Messages in this thread | | | Date | Wed, 13 Sep 2017 03:42:19 +0100 | From | Will Deacon <> | Subject | Re: [PATCH v2] arm64: fix unwind_frame() for filtered out fn for function graph tracing |
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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. > > 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 use that, and we could kill the graph member of struct stackframe completely?
Will
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