On 2018-11-06, Steven Rostedt <rost...@goodmis.org> wrote: > On Sun, 4 Nov 2018 22:59:13 +1100 > Aleksa Sarai <cyp...@cyphar.com> wrote: > > > The same issue is present in __save_stack_trace > > (arch/x86/kernel/stacktrace.c). This is likely the only reason that -- > > as Steven said -- stacktraces wouldn't work with ftrace-graph (and thus > > with the refactor both of you are discussing). > > By the way, I was playing with the the orc unwinder and stack traces > from the function graph tracer return code, and got it working with the > below patch. Caution, that patch also has a stack trace hardcoded in > the return path of the function graph tracer, so you don't want to run > function graph tracing without filtering.
Neat! > diff --git a/kernel/trace/trace_functions_graph.c > b/kernel/trace/trace_functions_graph.c > index 169b3c44ee97..aaeca73218cc 100644 > --- a/kernel/trace/trace_functions_graph.c > +++ b/kernel/trace/trace_functions_graph.c > @@ -242,13 +242,16 @@ ftrace_pop_return_trace(struct ftrace_graph_ret *trace, > unsigned long *ret, > trace->calltime = current->ret_stack[index].calltime; > trace->overrun = atomic_read(¤t->trace_overrun); > trace->depth = index; > + > + trace_dump_stack(0); Right, this works because save_stack is not being passed a pt_regs. But if you pass a pt_regs (as happens with bpf_getstackid -- which is what spawned this discussion) then the top-most entry of the stack will still be a trampoline because there is no ftrace_graph_ret_addr call. (I'm struggling with how to fix this -- I can't figure out what retp should be if you have a pt_regs. ->sp doesn't appear to work -- it's off by a few bytes.) I will attach what I have at the moment to hopefully explain what the issue I've found is (re-using the kretprobe architecture but with the shadow-stack idea). -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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