Re: [PATCH 1/3] arm64: stacktrace: recover return address for first entry

[Will Deacon <will@kernel.org>]

On Fri, Mar 24, 2023 at 01:49:56PM +0000, Mark Rutland wrote:
> The function which calls the top-level backtracing function may have
> been instrumented with ftrace and/or kprobes, and hence the first return
> address may have been rewritten.
> 
> Factor out the existing fgraph / kretprobes address recovery, and use
> this for the first address. As the comment for the fgraph case isn't all
> that helpful, I've also dropped that.
> 
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Kalesh Singh <kaleshsingh@google.com>
> Cc: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
> Cc: Mark Brown <broonie@kernel.org>
> Cc: Will Deacon <will@kernel.org>
> ---
>  arch/arm64/kernel/stacktrace.c | 52 +++++++++++++++++++---------------
>  1 file changed, 29 insertions(+), 23 deletions(-)
> 
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index 83154303e682c..219ce0668a3dd 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -69,6 +69,31 @@ static __always_inline void unwind_init_from_task(struct unwind_state *state,
>  	state->pc = thread_saved_pc(task);
>  }
>  
> +static __always_inline int
> +unwind_recover_return_address(struct unwind_state *state)
> +{
> +	struct task_struct *tsk = state->task;
> +
> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> +	if (tsk->ret_stack &&
> +	    (state->pc == (unsigned long)return_to_handler)) {
> +		unsigned long orig_pc;
> +		orig_pc = ftrace_graph_ret_addr(tsk, NULL, state->pc,
> +						(void *)state->fp);
> +		if (WARN_ON_ONCE(state->pc == orig_pc))
> +			return -EINVAL;
> +		state->pc = orig_pc;
> +	}
> +#endif /* CONFIG_FUNCTION_GRAPH_TRACER */
> +
> +#ifdef CONFIG_KRETPROBES
> +	if (is_kretprobe_trampoline(state->pc))
> +		state->pc = kretprobe_find_ret_addr(tsk, (void *)state->fp, &state->kr_cur);
> +#endif /* CONFIG_KRETPROBES */
> +
> +	return 0;
> +}

This generates an unused variable warning with defconfig:

arch/arm64/kernel/stacktrace.c: In function ‘unwind_recover_return_address’:
arch/arm64/kernel/stacktrace.c:78:22: warning: unused variable ‘tsk’ [-Wunused-variable]
   78 |  struct task_struct *tsk = state->task;
      |                      ^~~
 

Will

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