linux-kernel - Re: [PATCH] arm64: stacktrace: fix the usage of ftrace_graph_ret

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Date: Tue, 18 Jun 2024 12:50:31 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Puranjay Mohan <puranjay@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>, Will Deacon
 <will@...nel.org>, Mark Rutland <mark.rutland@....com>, "Madhavan T.
 Venkataraman" <madvenka@...ux.microsoft.com>, Kalesh Singh
 <kaleshsingh@...gle.com>, chenqiwu <qiwuchen55@...il.com>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 puranjay12@...il.com
Subject: Re: [PATCH] arm64: stacktrace: fix the usage of
 ftrace_graph_ret_addr()

On Tue, 18 Jun 2024 16:23:42 +0000
Puranjay Mohan <puranjay@...nel.org> wrote:

> ftrace_graph_ret_addr() takes an 'idx' integer pointer that is used to
> optimize the stack unwinding process. arm64 currently passes `NULL` for
> this parameter which stops it from utilizing these optimizations.

It no longer is an optimization (in linux-next). If it's not included,
it doesn't bother to find what the "return_to_handler" actually points to.

> 
> Further, the current code for ftrace_graph_ret_addr() will just return
> the passed in return address if it is NULL which will break this usage.
> 
> Pass a valid integer pointer to ftrace_graph_ret_addr() similar to
> x86_64's stack unwinder.

In the next merge window, this will not work. Besides the comment about
"optimization" not the real reason for this change...

Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>

-- Steve


> 
> Signed-off-by: Puranjay Mohan <puranjay@...nel.org>
> ---
>  arch/arm64/kernel/stacktrace.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index 6b3258860377..2729faaee4b4 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -25,6 +25,7 @@
>   *
>   * @common:      Common unwind state.
>   * @task:        The task being unwound.
> + * @graph_idx:   Used by ftrace_graph_ret_addr() for optimized stack unwinding.
>   * @kr_cur:      When KRETPROBES is selected, holds the kretprobe instance
>   *               associated with the most recently encountered replacement lr
>   *               value.
> @@ -32,6 +33,7 @@
>  struct kunwind_state {
>  	struct unwind_state common;
>  	struct task_struct *task;
> +	int graph_idx;
>  #ifdef CONFIG_KRETPROBES
>  	struct llist_node *kr_cur;
>  #endif
> @@ -106,7 +108,7 @@ kunwind_recover_return_address(struct kunwind_state *state)
>  	if (state->task->ret_stack &&
>  	    (state->common.pc == (unsigned long)return_to_handler)) {
>  		unsigned long orig_pc;
> -		orig_pc = ftrace_graph_ret_addr(state->task, NULL,
> +		orig_pc = ftrace_graph_ret_addr(state->task, &state->graph_idx,
>  						state->common.pc,
>  						(void *)state->common.fp);
>  		if (WARN_ON_ONCE(state->common.pc == orig_pc))