linux-kernel - Re: [PATCH] function_graph: Add READ_ONCE() when accessing fgraph

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Date: Thu, 13 Jun 2024 23:43:37 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux trace kernel
 <linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>, Mathieu
 Desnoyers <mathieu.desnoyers@...icios.com>, Naresh Kamboju
 <naresh.kamboju@...aro.org>
Subject: Re: [PATCH] function_graph: Add READ_ONCE() when accessing
 fgraph_array[]

On Thu, 13 Jun 2024 09:52:23 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> 
> In function_graph_enter() there's a loop that looks at fgraph_array[]
> elements which are fgraph_ops. It first tests if it is a fgraph_stub op,
> and if so skips it, as that's just there as a place holder. Then it checks
> the fgraph_ops filters to see if the ops wants to trace the current
> function.
> 
> But if the compiler reloads the fgraph_array[] after the check against
> fgraph_stub, it could race with the fgraph_array[] being updated with the
> fgraph_stub. That would cause the stub to be processed. But the stub has a
> null "func_hash" field which will cause a NULL pointer dereference.
> 
> Add a READ_ONCE() so that the gops that is compared against the
> fgraph_stub is also the gops that is processed later.
> 
> Link: https://lore.kernel.org/all/CA+G9fYsSVJQZH=nM=1cjTc94PgSnMF9y65BnOv6XSoCG_b6wmw@mail.gmail.com/
> 
> Fixes: cc60ee813b503 ("function_graph: Use static_call and branch to optimize entry function")
> Reported-by: Naresh Kamboju <naresh.kamboju@...aro.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>

Looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thanks.

> ---
>  kernel/trace/fgraph.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> index 8317d1a7f43a..fc205ad167a9 100644
> --- a/kernel/trace/fgraph.c
> +++ b/kernel/trace/fgraph.c
> @@ -641,7 +641,7 @@ int function_graph_enter(unsigned long ret, unsigned long func,
>  	{
>  		for_each_set_bit(i, &fgraph_array_bitmask,
>  					 sizeof(fgraph_array_bitmask) * BITS_PER_BYTE) {
> -			struct fgraph_ops *gops = fgraph_array[i];
> +			struct fgraph_ops *gops = READ_ONCE(fgraph_array[i]);
>  			int save_curr_ret_stack;
>  
>  			if (gops == &fgraph_stub)
> -- 
> 2.43.0
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>