Re: [PATCH v3 1/2] kretprobe: produce sane stack traces
From: Aleksa Sarai <hidden>
Date: 2018-11-08 07:46:36
Also in:
linux-kselftest, lkml, netdev
On 2018-11-06, Steven Rostedt [off-list ref] wrote:
On Sun, 4 Nov 2018 22:59:13 +1100 Aleksa Sarai [off-list ref] wrote:quoted
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!
quoted hunk ↗ jump to hunk
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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