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