entry) - sizeof(entry->caller)) + size,
> +				    trace_ctx);
>  	if (!event)
>  		goto out;
>  	entry = ring_buffer_event_data(event);

It does! Thanks for the explanation and for the fix. I wonder why nobody
noticed and complained about that since v5.6.

Acked-by: Vasily Gorbik <gor@linux.ibm.com>