Re: [PATCH 6/8] unwind: arm64: add reliable stacktrace support for arm64

[Prasanna Kumar T S M <ptsm@linux.microsoft.com>]

On 28-01-2025 03:03, Weinan Liu wrote:
> To support livepatch, we need to add arch_stack_walk_reliable to
> support reliable stacktrace according to
> https://docs.kernel.org/livepatch/reliable-stacktrace.html#requirements
>
> report stacktrace is not reliable if we are not able to unwind the stack
> by sframe unwinder and fallback to FP based unwinder
>
> Signed-off-by: Weinan Liu <wnliu@google.com>
> ---
>   arch/arm64/include/asm/stacktrace/common.h |  2 +
>   arch/arm64/kernel/stacktrace.c             | 47 +++++++++++++++++++++-
>   2 files changed, 47 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/stacktrace/common.h b/arch/arm64/include/asm/stacktrace/common.h
> index 19edae8a5b1a..26449cd402db 100644
> --- a/arch/arm64/include/asm/stacktrace/common.h
> +++ b/arch/arm64/include/asm/stacktrace/common.h
> @@ -26,6 +26,7 @@ struct stack_info {
>    * @stacks:      An array of stacks which can be unwound.
>    * @nr_stacks:   The number of stacks in @stacks.
>    * @cfa:         The sp value at the call site of the current function.
> + * @unreliable:  Stacktrace is unreliable.
>    */
>   struct unwind_state {
>   	unsigned long fp;
> @@ -36,6 +37,7 @@ struct unwind_state {
>   	int nr_stacks;
>   #ifdef CONFIG_SFRAME_UNWINDER
>   	unsigned long cfa;
> +	bool unreliable;
>   #endif
>   };
>   
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index c035adb8fe8a..eab16dc05bb5 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -310,11 +310,16 @@ kunwind_next(struct kunwind_state *state)
>   	case KUNWIND_SOURCE_TASK:
>   	case KUNWIND_SOURCE_REGS_PC:
>   #ifdef CONFIG_SFRAME_UNWINDER
> -	err = unwind_next_frame_sframe(&state->common);
> +	if (!state->common.unreliable)
> +		err = unwind_next_frame_sframe(&state->common);
>   
>   	/* Fallback to FP based unwinder */
> -	if (err)
> +	if (err || state->common.unreliable) {
>   		err = kunwind_next_frame_record(state);
> +		/* Mark its stacktrace result as unreliable if it is unwindable via FP */
> +		if (!err)
> +			state->common.unreliable = true;
> +	}
>   #else
>   	err = kunwind_next_frame_record(state);
>   #endif
> @@ -446,6 +451,44 @@ noinline noinstr void arch_stack_walk(stack_trace_consume_fn consume_entry,
>   	kunwind_stack_walk(arch_kunwind_consume_entry, &data, task, regs);
>   }
>   
> +#ifdef CONFIG_SFRAME_UNWINDER
> +struct kunwind_reliable_consume_entry_data {
> +	stack_trace_consume_fn consume_entry;
> +	void *cookie;
> +	bool unreliable;
> +};
> +
> +static __always_inline bool
> +arch_kunwind_reliable_consume_entry(const struct kunwind_state *state, void *cookie)
> +{
> +	struct kunwind_reliable_consume_entry_data *data = cookie;
> +
> +	if (state->common.unreliable) {
> +		data->unreliable = true;
> +		return false;
> +	}
> +	return data->consume_entry(data->cookie, state->common.pc);
> +}
> +
> +noinline notrace int arch_stack_walk_reliable(
> +				stack_trace_consume_fn consume_entry,
> +				void *cookie, struct task_struct *task)
> +{
> +	struct kunwind_reliable_consume_entry_data data = {
> +		.consume_entry = consume_entry,
> +		.cookie = cookie,
> +		.unreliable = false,
> +	};
> +
> +	kunwind_stack_walk(arch_kunwind_reliable_consume_entry, &data, task, NULL);
> +
> +	if (data.unreliable)
> +		return -EINVAL;
> +
> +	return 0;
> +}
> +#endif
> +
>   struct bpf_unwind_consume_entry_data {
>   	bool (*consume_entry)(void *cookie, u64 ip, u64 sp, u64 fp);
>   	void *cookie;

Why not fold the previous patch and this into one?

But the code looks good to me.

Reviewed-by: Prasanna Kumar T S M <ptsm@linux.microsoft.com>.