Re: [PATCH v10 07/36] function_graph: Allow multiple users to attach to function graph

Steven Rostedt
2024-05-25T01:31:23Z

On Tue, 7 May 2024 23:09:22 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> @@ -109,6 +244,21 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func,
> if (!current->ret_stack)
> return -EBUSY;
>
> + /*
> + * At first, check whether the previous fgraph callback is pushed by
> + * the fgraph on the same function entry.
> + * But if @func is the self tail-call function, we also need to ensure
> + * the ret_stack is not for the previous call by checking whether the
> + * bit of @fgraph_idx is set or not.
> + */
> + ret_stack = get_ret_stack(current, current->curr_ret_stack, &offset);
> + if (ret_stack && ret_stack->func == func &&
> + get_fgraph_type(current, offset + FGRAPH_FRAME_OFFSET) == FGRAPH_TYPE_BITMAP &&
> + !is_fgraph_index_set(current, offset + FGRAPH_FRAME_OFFSET, fgraph_idx))
> + return offset + FGRAPH_FRAME_OFFSET;
> +
> + val = (FGRAPH_TYPE_RESERVED << FGRAPH_TYPE_SHIFT) | FGRAPH_FRAME_OFFSET;
> +
> BUILD_BUG_ON(SHADOW_STACK_SIZE % sizeof(long));

I'm trying to figure out what the above is trying to do. This gets called
once in function_graph_enter() (or function_graph_enter_ops()). What
exactly are you trying to catch here?

Is it from this email:

https://lore.kernel.org/all/20231110105154.df937bf9f200a0c16806c522@kernel.org/

As that's the last version before you added the above code.

But you also noticed it may not be needed, but triggered a crash without it
in v3:

https://lore.kernel.org/all/20231205234511.3839128259dfec153ea7da81@kernel.org/

I removed this code in my version and it runs just fine. Perhaps there was
another bug that this was hiding that you fixed in later versions?

-- Steve