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SubjectRe: [PATCH] s390/livepatch: Implement reliable stack tracing for the consistency model
On Wed, Jul 10, 2019 at 12:59:18PM +0200, Miroslav Benes wrote:
> The livepatch consistency model requires reliable stack tracing
> architecture support in order to work properly. In order to achieve
> this, two main issues have to be solved. First, reliable and consistent
> call chain backtracing has to be ensured. Second, the unwinder needs to
> be able to detect stack corruptions and return errors.
>
> The "zSeries ELF Application Binary Interface Supplement" says:
>
> "The stack pointer points to the first word of the lowest allocated
> stack frame. If the "back chain" is implemented this word will point to
> the previously allocated stack frame (towards higher addresses), except
> for the first stack frame, which shall have a back chain of zero (NULL).
> The stack shall grow downwards, in other words towards lower addresses."
>
> "back chain" is optional. GCC option -mbackchain enables it. Quoting
> Martin Schwidefsky [1]:

This reference footnote seems to be missing at the bottom of the patch
description.

> diff --git a/arch/s390/include/asm/unwind.h b/arch/s390/include/asm/unwind.h
> index d827b5b9a32c..1cc96c54169c 100644
> --- a/arch/s390/include/asm/unwind.h
> +++ b/arch/s390/include/asm/unwind.h
> @@ -45,6 +45,25 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
> bool unwind_next_frame(struct unwind_state *state);
> unsigned long unwind_get_return_address(struct unwind_state *state);
>
> +#ifdef CONFIG_HAVE_RELIABLE_STACKTRACE
> +void __unwind_start_reliable(struct unwind_state *state,
> + struct task_struct *task, unsigned long sp);
> +bool unwind_next_frame_reliable(struct unwind_state *state);
> +
> +static inline void unwind_start_reliable(struct unwind_state *state,
> + struct task_struct *task)
> +{
> + unsigned long sp;
> +
> + if (task == current)
> + sp = current_stack_pointer();
> + else
> + sp = task->thread.ksp;
> +
> + __unwind_start_reliable(state, task, sp);
> +}
> +#endif
> +

(Ah, cool, I didn't realize s390 ported the x86 unwind interfaces. We
should look at unifying them someday.)

Why do you need _reliable() variants of the unwind interfaces? Can the
error checking be integrated into unwind_start() and unwind_next_frame()
like they are on x86?

> +#ifdef CONFIG_HAVE_RELIABLE_STACKTRACE
> +void __unwind_start_reliable(struct unwind_state *state,
> + struct task_struct *task, unsigned long sp)
> +{
> + struct stack_info *info = &state->stack_info;
> + struct stack_frame *sf;
> + unsigned long ip;
> +
> + memset(state, 0, sizeof(*state));
> + state->task = task;
> +
> + /* Get current stack pointer and initialize stack info */
> + if (get_stack_info_reliable(sp, task, info) ||
> + !on_stack(info, sp, sizeof(struct stack_frame))) {
> + /* Something is wrong with the stack pointer */
> + info->type = STACK_TYPE_UNKNOWN;
> + state->error = true;
> + return;
> + }
> +
> + /* Get the instruction pointer from the stack frame */
> + sf = (struct stack_frame *) sp;
> + ip = READ_ONCE_NOCHECK(sf->gprs[8]);
> +
> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> + /* Decode any ftrace redirection */
> + if (ip == (unsigned long) return_to_handler)
> + ip = ftrace_graph_ret_addr(state->task, &state->graph_idx,
> + ip, NULL);
> +#endif

The return_to_handler and ifdef checks aren't needed. Those are done
already by the call.

Also it seems a bit odd that the kretprobes check isn't done in this
function next to the ftrace check.

> +
> + /* Update unwind state */
> + state->sp = sp;
> + state->ip = ip;
> +}
> +
> +bool unwind_next_frame_reliable(struct unwind_state *state)
> +{
> + struct stack_info *info = &state->stack_info;
> + struct stack_frame *sf;
> + struct pt_regs *regs;
> + unsigned long sp, ip;
> +
> + sf = (struct stack_frame *) state->sp;
> + sp = READ_ONCE_NOCHECK(sf->back_chain);
> + /*
> + * Idle tasks are special. The final back-chain points to nodat_stack.
> + * See CALL_ON_STACK() in smp_start_secondary() callback used in
> + * __cpu_up(). We just accept it, go to else branch and look for
> + * pt_regs.
> + */
> + if (likely(sp && !(is_idle_task(state->task) &&
> + outside_of_stack(state, sp)))) {
> + /* Non-zero back-chain points to the previous frame */
> + if (unlikely(outside_of_stack(state, sp)))
> + goto out_err;
> +
> + sf = (struct stack_frame *) sp;
> + ip = READ_ONCE_NOCHECK(sf->gprs[8]);
> + } else {
> + /* No back-chain, look for a pt_regs structure */
> + sp = state->sp + STACK_FRAME_OVERHEAD;
> + regs = (struct pt_regs *) sp;
> + if ((unsigned long)regs != info->end - sizeof(struct pt_regs))
> + goto out_err;
> + if (!(state->task->flags & (PF_KTHREAD | PF_IDLE)) &&
> + !user_mode(regs))
> + goto out_err;
> +
> + state->regs = regs;
> + goto out_stop;
> + }
> +
> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> + /* Decode any ftrace redirection */
> + if (ip == (unsigned long) return_to_handler)
> + ip = ftrace_graph_ret_addr(state->task, &state->graph_idx,
> + ip, (void *) sp);
> +#endif
> +
> + /* Update unwind state */
> + state->sp = sp;
> + state->ip = ip;
> + return true;
> +
> +out_err:
> + state->error = true;
> +out_stop:
> + state->stack_info.type = STACK_TYPE_UNKNOWN;
> + return false;
> +}
> +#endif

For the _reliable() variants of the unwind interfaces, there's a lot of
code duplication with the non-reliable variants. It looks like it would
be a lot cleaner (and easier to follow) if they were integrated.

Overall it's looking good though.

--
Josh

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