Messages in this thread | | | Subject | Re: [PATCH v2] arm64: fix unwind_frame() for filtered out fn for function graph tracing | From | Pratyush Anand <> | Date | Wed, 13 Sep 2017 10:29:19 +0530 |
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On Wednesday 13 September 2017 08:12 AM, Will Deacon wrote: > On Tue, Sep 12, 2017 at 10:54:28AM +0100, James Morse wrote: >> Hi Pratyush, >> >> On 01/09/17 06:48, Pratyush Anand wrote: >>> do_task_stat() calls get_wchan(), which further does unbind_frame(). >>> unbind_frame() restores frame->pc to original value in case function >>> graph tracer has modified a return address (LR) in a stack frame to hook >>> a function return. However, if function graph tracer has hit a filtered >>> function, then we can't unwind it as ftrace_push_return_trace() has >>> biased the index(frame->graph) with a 'huge negative' >>> offset(-FTRACE_NOTRACE_DEPTH). >>> >>> Moreover, arm64 stack walker defines index(frame->graph) as unsigned >>> int, which can not compare a -ve number. >>> >>> Similar problem we can have with calling of walk_stackframe() from >>> save_stack_trace_tsk() or dump_backtrace(). >>> >>> This patch fixes unwind_frame() to test the index for -ve value and >>> restore index accordingly before we can restore frame->pc. >> >> I've just spotted arm64's profile_pc, which does this: >> From arch/arm64/kernel/time.c:profile_pc(): >>> #ifdef CONFIG_FUNCTION_GRAPH_TRACER >>> frame.graph = -1; /* no task info */ >>> #endif >> >> Is this another elaborate way of hitting this problem? >> >> I guess the options are skip any return-address restore in the unwinder if >> frame.graph is -1. (and profile_pc may have a bug here). Or, put >> current->curr_ret_stack in there.
I think we should go with latter, ie assign frame.graph = current->curr_ret_stack in profile_pc().
>> >> profile_pc() always passes tsk=NULL, so the unwinder assumes its current... >> kernel/profile.c pulls the pt_regs from a per-cpu irq_regs variable, that is >> updated by handle_IPI ... so it looks like this should always be current... > > Hmmm... is profile_pc the *only* case where frame->graph isn't equal to > tsk->curr_ret_stack in unwind_frame? If so, maybe unwind_frame should just
Yes, it is the only place.
> use that, and we could kill the graph member of struct stackframe completely? >
Humm, not sure, we initialize frame->graph out of the while loop in unwind_frame()'s caller and then keep in decrementing it in looped function.
-- Regards Pratyush
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