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Re: [PATCH v2] arm64: fix unwind_frame() for filtered out fn for function graph tracing

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  Re: [PATCH v2] arm64: fix unwind_frame() for filtered out fn for  function graph tracing James Morse <james.morse@arm.com> - 2017-09-12 12:00 +0200
    Re: [PATCH v2] arm64: fix unwind_frame() for filtered out fn for  function graph tracing Will Deacon <will.deacon@arm.com> - 2017-09-13 04:50 +0200
      Re: [PATCH v2] arm64: fix unwind_frame() for filtered out fn for  function graph tracing Pratyush Anand <panand@redhat.com> - 2017-09-13 07:00 +0200

#1730793 — Re: [PATCH v2] arm64: fix unwind_frame() for filtered out fn for function graph tracing

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...


Thanks,

James


> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index 09d37d66b630..4c47147d0554 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -75,6 +75,9 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>  	if (tsk->ret_stack &&
>  			(frame->pc == (unsigned long)return_to_handler)) {
> +		if (frame->graph < 0)
> +			frame->graph += FTRACE_NOTRACE_DEPTH;
> +
>  		/*
>  		 * This is a case where function graph tracer has
>  		 * modified a return address (LR) in a stack frame
> 

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#1731345

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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#1731372

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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