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From:   Balbir Singh <bsingharora@gmail.com>
Date:   Wed, 14 Feb 2018 09:01:02 +1100
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Subject: Re: [PATCH] powerpc/xmon: Dont register sysrq key when kernel param xmon=off
To:     Vaibhav Jain <vaibhav@linux.vnet.ibm.com>
Cc:     "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@lists.ozlabs.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Nicholas Piggin <npiggin@gmail.com>,
        Douglas Miller <dougmill@linux.vnet.ibm.com>,
        Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
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On Mon, Feb 12, 2018 at 11:35 PM, Vaibhav Jain
<vaibhav@linux.vnet.ibm.com> wrote:
> Thanks for reviewing this patch Balbir
>
> Balbir Singh <bsingharora@gmail.com> writes:
>
>> Any specific issue you've run into without this patch?
> Without this patch since xmon is still accessible via sysrq and there is
> no indication/warning on the xmon console mentioning that its is not
> fully functional. Specifically xmon-console would still allow user to
> set instruction/data breakpoint eventhough they wont work and will
> result in a kernel-oops.
>
> Below is command log illustrating this problem on one of my test system
> where I tried setting an instruction breakpoint on cmdline_proc_show()
> with xmon=off:
>
> ~# cat /proc/cmdline
> root=UUID=248ad10e-a272-4187-8672-5b25f701e8b9 ro xmon=off
>
> ~# echo 'x' > /proc/sysrq-trigger
> [  458.904802] sysrq: SysRq : Entering xmon
>
> [ snip ]
>
> 78:mon> ls cmdline_proc_show
> cmdline_proc_show: c0000000004196e0
> 78:mon> bi c0000000004196e0
> 78:mon> x
>
> ~# cat /proc/cmdline
> [  505.618702] Oops: Exception in kernel mode, sig: 5 [#1]
> [ snip ]
> [  505.620082] NIP [c0000000004196e4] cmdline_proc_show+0x4/0x60
> [  505.620136] LR [c0000000003b1db0] seq_read+0x130/0x5e0
> [  505.620177] Call Trace:
> [  505.620202] [c000200e5078fc00] [c0000000003b1d74] seq_read+0xf4/0x5e0 (unreliable)
> [  505.620267] [c000200e5078fca0] [c00000000040cae0] proc_reg_read+0xb0/0x110
> [  505.620322] [c000200e5078fcf0] [c00000000037687c] __vfs_read+0x6c/0x1b0
> [  505.620376] [c000200e5078fd90] [c000000000376a7c] vfs_read+0xbc/0x1b0
> [  505.620430] [c000200e5078fde0] [c00000000037724c] SyS_read+0x6c/0x110
> [  505.620485] [c000200e5078fe30] [c00000000000b320] system_call+0x58/0x6c
> [  505.620536] Instruction dump:
> [  505.620570] 3c82ff2a 7fe3fb78 38a00000 3884dee0 4bf98c05 60000000 38210030 e8010010
> [  505.620656] ebe1fff8 7c0803a6 4e800020 3c4c00d6 <38422120> 7c0802a6 f8010010 f821ff91
> [  505.620728] ---[ end trace eaf583921860b3de ]---
> [  506.629019]
> Trace/breakpoint trap
> ~#
>
>
>> I presume running xmon=off indicates we don't want xmon to take over in case of
>> panic/die/oops,
> I believe that when xmon console is available it should be fully
> functional rather than partially, otherwise it gets really confusing to
> the user as to why Instruction/Data break points arent working.
>

OK, so kernel breakpoints are broken with xmon=off and lead to oops as
opposed to passing them on to a kprobe handler perhaps?

Balbir Singh.