Re: [PATCH 10/20] function_graph: Have the instances use their own ftrace_ops for filtering

Steven Rostedt
2024-05-31T02:31:34Z

On Fri, 24 May 2024 22:37:02 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
>
> Allow for instances to have their own ftrace_ops part of the fgraph_ops
> that makes the funtion_graph tracer filter on the set_ftrace_filter file
> of the instance and not the top instance.
>
> Note that this also requires to update ftrace_graph_func() to call new
> function_graph_enter_ops() instead of function_graph_enter() so that
> it avoid pushing on shadow stack multiple times on the same function.

So I found a major design flaw in this patch.

>
> Co-developed with Masami Hiramatsu:
> Link: https://lore.kernel.org/linux-trace-kernel/171509102088.162236.15758883237657317789.stgit@devnote2
>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---

> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> index 8da0e66ca22d..998558cb8f15 100644
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -648,9 +648,24 @@ void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
> struct ftrace_ops *op, struct ftrace_regs *fregs)
> {
> struct pt_regs *regs = &fregs->regs;
> - unsigned long *stack = (unsigned long *)kernel_stack_pointer(regs);
> + unsigned long *parent = (unsigned long *)kernel_stack_pointer(regs);
> + struct fgraph_ops *gops = container_of(op, struct fgraph_ops, ops);
> + int bit;
> +
> + if (unlikely(ftrace_graph_is_dead()))
> + return;
> +
> + if (unlikely(atomic_read(&current->tracing_graph_pause)))
> + return;
>
> - prepare_ftrace_return(ip, (unsigned long *)stack, 0);
> + bit = ftrace_test_recursion_trylock(ip, *parent);
> + if (bit < 0)
> + return;
> +
> + if (!function_graph_enter_ops(*parent, ip, 0, parent, gops))

So each registered graph ops has its own ftrace_ops which gets
registered with ftrace, so this function does get called in a loop (by
the ftrace iterator function). This means that we would need that code
to detect the function_graph_enter_ops() getting called multiple times
for the same function. This means each fgraph_ops gits its own retstack
on the shadow stack.

I find this a waste of shadow stack resources, and also complicates the
code with having to deal with tail calls and all that.

BUT! There's good news! I also thought about another way of handling
this. I have something working, but requires a bit of rewriting the
code. I should have something out in a day or two.

-- Steve

> + *parent = (unsigned long)&return_to_handler;
> +
> + ftrace_test_recursion_unlock(bit);
> }
> #endif
>