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SubjectRe: [PATCH] arm64: fix unwind_frame() for filtered out fn via set_graph_notrace
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On Tuesday 29 August 2017 10:33 PM, James Morse wrote:
> Hi Pratyush,
>
> (Nit: get_maintainer.pl will give you the list of people to CC, you're reliant
> on the maintainers being eagle-eyed to spot this!)

I noticed it after sending. I do use it, but there was a bug in my cccmd
script. I have fixed it. I hope, it won't miss from next time.

>
> On 28/08/17 13:53, Pratyush Anand wrote:
>> Testcase:
>> cd /sys/kernel/debug/tracing/
>> echo schedule > set_graph_notrace
>> echo 1 > options/display-graph
>> echo wakeup > current_tracer
>> ps -ef | grep -i agent
>>
>> Above commands result in
>> PANIC: "Unable to handle kernel paging request at virtual address
>> ffff801bcbde7000"
>
> This didn't panic for me, it just killed the running task. (I guess you have
> panic-on-oops set)
>

yes..

>
>> vmcore analysis:
>> 1)
>> crash> bt
>> PID: 1561 TASK: ffff8003cb7e4000 CPU: 0 COMMAND: "ps"
>> #0 [ffff8003c4ff77b0] crash_kexec at ffff00000816b9b8
>> #1 [ffff8003c4ff77e0] die at ffff000008088b34
>> #2 [ffff8003c4ff7820] __do_kernel_fault at ffff00000809b830
>> #3 [ffff8003c4ff7850] do_bad_area at ffff000008098b90
>> #4 [ffff8003c4ff7880] do_translation_fault at ffff0000087c6cdc
>> #5 [ffff8003c4ff78b0] do_mem_abort at ffff000008081334
>> #6 [ffff8003c4ff7ab0] el1_ia at ffff000008082cc0
>> PC: ffff00000808811c [unwind_frame+300]
>> LR: ffff0000080858a8 [get_wchan+212]
>> SP: ffff8003c4ff7ab0 PSTATE: 60000145
>> X29: ffff8003c4ff7ab0 X28: 0000000000000001 X27: 0000000000000000
>> X26: 0000000000000000 X25: 0000000000000000 X24: 0000000000000000
>> X23: ffff8003c1c20000 X22: ffff000008c73000 X21: ffff8003c1c1c000
>> X20: 000000000000000f X19: ffff8003c1bc7000 X18: 0000000000000010
>> X17: 0000000000000000 X16: 0000000000000001 X15: ffffffffffffffed
>> X14: 0000000000000010 X13: ffffffffffffffff X12: 0000000000000004
>> X11: 0000000000000000 X10: 0000000002dd14c0 X9: 1999999999999999
>> X8: 000000000000003f X7: 00008003f71b0000 X6: 0000000000000018
>> X5: 0000000000000000 X4: ffff8003c1c1fd20 X3: ffff8003c1c1fd10
>> X2: 00000017ffe80000 X1: ffff8003c4ff7af8 X0: ffff8003cbf67000
>> #7 [ffff8003c4ff7b20] do_task_stat at ffff000008304f0c
>> #8 [ffff8003c4ff7c60] proc_tgid_stat at ffff000008305b48
>> #9 [ffff8003c4ff7ca0] proc_single_show at ffff0000082fdd10
>> #10 [ffff8003c4ff7ce0] seq_read at ffff0000082b27bc
>> #11 [ffff8003c4ff7d70] __vfs_read at ffff000008289e54
>> #12 [ffff8003c4ff7e30] vfs_read at ffff00000828b14c
>> #13 [ffff8003c4ff7e70] sys_read at ffff00000828c2d0
>> #14 [ffff8003c4ff7ed0] __sys_trace at ffff00000808349c
>> PC: 00000006 LR: 00000000 SP: ffffffffffffffed PSTATE: 0000003f
>> X12: 00000010 X11: ffffffffffffffff X10: 00000004 X9: ffff7febe8d0
>> X8: 00000000 X7: 1999999999999999 X6: 0000003f X5: 0000000c
>> X4: ffff7fce9c78 X3: 0000000c X2: 00000000 X1: 00000000
>> X0: 00000400
>>
>> (2) Instruction at ffff00000808811c caused IA/DA.
>>
>> crash> dis -l ffff000008088108 6
>> /usr/src/debug/xxxxxxxxxxxx/xxxxxxxxxx/arch/arm64/kernel/stacktrace.c:
>> 84
>> 0xffff000008088108 <unwind_frame+280>: ldr w2, [x1,#24]
>> 0xffff00000808810c <unwind_frame+284>: sub w6, w2, #0x1
>> 0xffff000008088110 <unwind_frame+288>: str w6, [x1,#24]
>> 0xffff000008088114 <unwind_frame+292>: mov w6, #0x18 // #24
>> 0xffff000008088118 <unwind_frame+296>: umull x2, w2, w6
>> 0xffff00000808811c <unwind_frame+300>: ldr x0, [x0,x2]
>>
>> Corresponding c statement is
>> frame->pc = tsk->ret_stack[frame->graph--].ret;
>>
>> (3) So, it caused data abort while executing
>> 0xffff00000808811c <unwind_frame+300>: ldr x0, [x0,x2]
>>
>> x0 + x2 = ffff8003cbf67000 + 00000017ffe80000 = ffff801bcbde7000
>> Access of ffff801bcbde7000 resulted in "Unable to handle kernel paging
>> request"
>>
>> from above data: frame->graph = task->curr_ret_stack which should be,
>> x2 / 0x18 = FFFF0000, which is -FTRACE_NOTRACE_DEPTH.
>
> (0x18 is the size of struct ftrace_ret_stack for your config?)

yes.

>
>
>> OK, so problem is here:
>> do_task_stat() calls get_wchan(). Here p->curr_ret_stack is
>> -FTRACE_NOTRACE_DEPTH in the failed case.
>
>> It means, when do_task_stat()
>> has been called for task A (ps in this case) on CPUm,task A was in mid
>> of execution on CPUn,
>
> get_wchan() on a running processes can't work: the stack may be modified while
> we walk it.
> From arch/arm64/kernel/process.c::get_wchan():
>> if (!p || p == current || p->state == TASK_RUNNING)
>> return 0;
>
> As far as I can see, if task A is running on CPU-N then CPU-Ms attempt to
> get_wchan() it will return 0.

You seems right.

>
>
>> and was in the mid of mcount() execution where
>> curr_ret_stack had been decremented in ftrace_push_return_trace() for
>> hitting schedule() function, but it was yet to be incremented in
>> ftrace_return_to_handler().
>
> So if the function-graph-tracer has hooked the return values on a stack and hit
> a filtered function, (schedule() in your example), we can't unwind it as
> ftrace_push_return_trace() has biased the index with a 'huge negative' offset to
> flag it as 'this should be filtered out'.
>
> The arm64 stack walker isn't aware of this and interprets it as an unsigned
> index. Nasty!
>

Hummm...frame.graph always takes tsk->curr_ret_stack which is int. It has been
assigned as -1 in arch/arm64/kernel/time.c. frame.graph should be defined as int.


>
>> Similar problem we can have with calling of walk_stackframe() from
>> save_stack_trace_tsk() or dump_backtrace().
>>
>> This patch fixes unwind_frame() to not to manipulate frame->pc for
>
> Nit: strictly this is is 'restore frame->pc from ftrace's ret_stack', but I
> think we still need to do this restore...
>
>
>> function graph tracer if the function has been marked in
>> set_graph_notrace.
>
> Nit: ftrace describes these as 'filtered out', set_graph_notrace is the debugfs
> interface.
>
>
>> This patch fixes: 20380bb390a4 (arm64: ftrace: fix a stack tracer's
>> output under function graph tracer)
>
>> Signed-off-by: Pratyush Anand <panand@redhat.com>
>
>
>> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
>> index 09d37d66b630..e79035d673b3 100644
>> --- a/arch/arm64/kernel/stacktrace.c
>> +++ b/arch/arm64/kernel/stacktrace.c
>> @@ -74,7 +74,8 @@ 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)) {
>> + (frame->pc == (unsigned long)return_to_handler) &&
>
>> + (frame->graph > -1)) {
>
> This should give you a compiler warning: its declared as an unsigned int in
> struct stackframe.

I did not get this warning :(
# gcc --version
gcc (GCC) 4.8.5 20150623

>
>
> ... but with this patch /proc/<pid>/wchan now reports:
>> cat /proc/1/wchan
>> return_to_handler
>
> So it looks like 'filtered out' doesn't mean 'not hooked'. I think a more
> complete fix is to test if frame->graph is negative and add FTRACE_NOTRACE_DEPTH
> back to it:
>

Yep, it should allow to read correct /proc/.../wchan.


> From kernel/trace/ftrace_functions_graph.c::trace_pop_return_trace():
>> index = current->curr_ret_stack;
>>
>> /*
>> * A negative index here means that it's just returned from a
>> * notrace'd function. Recover index to get an original
>> * return address. See ftrace_push_return_trace().
>> *
>> * TODO: Need to check whether the stack gets corrupted.
>> */
>> if (index < 0)
>> index += FTRACE_NOTRACE_DEPTH;
>
> (looks like this is the only magic offset)
>
>
> I don't think we need to preserve the vmcore debugging in the kernel log, could
> you cut the commit message down to describe the negative curr_ret_stack being
> interpreted as a signed value instead of skipped by unwind_frame(), then have
> the reproducer and a chunk of the splat.
>
>
> Thanks for getting to the bottom of this, it looks like this was a mammoth
> debugging session!
>

Certainly few hours :-).

Thanks for your review.

--
Regards
Pratyush

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