[PATCH] arm64: ftrace: fix function_graph tracer panic

[takahiro.akashi@linaro.org (AKASHI Takahiro)]

 > Do I take this as an ack?

Yes, but

On 10/02/2015 12:27 AM, Catalin Marinas wrote:
> On Thu, Oct 01, 2015 at 03:11:29PM +0900, AKASHI Takahiro wrote:
>> On 09/30/2015 11:49 AM, Li Bin wrote:
>>> When function graph tracer is enabled, the following operation
>>> will trigger panic:
>>>
>>> mount -t debugfs nodev /sys/kernel
>>> echo next_tgid > /sys/kernel/tracing/set_ftrace_filter
>>> echo function_graph > /sys/kernel/tracing/current_tracer
>>> ls /proc/
>>>
>>> ------------[ cut here ]------------
>>> [  198.501417] Unable to handle kernel paging request at virtual address cb88537fdc8ba316
>>> [  198.506126] pgd = ffffffc008f79000
>>> [  198.509363] [cb88537fdc8ba316] *pgd=00000000488c6003, *pud=00000000488c6003, *pmd=0000000000000000
>>> [  198.517726] Internal error: Oops: 94000005 [#1] SMP
>>> [  198.518798] Modules linked in:
>>> [  198.520582] CPU: 1 PID: 1388 Comm: ls Tainted: G
>>> [  198.521800] Hardware name: linux,dummy-virt (DT)
>>> [  198.522852] task: ffffffc0fa9e8000 ti: ffffffc0f9ab0000 task.ti: ffffffc0f9ab0000
>>> [  198.524306] PC is at next_tgid+0x30/0x100
>>> [  198.525205] LR is at return_to_handler+0x0/0x20
>>> [  198.526090] pc : [<ffffffc0002a1070>] lr : [<ffffffc0000907c0>] pstate: 60000145
>>> [  198.527392] sp : ffffffc0f9ab3d40
>>> [  198.528084] x29: ffffffc0f9ab3d40 x28: ffffffc0f9ab0000
>>> [  198.529406] x27: ffffffc000d6a000 x26: ffffffc000b786e8
>>> [  198.530659] x25: ffffffc0002a1900 x24: ffffffc0faf16c00
>>> [  198.531942] x23: ffffffc0f9ab3ea0 x22: 0000000000000002
>>> [  198.533202] x21: ffffffc000d85050 x20: 0000000000000002
>>> [  198.534446] x19: 0000000000000002 x18: 0000000000000000
>>> [  198.535719] x17: 000000000049fa08 x16: ffffffc000242efc
>>> [  198.537030] x15: 0000007fa472b54c x14: ffffffffff000000
>>> [  198.538347] x13: ffffffc0fada84a0 x12: 0000000000000001
>>> [  198.539634] x11: ffffffc0f9ab3d70 x10: ffffffc0f9ab3d70
>>> [  198.540915] x9 : ffffffc0000907c0 x8 : ffffffc0f9ab3d40
>>> [  198.542215] x7 : 0000002e330f08f0 x6 : 0000000000000015
>>> [  198.543508] x5 : 0000000000000f08 x4 : ffffffc0f9835ec0
>>> [  198.544792] x3 : cb88537fdc8ba316 x2 : cb88537fdc8ba306
>>> [  198.546108] x1 : 0000000000000002 x0 : ffffffc000d85050
>>> [  198.547432]
>>> [  198.547920] Process ls (pid: 1388, stack limit = 0xffffffc0f9ab0020)
>>> [  198.549170] Stack: (0xffffffc0f9ab3d40 to 0xffffffc0f9ab4000)
>>> [  198.582568] Call trace:
>>> [  198.583313] [<ffffffc0002a1070>] next_tgid+0x30/0x100
>>> [  198.584359] [<ffffffc0000907bc>] ftrace_graph_caller+0x6c/0x70
>>> [  198.585503] [<ffffffc0000907bc>] ftrace_graph_caller+0x6c/0x70
>>> [  198.586574] [<ffffffc0000907bc>] ftrace_graph_caller+0x6c/0x70
>>> [  198.587660] [<ffffffc0000907bc>] ftrace_graph_caller+0x6c/0x70
>>> [  198.588896] Code: aa0003f5 2a0103f4 b4000102 91004043 (885f7c60)
>>> [  198.591092] ---[ end trace 6a346f8f20949ac8 ]---
>>>
>>> This is because when using function graph tracer, if the traced
>>> function return value is in multi regs ([0x-07]), return_to_handler

typo: 0x-07 => x0-x7

and pre/post-indexed addressing stp&ldp may save add&sub instructions, but
it's a matter of preference.

-Takahiro AKASHI

>>> may corrupt them. So in return_to_handler, the parameter regs should
>>> be protected properly.
>>
>> You're right. we should preserve x0-x7 around a call to ftrace_return_to_handler()
>> just in case they might be used as a "composite type" (ie. struct) of return value.
>
> Do I take this as an ack?
>
> I applied the patch locally and I'm going to send a pull request
> tomorrow.
>
> Thanks.
>